EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Vince on July 05, 2020, 11:48:37 pm
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Hi there,
Back at the lab after 18 months away... was waiting for new house to be built, got the keys, still some work to do in there and lab not reinstalled just yet, but I am too impatient so I started working on stuff anyway... on the big vice bench... but well, once you put a soldering iron on it, multimeter, a power strip and a scope, you forget about it and just enjoy working on stuff. Hoping to have the proper lab set up within a few weeks, though...
Anyway, I just bought this GOULD 1604 scope the other day, to complement my seemingly ever growing collection of old scopes.
It's a hybrid analog + digital scope, love this type of scope, quite versatile. 4 " full "channels (I mean not a " 2+2 " ), with independent controls.
Spec wise it's horrible at 20MHz analog BW and just as much sample rate, but who cares... didn't buy it to make it my main scope he !
Doesn't have much in the interface department either. No serial port, no GPIB.
Actually even the interfaces are weird, nothing I have ever seen. It all seems proprietary. As you can see there is what looks like, in the bottom right corner, like a DB15 connector that just reads "analog", go figure it out.
Then there are two expansion slots, 3 rows high density connectors, that look like board to board connectors... not cable connectors.
They must be general purpose expansion slots, they come straight from the main board. They are just labeled "Interface 1 " and "interface 2".
On my scope as you can see, one of these slots is populated, with a grey plastic box, which just reads " Processor Interface (Memory) ".
A memory expansion card maybe ? But why does it carry this weird looking connector on it, looks like an old RJ11 telephone plug !!!
A modem link maybe, for remote maintenance or what not ? Memory or modem, not clear...
Needless to say I rushed to open it up ! Pictures below. There is indeed some memory on it, 2x 32KB RAM chips and an EPROM. Around the "phone" socket, I see a few 74 series logic chips. There is a backup battery which of course has leaked...but luckily it seems it didn't damage any tracks or pads around it :phew:
Battery reads "MEMPAC" on it.. so surely there to save the contents of the RAM chips... why would you have a ROM chip and battery backup on a memory expansion card...not to mention the phone socket. There must be much more to it than just memory expansion.
Don't know. Maybe the ROM adds some code for added functionality, the SRAM is required to implement that functionality, and the battery is used to save some parameters associated with that functionality/feature. If anyone knows...
Have mostly Tek scopes, this is my first GOULD. They are hard to come by here in France, despite GOULD being German hence my neighbour. Well I thought it was German but it says "Made in the UK " on the scope as well as the expansion module... so not so sure now ! :o
I remember back in the day, I was in awe when I saw ads in magazines for these scopes. Their front panel looked like a space ship, and they were always pictured with a cool built-in printer, which I admired so much ! So I always had these scopes at back of my mind... and when I saw this one the other day, for only 50 Euros, I couldn't resist ! Usually good deals are sold within a couple hours typically, but this time I got lucky, responded within 30 minutes and secured it :D
Sadly I should not have assumed that they all came with the built-in printer... mine doesn't have it ! :( I guess I should have asked the seller for more pictures.. but the problem is that whenever I do that, by the time you get the pictures, the seller comes back and says sorry it's sold, to someone who bought it straight away, didn't ask for pics or additional information. So the rule is : whenever you find something interesting : respond immediately, ideally within 2 hours, don't discuss the price, and do NOT ask for anything, just say "hi I want it, what address do I send the cheque at ? ". Anything else, and you can say goodbye to the instrument :(
Still, even without printer I like this scope, it's cool to have something a bit "exotic", out of the ordinary, both in front panel design, and internal design, both electronic and mechanical.
Obviously for 50 Euros, it's defective. Seller said he was about to replace the backup battery (he supplied the brand new battery to me, nice of him), was using the scope, was working just fine, then suddenly it died on him, and was never to turn on again !
So I don't know how true that is... did he mess with the scope, fucked it up and would not admit it.. hmmm.... maybe it's a rabbit hole, maybe he did some damage that I have no idea about, and it's a hopeless case. We shall see ! ....
But for now, let's assume he was honest ans said it as it is...
I am posting this here because GOULD scopes aren't that common I feel, compared to Tek scopes for sure, so some content on these things can't hurt and might help others later on... ALSO, since I know nothing about GOULD scopes, I am hoping my German friends (English ?! I am confused now ! ) might be able to offer some insight / help on this repair...
I guess to start with, we can assume the problem is a failed power supply, or shorted power rail on some board.
I opened the scope just a few minutes ago, and took a few piccies, below. You know as much as I do right now ! :P
Luckily I was able to find the service manual for this scope. Crappy scanned copy as often, but still, better than no schematics at all of course. I attached below a PDF containing the 3 pages pertaining to the PSU. It's a "mixed" design : mostly linear, with a big mains toroîdal transformer (pure porn to look at, love these things...), with a few secondary windings. Most lead to tripod linear regulators, very basic design, but one winding supplies a (rather simplistic) switch mode regulator that provides the 5V rail for the digital stuff. On the last 2 pages I see an alternative design which is 100% linear and does away with the switching regulator. No idea what version I have in my scope, but I guess I shall soon find out ! ...
Looking inside the scope, the construction is indeed very different from the Tek scopes I am used to, especially the 2000 series since that's what's closer in spirit to this GOULD scope. To begin with, it seems more modular, which is a good thing, helps with the servicing... There is a dedicated board for the HV CRT related stuff, and a dedicated board for the power supply, great !! :-+
Had a quick look around, and I spotted a bucket load of these horrible, dreaded troublesome dipped tantalum caps !!! :palm:
So it's looking good so far, eh ?
Power supply is very simple in design, it's modular so I can take it out and work on it outside the scope easily. So should not take long to figure out if it works or not, and if not, should be quick and easy to fix. If the PSU is good, then next suspect of course are all those tantalum caps. So will do some dismantling to get better access to the top side of the main board, so I can find and locate/test all of them. Obviously ideally would be good to replace them all, but there seems to be so many of them that it could be a very daunting task indeed ! For now, the aim is just to troubleshoot / repair the scope, not to "restore" it, so I will just look for the failed one(s).
Anyway, enough talking, let's get to work now !!! :box:
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Edited my first message to talk about the expansion/interface stuff at the back, here are the related piccies...
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OK...pulled the power supply board. It's well designed, easy to free, and enough slack in the cables that you can lay the board flat on the bench next to the scope, while working on it. Really couldn't dream of having it any easier than that.
Had a closer look at the PSU schematics... guess I was wrong. There are no two different versions of that board. The second "version" is just the old hard to read original schematic, that has been redrawn on CAD so it's easy to read, and also indicate component values and chip references, which the original schematic did not show. The redrawn schematic does still have the switching supply but it's on separate page so I overlooked it at first... but it's there.
My board does match the schematics. However I had headaches because of the pinout of the output connector ! The old and new schematics don't perfectly agree on what pin goes where. And my board is a mess too : there are 11 wires, all nicely labeled on the board silk screen, to match the schematic : ' P1 ' up to ' P11 '. Except that on my board, P1 does not go to pin 1 of the connector, P2 does not go to pin 2, etc etc... had to redraw the pîn out so I could actually go probe all the pins with the voltmeter.
So, powered up the supply, unloaded/disconnected, and.... as I kinda feared... it's all good. All voltages are good, and ripple is negligible. Sure it's unloaded but still....
So, definitely nothing wrong enough on that board to cause it not to power up. Problem must be on the scope somewhere, not in the PSU.
I guess it makes sense... since the seller said it died all of a sudden, it's consistent with a dipped tantalum giving up the ghost abruptly as they do, shorting some power rail.
The PSU board was missing half of its mounting screws BTW, so the seller definitely had a go at fixing it...
Once I knew the PSU was good, I decided to plug it back into the scope and see what happened exactly... one two three... FIRE ! >:D
It's not ALL dead, I mean some stuff is going on at least :
1) Graticule illumination works
2) The green LED next to the power switch works
3) When I turn the scope ON, for a split second the LED that indicates that the trigger .. triggered, flashed. Same when I power the scope OFF.
None of the other LEDs work, so not sure why this one would show signs of life.. maybe it is a special one, might be used by the CPU in some service mode to flash error codes Morse style.. no idea. Or maybe it's just a glitch due to how the front panel board hardware is designed, and it just doesn't mean anything useful. Yeah, most probably that...
There is no relay "dance" at power up.
So I would say that the CPU board is cooked, or that at least the digital 5V rail feeding the CPU is shorted by some failed cap, whatever board that cap might be on, and its dragging the CPU board with it.
So next step I guess is to check the impedance of each power rail to see if I can see if one of them is shorted. If I don't see anything obvious then I will power the scope and check voltages while it's running, to see if one of the rails is being pulled low or something. I kinda missed the detective work, 18 months without a lab to work on stuff, I missed that so much ! But I am back in business now !!! 8)
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Ohhhh... did some measurements on the power rails, looks a bit weird ! I am opened to suggestions...
I first measured the impedance of each rail, well, resistance. I don't have any nominal values to compare with, but there are no dead shorts.
+5V, -5,2V, +12V, -12V .. they all read about the same, somewhere between 75 to 95 ohms. Not a dead short but might still be low, not sure... what do you think ?
The +180V rail reads NEGATIVE 1Mohm... no kidding ! :-DD
I guess it must supply the CRT one way or another, and the residual charge of the tube sneaks its way back to the power connector ?!... was funny anyway...
Then I plugged the PSU back and powered up the scope to measure real world voltages, loaded this time.
That's where I get interesting and puzzling things ! ???
+ 12V and -12V still read spot on, no worries there.
Problems start with these rails :
1) -5,2V reads super low at -4,87V !!! -5,2V I guess is to supply some ECL logic chips. Now I am not too familiar with ECL voltage tolerances, but -4,87 seems way off the scale to me ! So I guess any ECL chips in the scope are not working properly, or at all. Now whether that would keep the scope from power up, I don't know. I will have to go through all the schematic pages to try and locate all the ECL chips to see what they actually do.
So, maybe a bad cap on that rail, or a failed ECL chip if being shorted is among their fallure modes, I don't know.
2) The really weird one now, yeah I saved it for dessert as you do : the +5V for all the digital stuff, reads way.... way HIGH, not low ! It reads 5,40V !!! :o
How is that even possible ! :-/O
This one puzzles me big time, so if any knowledgeable person has any clue, or maybe you had that happen to you and you managed to figure it out.... I AM ALL EARS ! :scared:
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Where was your ground reference for those measurements?
If your -5.2V is overloaded and you're taking reference at the end of the loaded line the voltage drop at the end of your ground line adds to the measurement on the +5V rail.
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Hi Chacal Nocturne, nice to see you're still around ! :D
My TDS 544A says hi, says he is still so very grateful for all the hard work you did to bring it back from the dead ! :-+
Ground reference ?! I just used the chassis, right next to the PSU board.
As for the -5,2V rail, it was a red herring. I had a good look around and could not see any ECL chip, only a sea of 74HC series chips and a few 4000 series CMOS chips. Went through all the schematic pages, as well as the parts list, and no ECL. I guess it makes sense... sample rate is only 20MS/s so who needs ECL to handle that slow a data rate. The -5,2V rail is in fact only used for some of the analog stuff, nothing that would keep the scope from booting I would think ! So -5,2V was nothing to do with ECL, it was just a coincidence. The manual only has a short paragraph discussing the power supply, but at least it gives the tolerances for each rail. That rail has a wide range and is allowed to from -5,2V down to -4,80 V ! So since I measured -4,87 V under load, technically it should still be OK...
So I will stop worrying about that rail for now, and rather concentrate on the +5V digital rail which most definitely is not correct at 5,40 Volts !
Manual says 5,20 Maximum allowed, so we are way off the scale here. Hopefully not bad enough that it would cause permanent damage to all those 74 series chips ?! Hmmm... maybe it's tool late and they are all dead... will need to pull a datasheet for one of them and remind me of the maximum tolerable voltage that they can safely handle before permanent damage occurs...
So I guess the question now is : HOW is it possible for the 5V rail to read spot on, 5,0V when the PSU is UNloaded, but it reads 5,40, HIGHER when loaded ! How is that even possible ?! I don't have a clue ! Maybe "cross talk" with a higher voltage rail, like like the +12V for example... through a partially failed cap ? So I would need to find in the schematics where the 12V and 5V could possible "couple" through a tantalum cap... could be tedious I guess.
Ever seen such a case ?
I guess it's over for the simple repair... it's getting more involved now, so the vice becnh is not so adequate anymore... might be better to wait a few weeks for me to get around to reinstall the lab so I can work normally.
The problem hits the 5V rail only, and oh strange, it's the only rail that produced with a fancy switching regulator... so I guess a possibility is that maybe this regulator is misbehaving, giving proper 5V with no load, but increasing voltage when under load ? ... So I would need to pull the PSU board again, and test it with a dummy load on the bench, to see how the regulator behaves as I progressively increased the load.
If the PSU is 100% ruled out, then I can concentrate on the scope itself. I could measure the current draw on the 5V rail... though I would need a tiny DC current clamp for that, which I do not have.. need to get one. I could also supply the 5V rail with an external lab power supply, so I can control the voltage and monitor the current draw, might give some clues.
It's quite puzzling indeed... but it makes it all the more interesting ! 8)
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I've had a complicated move too... Still not finished nor recovered from it really.
I don't have a lab either as such one end of the large dining table serves me. Mrs. is tolerant to some threshold the trouble is the amount of hysteresis once beyond.
Your Gould seems to have a killer combo for capacitors the blue Philips axials and tantalum drops.
I'd apply the usual advice before actually thinking, remove all boards, check interconnect re-seat socketed stuff drown in contact cleaner...
Visual check on any low value resistors on daughter boards that may have fused and give away a short downstream.
Then I'd look towards the CPU (and other) reset circuitry, I wouldn't be surprised if you had one of those blue capacitors causing trouble there.
Also beware of the "MEMPAC" battery a lot of equipment does have boot issues with dead batteries. That's if this module is absolutely needed.
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Oh, the dining table ! I guess I am not so bad then with my vice bench, at least it's a bench of sorts !
Praying for you not to inadvertently exceed the unknown amount of hysteresis built into the missus ! :)
Piccie of the bench... right in the middle of the living room, which is completely filled with lab stuff, the bed and all the construction tools and paint for the walls.
Only the living room is done for now, all the other rooms are unusable as is.
Thanks for the good advice, will do...
Didn't know about the blue axial Philips caps ! Will watch them closely then...
Yesterday I refurbished an old '70s Philips SECAM / TV pattern generator, a PP 5514. It is full of these caps (I guess it makes sense for them to use their own stuff eh !), but somehow the instrument still worked just fine 45 years later ! :o I was a bit surprised I must admit...
For sale on LBC :
https://www.leboncoin.fr/bricolage/1809932477.htm/ (https://www.leboncoin.fr/bricolage/1809932477.htm/)
I had yet another look at the main board and I think I might be onto something worthwhile... stay tuned ! :D
... hopefully not yet another red herring...
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Making progress... spent 10 hours on the thing non stop today, but making progress... though ultimately I fear this is a lost case, but well... I tried.
So, first things first.
1) I was looking at the main board and noticed what looked like a Zener diode to me : glass body painted light blue with black markings. The blue paint had been scratched off partially, revealing the glass body. Also, the pins were bend, pushed onto the board. So I thought OK maybe it's damaged/defective...
I looked it up in the schematics. It's part of the RTC backup circuitry. It's located right next to where the MEMPAC battery used to be. Schematic below, diode highlighted in yellow, along with the MEMPAC.
So, it's not a Zener then, it's a low drop Shottky diode inline with the 5V rail. It feeds a 74LS123, dual monostable chip. One output goes to the CPU area, the other to the acquisition area. I thought well great, if that diode is defective, let's say open circuit because say the bent pins broke the bonding inside.. then the 5V would not get to the CPU nor power the monostable chip hence CPU would not get appropriate signals, not sure if it's reset or some other equally needed signal.
Sadly the diode checks just fine. It drops like 150mV. So 5,4V in, and 5,2V or so out.
I don't know how many chips get this "low" 5V rail, but I probed around at random and came across a couple. The rest of the chips, the vast majority of them, get the "full" 5V, upstream of the diode not downstream.
While I was in there, I installed the new MEMPAC battery that was supplied by the seller. Not sure why he didn't fit it. Instead, he removed the old one and soldered two very long wires in the holes/pads, to relocate the battery somewhere else I presume. Don't know why. I mean the battery is easy to get to, no reason to relocate it. Took me only a few minutes to remove his wires, clean the pads and cleanly fit the new battery.
Unfortunately that did not fix anything, but well nor did I expect it to ! But it needed doing, so it's done...
2) Then I decided to not waste time chasing faulty capacitors, because a) there are so many of them that the scope needs total dismantling and hours and hours of work and b) there is no reason to believe that there is a bad cap for now, since none of the rails are shorted and the PSU can maintain proper voltage just fine, and ripple is fine. If any of the caps were bad, the corresponding rail would sag at the least... whereas my troublesome 5V rail does not sag, just the opposite, it's WAY HIGH, and I just don't see how a shorted cap could make the rail INCREASE in voltage ?!.......
So, since 5,4V is out of spec by a long shot, and that I can't expect the scope to work with that high a 5V rail... I made it my top priority to figure out why it is so. No point spending time on any other issue until the 5,4V problem is fixed.
Will try to cut it shorter than usual... the 5V rail is the only one that's generated using a fancy switching regulator, so I tried to figure out how that could misbehave. We know that when UN_loaded, we do get our 5V just fine, well 5,10 V as that's the minimum voltage that regulator, a L296, is capable of outputting.
So how comes it gives 5,40 when loaded, plugged to the scope ?
There is a "remote sense" wire / feedback in there. So possibly this was misbehaving and causing the regulator to increase the voltage to 5,40V, somehow. See schematic below. There is a resistor, R18 that connects the feedback input pin to the output, and also goes to the sense line. I located it on the PSU board, just to see if it was well... and it was not : it's a tiny 1/8W jobbie, and buried in the middle of lots of very tall components, hard to see and even more so to get to with a soldering iron. But, I shed some light in there with a torch, and managed a decent close-up shot : can't you see anything wrong in there ?! ;) Yes... one of the two pins has NO solder on it whatsoever ! Suspicious... it was removed cleanly, probably by the seller who had a go at fixing the scope. So I redid the solder joint, and hey presto : didn't change a thing, still 5,40V ! :-//
Was worth a shot anyway...
So I persevered in that same vein, but instead decided to disable the remote sense altogether, just to see what would happen. I mean the 5V rail draws only 2,5A according to the schematic (and the regulator chip can only output 4A max anyway), which is spread across two wires so... in practice I didn't measure any significant voltage drop across the cable, even though indeed it is quite long. I get 5,40V at one end of the cable on the main board, and also 5,40V at the other end of the cable, on the PSU board. So I thought what the heck, should be alright, at least for troubleshooting purposes.
And alright it turned out to be : problem solved ! I now get 5,10 out of the PSU, and 5,10 also on the main board.... all my lovely 74HC chips get 5,05 Volts or so, depending which particular one you probe, because the board is quite large of course.
The chips which are supplied by the RTC circuitry of course get a bit less than 5V due to the Shottky diode drop that's inline, so they get about 4,95V. Still, it's well within spec !!! :D
So the biggest most urgent and mysterious problem is fixed now, gone is the 5,4 V, back is the 5,0 V, happy the chips are now !
I checked in some random 74HC datasheet. The operating range for the HC series is up to 6.0 Volts ! So no worries,, no way 5,4 killed them. Big relief then, since there are a gazillion of these chips in the scope !
However nothing changed, scope still brain dead sadly. I guess the other chips in the scope did not survive the 5,40 V, it was to be expected..... :(
Static RAM chips, EPROM chips, the CPU of course, the few big custom ASIC chips (gate arrays to do with the acquisition side of things), at the very least :-(
CPU is a good old 6809 MCU. To be precise, a EF68B09 . Google failed to find me a datasheet for this particular one. All it found was that of the EF6809. No idea what the difference is, never mind if it is relevant to my problem here. And even that data sheet is useless, it's an old scanned copy from Thomson, that's absolutely impossible to decipher, especially the "absolute ratings" table which I was interested in, as luck would have it.
RAM and EPROM chips, have yet to look at their datasheet to see what voltage range they can survive....
The ASIC chips no idea. Marking on them is weird, probably custom GOULD part numbers that mean nothing, so I will never know what it is.
So to sum it up
1) GOOD NEWS : power supply is fixed, mystery solved, now all the the rails are spot on and sound, so I can dig further.
2) BAD NEWS : did not solve the problem, still brain dead , and likely to remain so, as probably some circuits got fried.
So I am done with the easy stuff. Now if I want to make more progress, assuming any can be made that is, it will be more involved... I need to get proper access to the main board so I can probe around easily. So need to pull the entire CRT assembly, the front panel assembly, and the big mains toroidal transformer. All these need to go ! Ideally I would need to be able to remove the main board from the chassis, so I can lay it flat on the bench, standalone, with just the PSU attached next to it.
If I can do that, then the plan might be :
1) Check the clock on the CPU, and some control signals to see if it's running and doing something.
2) Check data lines on the RAM chips to see if they actually still bother putting data on the bus...
3) Check some caps : again it's clear that the vast majority of these tantalum caps are still good (enough) since the rails are all fine and dandy now. However, a small minority of electrolytics, from what I gather here and there while going through the schematics, are used for other duties than just basic supply decoupling. Some are used in timing RC networks. Good example is the RTC circuit discussed earlier. Two RC networks for the two monostables. At least one of them uses an electrolytic. Sure enough if a cap went bad in such an application, it would not short the power rail of course, so would go unnoticed, yet would cause bad timings hence possibly a brain dead board.... so yes that is a good path to follow. So I need to go through all the schematics one by one, to locate all the electrolytics that are used for something other than decoupling. The parts list is nice and clearly state it, when a cap is a tantalum, or an aluminium electrolytic.
OK so I need to do a lot of dismantling now... not enough space on the bench for that, so might motivate me to move forward in my construction duties in the house... the sooner I get to reconstruct the lab, the sooner I can work comfortably/properly on this scope ! If I move my butt, maybe I can get there in 2 or 3 weeks... in the mean time, thanks for watching, and see you in the next episode of " GOULD 1604, the resurrection !!! " 8)
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Pfff..... the forum just won't let me post pictures for some reason ! :(
I can select them, it does upload them to the server, looks all good, then once I have uploaded a few piccies, only 4 or 5 max, all well below 400KB each in any case, so all within the restrictions that apply... well then I click the "POST" button, and what I get 90% of the time is this error message :
An Error Has Occurred!
Your attachment couldn't be saved. This might happen because it took too long to upload or the file is bigger than the server will allow.
Please consult your server administrator for more information.
Weird.
Will keep trying, might work one day... :-//
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....
There you go ! The 25th attempt worked, here are the piccies at last !
Never despair...
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Hi Vince....following along.....
You need a bigger bench ! :scared:
Does the scope have a beam finder and does it work ?
Oh and about forum image management, it currently stinks !
Only post one or two in each post and it should work OK.
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Hi Tautech ! Nice to see another familiar face on here ! :D
Yeah bigger and proper bench is on its way, as I said it's a matter of a few weeks tops ! >:D
No, no beam finder on this scope... best I have is the dreaded "autoset" button.
But as I explained in my introductory message, the front panel is 100% unresponsive. I can play with any and all button on the front panel, not a sign of life. No relay dancing at power up, no nothing other than the "trigger'd" LED in the trigger section, that blinks briefly twice at power up, and once at power off.
Green LED next to the power on mechanical switch, works, as does the analog pot controlling the graticule illumination... but that's about it.
All the digital stuff on the front panel is brain dead.
There is no trace on the screen, not even a brief glimpse at power up or power off.
The analog part of the scope is under control of the digital stuff, so since the digital stuff is not working... I don't get analog scope either. It's all or nothing... and this case it's nothing at all !
4H30 AM here but I guess down under it's just tea time...
Image management is worse than I though : I just noticed a weird bug where if I click on the thumbnail of the second pic of the work bench, to enlarge it, instead of a bench it loads the pic of the switching regulator schematic !? :o I think the server is well confused...
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Ensure all the PCB interconnects are solid conductive and plugs and sockets have zero dry joints.
Triple check for any corrosion of traces near electrolytics.
Sounds like something possibly very simple Vince. :-\
Mid winter downunder Vince and wet and shitty day here.
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Yes Tautech let's hope it's an easy one ! :)
Made some progress this evening. Meant to "reseat" the front panel to see if that would help, so started to look how it was put together.
Piccies below.
Turns out it's as marvelous as one can get hope for : you do not remove the front panel... the ENTIRE front of the scope, the whole metal work for the front of the instrument I mean, comes off as one unit ! This gives much better access to the main board, which does bring a smile.
Then, the front panel electronics is made of two similarly sized boards stacked on one another using some plastic spacers. One board carries all the switched and LEDs and nothings else, no components. It is connected to the other board with flax flex cables (4 of them), 25 pin each.. yes, 100 pins total, times for both sides...they all looked brand new, nice and shiny, but reflowed them all anyway, just in case...
Then the other board, which carries all the electronics, is a two layer board. However, 100% of the components are on one side only.. the side facing you !
So everything is right in front of you, perfect, easy access. Also the 20way ribbon cable that connects that board to the main board of the scope, is just long enough that you can flip the front panel flat on the bench in front of the scope, yet still have enough slack to connect the cable to the main board. So you can probe around the front panel to your heart content while the scope is running ! I just don't see how it could be made any better, it's lovely ! :-+
The front panel doesn't have it's MCU to control everything locally with a serial to the scope. Nope. It's all fashioned here : the board on the front panel is just a bunch of muxes, decoders, LED drivers, shit registers, to make it easier for the CPU on the main board, but the main CPU is still directly controlling the front panel, there is no middle man here.
The 3 largest chips on the board are socketed, using nice gas tight sockets which really didn't need disturbing I think... but well while I was in there I reseated the chips anyway, sue me...
So after that, I powered up the scope to see if it made any difference what so ever... none, status quo, back to square one.
But I don't care, progress has been made. It's now highly unlikely that it's bad contact between the main board and the front panel. Also we know that the front panel is not of the smart type, it's directly under control of the main board so... unresponsive front panel means unresponsive main board as well !
I attached below the two schematics pages (one page for each board) for the front panel.
Oh, and there is one excellent news in the front panel department : in my introductory message I somehow forgot to mention a problem with that scope : the 5 pots that control the "variable" feature of the 4 channels and the time base.. they are just regular single turn pots with a detent when fully CW, as usual... except on my scope, all 5 of them where turning freely ! 360°, round and round and round, no stop ! :o I was fearing a big bill, but not so ! It got fixed while dismantling the front panel : I noticed that the second board was not sitting properly on top of the other board. I t was at a slightly angle for some reason... turned out the board that carries the electronics and those 5 pots, somehow had been abused, pried or something... seller probably tried to remove the front panel the wrong way... Well turns out the shaft of the pots does NOT protrude outside. The knobs do not connect to them. Instead, there is a little one inch long plastic "extender" that connect the shafts and the knobs. I think what happened is the guy pried the board, which made the plastic extenders pop out, move a bit and get out of "sync" (they are keyed, they need to be aligned properly with the shafts or they won't go in anymore). When he released tension on the board, the plastic bits where not aligned anymore and therefore the pots could not fit them any more, which kept the board from getting back in to place. Anyway, took just a few seconds of wiggling the plastic extended to make them fit the shafts again, and hey presto the board is now sitting flat and all 5 pots work just fine again ! Problem fixed in 2 minutes for zero Euro, I like that ! :)
So this is looking good, I like that... fixed this 5 pot problem, PSU is fixed, all rails OK now, and with the front of the scope removed, I have better access to the main board, great.
Can now probe the front panel board easily while the scope is live, which will tell me quickly if the main board is at all alive and trying to control the front panel, or if there is zero activity.
So, I am not in a dead end yet, progress is being made ! :P
Also, looking at the chassis, it seems like it indeed should be easy if need be, if need be... to remove the main board from the chassis altogether. All sides look like they can come off separately.
Will post pictures in a separate post so as to avoid upsetting the server, which appears very susceptible...
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Schematics for the front panel are attached at the very bottom, after all the pics.
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I had a peep at the schematics.
Your "Processor Interface (Memory)" option is in there on page 128 as "Waveform processor interface pod option" (W-PIPO?), that adds you a realtime clock, non-volatile RAM, mystery ROM for whatever and a serial data link to the outside world (I'd guess out-only).
So there's another battery in there... just for backing-up the RAM on the CPU board, by schematics I doubt a dead battery there would cause a failure to boot here.
Still I'd put checking the /RESET and /HALT lines towards the top of the to-do list. It also appears that the U19 monostable could cut the 8MHz clock to CPU through U54 PAL when system is halted. (Could although I don't see much point in it...)
There's also the LINE detection from the PSU to the monostable that you may want to look at, I'd definitely make C1 a suspect.
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Hi Paul,
Your "Processor Interface (Memory)" option is in there on page 128 as "Waveform processor interface pod option" (W-PIPO?),
Yeah I saw that schematic last night and came to the same conclusion, as it looks so very much like what I have in my expansion module ! Too similar to be a mere coincidence ^^
that adds you a real time clock, non-volatile RAM, mystery ROM for whatever and a serial data link to the outside world (I'd guess out-only).
On page 65 of the PDF file (P64 of the manual), there is a short paragraph describing that module. It sheds some interesting light, and it all adds up nicely :
They say that it interfaces to an external Keypad ! Schematic for said keypad is also in the manual, look.
So the surprising RJ11 "telephone" plug on the module is not for a modem/remote maintenance... it's no doubt the serial link that goes to the external Keypad, mystery solved ! :-)
Also, the ROM... is there precisely to add some code/routines to handle the keypad.
As for the RAM, it's used to store waveform data. Battery is there of course to save the waveform.
As for the RTC, they say it is there, but don't say WHY it is there. From what I understand they just added "because they could" ! LOL Once you have a battery in there, why not throw an RTC in there as well, can't hurt ! LOL
However one could reasonably ask why the module doesn't simply use the battery that's on the main board... maybe the module was designed to work on a range of scopes, some of which maybe didn't have a battery. Whatever....
So there's another battery in there... just for backing-up the RAM on the CPU board, by schematics I doubt a dead battery there would cause a failure to boot here.
Not to back up the main board RAM, to back up the RAM in the module, which hold wave forms. Yeah I don't see how it would keep thje scope from booting either, especially since it's an optional module, so the scope is supposed to work just fine without it ! So, I un plugged the module (as you advised me to...),
and indeed it doesn't make any difference what so ever, good or bad.
Still I'd put checking the /RESET and /HALT lines towards the top of the to-do list. It also appears that the U19 monostable could cut the 8MHz clock to CPU through U54 PAL when system is halted. (Could although I don't see much point in it...)
There's also the LINE detection from the PSU to the monostable that you may want to look at, I'd definitely make C1 a suspect.
Thanks for the advice. Yep first things to do is to check if the CPU is running or not, and if not work backwards to see why not.
But in order to do that, I need to completely dismantle the scope, because all of the good/digital stuff is hidden either by the CRT assembly, or the big toroidal transformer. Luckily I don't think that unplugging the CRT assembly would make the main board unhappy, which would probably not even know/realize that it's missing ! LOL So I will just remove it altogether so I can probe around the digital stuff.
In the meantime, last night I probed the front panel a bit. According to the schematics, and the few words of explanation that the manual was kinf enough to provide, it's clear that the front panel is 100% "passive" so to speak. That is, it's 100% driven by the main board. If I unplug the front panel the main board would not even know about it and would just keep sending bit streams to the shift registers to drive the LEDS or read the keypad. So, it's clear that in no way the front panel can keep the main board from running, now way in hell.. I rule that out.
The CPU is constantly polling the K/B for key presses, there is no interrupt mechanism at all. That means that I should be able to see activity on the front panel, so I probed for that last night.... NADA ! Not a sausage, zero activity on all the lines, dead as a dodo.
So clearly the CPU is not running, I would say !
Looking at the schematics, I figured why I none the less get two LEDS that DO show signs of life, even though the CPU is dead :
- The "Power ON "LED : it's not controlled by the CPU, it's hard wired to the 5V supply on the front panel, that's it.
- The "Trig'ed " LED : probably because the CPU is too slow to drive it with the required responsiveness, it's not driven by the CPU. Instead, it's driven directly by the analog hardware/circuitry that handles the trigger stuff.
So mystery solved.
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OK I bit the bullet ! See pics... the scope just had a bad day ! :-DD
Things went as easily and smoothly as I had hoped. The CRT assembly (CRT itself, and its control board), I left in the chassis as is, didn't touch anything, and didn't further dismantle the chassis. All I had to do was to remove the power switch "remote control", for lack of a better word, that was screwed to the edge of the main board. Then removed a few screws that secured the main board from the chassis, and that's about it ! The CRT board is connected to the main board with just one single board to board connector. So just pull on the main board downwards, and the two separate easily, and you are left with one big main board in your hands ! ;D
Then I unbolted the big toroidal transformer from the back of the chassis, so I could re-route the wires in a way that would allow me to arrange / lay out all the bits of the scope on the bench in as practical and tidy a manner, as possible.
Also removed the vice from the corner of the bench, to free some much needed space and not have it in the way any more.
Set up a scope (one that works that is ! ;) ) on the ground by the bench, facing upwards, standing on its arse. Not ideal but does the job.
So now I have a dream access to any part of the main board, anything anywhere, it's all flat on the bench, nothing restricting access... and now I also have perfect access to any and all of these dreaded tantalum caps, a myriad of them !
And it all took only 15 minutes to do, so was well worth the investment in time indeed !
I just fired it up to see what would happen, if the main board would still be happy with no CRT assembly attached... and as far as I can see the answer as I hoped is YES : when I power it up, the front panel reacts 100% in the same way as it ever did !
Not a CRT specialist and didn't spend hours studying the CRT board schematic I confess, so I guess it's still possible that doing so fried something... but I think it's unlikely and even if it were the case, it would just be an analog problem which would be I guess quite easy to pinpoint as well as cheap and easy to fix.
Easier so at least, than fixing the current problem at hand...
Anyway, now I can start probing around all the digital stuff until I get sick of it !!! :box:
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Hmmm, can't beleive it, but I think I am making progress...
Haven't yet started probing anything, but a good visual inspection was fruitful !
First I reaseated all the chips that had sockets, though I was not expecting much from it because just like on the front panel, they used nice gas tight sockets everywhere, these are good... So indeed reseating didn't improve anything at all.
Then put my fingers everywhere to see if I could feel a chip that would be abnormally hot... but no. I guess I really ought to at least get a cheap 5/10 Euros IR thermometer, more accurate and convenient than using my fingers... Obviously a full blown IR camera would be nice, but too expensive for me.
Then, I looked closely at the MEMPAC I had just replaced, and hey presto you guessed it I am sure... there is corrosion damage ! The old MEMPAC leaked and did its thing no doubt. That must be why the seller removed the old battery, so he could work on the area to try to fix things up.
See pics. The damage is only on one side of the battery. Between it, and 2 RAM chips that are right by it as you can see. There are 3 parallel tracks that run between the battery and those RAM chips. So he tried fixing those tracks, as well as a couple pins on the RAM chips. Tracks have been scrapped here and there. sometime the bare copper is still visible, sometimes he tinned it, sometimes he put some green solder mask over it.
As for the RAM pins, only the very first pin, closest to the battery, got hit, it seems. So that would be pin 14 on one of the two chips, which is Ground, and on the other chip it's pin 15 that got reworked. It's a data bus pin. On both pins you can see that there is a big blob of solder that tries to bridge the gap between the pad and what was left of the pins... If you look at those pins on the other side of the board, well there is no pin to look at ! Another evidence that it got eaten away, the guy just removed what was left from the pad hole to clean things up a little, I guess.
So there you go ! The seller clearly lied to me ! " It was working fine then died on me all of a sudden " he said, yeah right !!!! :--
Clearly he tried to fix that scope, failed, but would not admit it. He just saw the leaked battery, removed it, tried to fix the corrosion as best he could, that didn't do it, so he gave up, put the scope back together hastily, and flogged it on-line... to me.
OK.... so my conclusions, next move...
1) When I removed the main board, for the FIRST time, all the screws were matching, still had their washer, and were all real tight. I mention this because so far as I dismantled the scope a bit more every time, I encountered only missing, mismatched, and loose screws, indicating clearly that the guy had already taken apart the scope. However the main board clearly he did not pull ! That's good. It means he had to do his corrosion inspection and repair, in situ, with crap visibility and terrific access. So I stand a good chance that he either missed some damage and/or that he failed to fix it properly. So by inspecting the area thoroughly, I might stand a chance to stumble across something he missed or screwed up, hence scope might work once I fix that.
2) With the main board still attached, he just could not get any access to the digital stuff, hence he most definitely didn't even try to trouble shoot the CPU area. I bet all he did was trying to address the corrosion damage, but that was not good enough so he gave up at that point. Which means I stand a chance of finding something in the CPU area, all hope is not lost and it's therefore well worth spending some time probing around that area of the board...
As for the corrosion damage, even though I of course must inspect that area very thoroughly, I none the less doubt that it is the root cause of the problem, seeing that the 2 RAM chips that got a pin or two eaten away, have nothing to do with the CPU area ! They are part of a bank of 3 RAM chips that serve the big ASIC acquisition chip (the two large square PLCC chips on the right side of the pictures). So I guess the worse that could happen is that at boot up, the CPU might try to do a R/W sanity check on those RAMs, via the acquisition chip... but then what ? Even if R/W-ing those RAM chips failed because of the corrosion damage, how on earth would that be a reason for the CPU to halt the boot process ?! It should just store an error code of some sort and call it done ! It's not a reason to brick the scope, especially since it's also an analog scope which could still be useful / work just fine with defective acquisition RAM chips !
So, that's why I suspect there must be more to it, than just rotten acquisition RAM chips... what do you bet ?! ....
As far as inspection goes, I really would like to remove the two affected RAM chips, so I can inspect the tracks and vias underneath them ! Problem is that they are not socketed ! Obviously removing them is a risky adventure, being large 40pin DIP packages, soldered on both sides. The risk of damaging pads and making things even much worse than it is already, is too great. :(
Maybe I could cut all the pins with side cutters, then I could safely remove the bits of pins individually, one by one, hence no damage to the pads... but I would have to then replace the chips, and they are not available anymore of course, other than from obscure vendors on ebay, which sell them for 20 euros each.. no thanks, I paid 50 Euros for the scope...
They are 6264 8KB chip, more precisely HM6264AP-10. 100ns chips. You can still find one or two 6264 DIP packages chips from Farnell etc, at only a few Euros, but how compatible are they with the old chip, no idea. I guess the pinouts must somewhat be standard, so not too worried about that. More worried about access time, since they don't do 100ns chips anymore. Slowest they sell is 70ns.... might upset the board, I don't know :-(
I guess for the price, it's worth giving it a try anyway... not much to lose.
Also, worse case.. I could reuse the mutilated old chips ?! I mean I could make a small PCB with headers that would replace the pins, and plug into a socket I would install on the main board. Then what remains of the cut pins on the old chip, I could just solder to the surface of that little adapter board, eh ?
I guess where there is a will there is a way !
On the plus side :
1) The corroded area is not that large, it's manageable.
2) The density of track and pads is not that great, compared to modern standards so to speak : only 3 fairly wide spaced tracks that run between the MEMPAC and those RAM chips.
3) Board appears to be only TWO layers ! So it's quite straight forward. No weird issue with layer XYZ deep in the stack, being disconnect because corrosion hit the inside of a via or something. It will be easy to test, and easy to fix.
4) Pulled the data sheet of those RAM chips, and ran to the "absolute ratings" table... the normal operating voltage of these chips goes up to 5,5V, like the 74HC chips that populate most the board. Excellent news ! So that means the RAM chips certainly did survive the faulty 5V rail that reached 5,40 V ! If the RAM chips are rated at 5,5V, then I guess so is the CPU and all the digital stuff on the board, which means that nothing got fried and that the scope might therefore stand a chance of RESURRECTING after all !!! :D
That's excellent news indeed ! So it's definitely worth spending some time on this board, it might actually be fixable ! :D
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Might be time to invest in a desoldering iron Vince ?
Removing a 40 pin DIP should be no problem. :)
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Yeah, that, and so many other equipment as well ! There is lots of stuff I would love to have in the lab ! One of my resolution for my new home and new lab... is to refrain fro buying constantly old broken gear, and rather spend that money to actually get all the tools and accessories that will actually add " value " to the overall lab experience...
But that will be for later... currently I am more broke than I ever been in my life, so currently I am fixing my old stuff so I can SELL it to make some money to help me with the very severe loss of income due to the virus.. though it's probably in vain since I heard last week that my company has just decided to lay off 55% off the staff by the end of october at the latest, to survive the virus crisis (Airbus sub-contractor here, my company lost 60% of their revenue because Airbus kicked out any and all of their subcontractors from all of their plants, great ! ).
So yeah, a vacuum desoldering tool would be great, like so many other things... some day, but clearly not NOW ! :scared:
That said from what I saw, even desoldering guns are not magical and sometimes they just can't do the job. When the pad holes are very tiny and the pin is really very tight in the hole, and soldered on both sides of the board, the gun is having a hard time sucking all the solder to the point where the pin would be 100% free.
One method I successfully tried on smaller DIP packages I remember, was when I had a hoard of small boards filled with 8 pin DIP opto-couplers. I wanted to salvage them of course, who wouldn't. But they were soldered on both sides, on each and every pin. Solder wick just didn't cut it. So I tried heating all the pins/pads at the same time, from the bottom (to try not overheat the chips), with my hot air soldering station. It worked a treat ! Once all the solder is melted, the chip comes off effortlessly, hence zero stress/damage to any of the pads, top or bottom. But it's a shitty tiny Atten 858D station with not much power and airflow. It did the job for a small 8 pin package, but for a 40 pin package I guess I would need a "proper" station like the Quick or the recent Atten clone at only 150 Euros. Need to get one of these.. another thing added to the "must get" list...
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Oh hell Vince, hope your job is safe.
Very uncertain times for sure and for most of us too.
Down here in NZ we are doing OK, sure there have been job losses and a general downturn but we are very isolated from many world events.
I don't have a desoldering gun and use similar techniques to you but the addition of fresh silver bearing solder can help a lot in getting stubborn parts out. Modern stuff with that Pb free muck is another story ! :--
I wouldn't write your hot air station off too quick as they do the job they are intended for providing the mass to get to heat is not too great.
A good sucker and refreshes with new solder works well most of the time. Some swear by braid but I use it rarely.
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The right amount of heat and a good manual sucker work for me most of the time. Best results if you can heat from one side (component) and suck from the other (solder) just as you remove the iron tip. I'll usually do that with the PCB propped on the edge. Fresh solder or at least flux will help.
The old trick (besides the special iron bits with a notch for each pin) when you want to heat all pins at once is to shape a rectangular frame with a piece of thick copper wire (2.5mm² i.e. if you have left overs from your house) the frame must come as close as possible to all the pins when you place it over the IC. Make sure no pins are bent on the solder side, then drown the frame and component pins in solder from the top side using a >50W iron, circle round a few times, you should be able to lift the IC right out remove the frame from IC while hot.
Consider threading a strip of Kapton or a couple of long strands of thin wire under the IC before starting to give you something to pull the IC up with.
Some preheat, hot air or I.R. from the solder side can help, beware to not go over the top with the temperatures on either side.
I use this method once in a while on TQFP when I feel that direct hot air is going to be trouble.
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Yeah sometimes it's good being isolated on a little island eh ! ;D
You have a talent to motivate me do things I was hesitant to doing... I like that in you, makes me progress ! ;D
I think you are right, I might give these DIP 40 RAM chips a go... it's only a two layer board, so jno ground plane, just two signal layers, and there are no copper fills either, so as far as thermal mass is concerned, it's not that much more challenging than a single layer board.
Thanks to my recent addiction for old tube Tek scopes, I now have that low melting point silver solder, so will help.
Also, I do have somewhere a tiny sample of that outrageously mega super immensely expensive "Chip Quick (tm) " "solder", Daved did a video on that years ago IIRC. You know that special alloy which has an extremely low melting point. The alloy then takes many seconds to ccol down, which gives you more time to wiggle high pin count chips off the board. Of course it's not solder as such, so you need to clean it off afterwards so you can replace it with proper solder. Anyway, have not used it yet, but I am sure it's in some drawer somewhere... just need to find it !
As for sucker versus braid, I use both, they work well. It's just that sometimes one is more suited than the other, so I used whatever works best for a given job. When I was young and didn't know any better, I hate BOTH the sucker and the braid, because I couldn't get anything done with either of them !
Retrospectively, there was nothing wrong with either of them on a fundamental level... it's just that I of course bought the cheap stuff, because you get whatever your local store has ( no internet back then of course...), and the sucker was crap of course, and the braid was not suited to electronics stuff, was way too big. But now I bought a more expensive sucker, and somehow it works an awful lot better and I love it. It never gets clogged, and has good suction power. The braid I use proper stuff now and it works wonderfully as well.
The braid is excellent, just use the proper size and lots of flux on the joints, and a large chisel tip to presses the braid on the joint and apply heat effectively.
Then you can just drag it quickly over a long series of joints and you are done. The main problem with braid is price ! It's a consumable. If you have lots of big joints, you can go through the stuff real fast, and it costs a fortune. It's plain copper and copper is expensive, so getting it from china or Mars or Neptune is not gonna change that fact : there is no such thing as cheap copper... Chinese copper or French copper is still copper and you gotta pay for it no matter where you get it from ! :-\
On the other hand the sucker you pay for only once, then you can suck as many big joints as you wish, it's free ! However the problem with the sucker is that it's extremely slow because you have to do one joint at a time, and "reload" the spring each and every time, takes time and gets tiring after a while. Also, one you release the trigger so to speak, the sudden release of the spring makes the boards "jump", it's a pain. The brain is perfectly smooth and much faster, just drag it in a smooth, quiet, effortless move... it's a joy.
Other problem with the suck is the size of the tip, it's enormous so not suited for modern tiny joints, as you can't get it to "cover" the joint, joint is too small. The fine braid OTHO covers small joints perfectly.
So that why I tend to use the sucker when I had big joints, and not too many of them. Typically, when I salvage components from an old TV or CRT monitor, where you have beefy power resistors, big transistor, transformers and such. Not many pins, but thick pins with large holes and lots of solder on each joint. This way I don't waste lots of expensive copper, and the big joints is where the big tip of the sucker is not a problem.
However if I have smaller joints, and many of them, like for example a large VFD display, two long rows of very thin pins / pads..... the sucker would take an hour and leave me with an aching wrist, whereas the braid does the job in an eye blink and in all comfort.
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Yep all that. ;D
With IC's as you probably know the pins are sometimes not straight and sit against one side of the hole and trap a little solder and so make extraction difficult. Using a tiny screwdriver with your iron can help hold a pin off the edge of the hole while it cools then you don't have the extraction problems.
I use a magnifying visor for all my soldering work now and it really helps spot these tiny traps. ;)
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Yep, I do the same. I first quickly drag the braid to suck all the solder from the rows of pins, then I check them one by one to see if they are free and if not, I reheat them and wiggle them with a pair of tweezers, then hold them in place in the center of the hole as I remove the iron tip.
@shakalnokturn : our posts collided ! You replied to me as I was already composing my reply to Tautech...
Thanks for all the good advice as usual ! :-+
Yeah, I really do need to get a better soldering gear... it's very high on my list of priorities for the new lab.
Currently I have my trusty 50 year old Weller iron, 24V AC, only 50Watts but somehow never failed me, more capable than it sounds on paper. Also, the temperature is regulated but not adjustable, you know it's the kind that's regulated by the iron tip itself, so if you want to use a different temperature, you need to buy a different tip that's calibrated for that temperature. One good thing about this iron other than its quality that made it last so many years, is the tips are still available 50 years later, and are real cheap, like a couple Euros a piece !
But I would like a newer iron, with adjustable temperature, ideally a " direct heat " type but I don't think Weller do this type so would need to buy elsewhere. Then I would want 90 Watts minimum to feel more comfortable.... but if I can afford a 130 Watt iron then why not, the better...
Then with that, a collection of tips so that I always have the best tip for the job, to get things done as easily, efficiently and cleanly as possible...
As for flux, after watching hours of videos on Louis Rossmann ' YT channel... I have become a flux addict ! As he is used to say : "There is no such thing as too much flux ! " ;D It's very expensive but I get Amtech flux through him in super large 30cc syringes and it really keeps cost down. I now use flux liberally all the time and it makes soldering and desoldering so much easier and cleaner. Why didn't they teach me the virtues of flux at school ? They don't have their priorities straight I am telling you ! ;D
Anyway... two things :
1) One of the RAM chips as you can see is surrounded by two big boys : the battery I just put in, and that big blue Philips axial electrolytic cap. I need to remove those two if I want to be able to grab the chip by its sides to pull on it.
2) HOUSTON, WE HAVE A PROBLEM !!! Just looked.... the pins on those ram chips.... they are all BENT flat on the PCB on the solder side !! |O
I guess the god of solder does not like me, is that it ?! :scared:
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2) HOUSTON, WE HAVE A PROBLEM !!! Just looked.... the pins on those ram chips.... they are all BENT flat on the PCB on the solder side !! |O
I guess the god of solder does not like me, is that it ?! :scared:
Oh no ! :palm:
Really :wtf: who really needs to bend the pins on a 40 pin DIP ? :o
Bugger, my sidecutters are twitching to make it easy for you.
Find some new IC's I guess. :-//
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Yeah that's tough luck ! :--
But I just had an idea that might do it for cheap and be quick to implement, since I am on a budget : once I have cut all the pins to safely remove the chip without damaging the board/pads, instead of doing a PCB adapter to fit it back, which costs money and time to do (as well as get delivered) since I have not done a PCB / CAD in 10+ years... I could maybe simply use a chunk of veroboard filled with pads, the size of the chip, then just solder a couple male to male header strips (I think I have some in stock...), the little bits of the strips that will protrude through the veroboard, the pin-less chip ought to fit snugly into, then I will just have a minimal gap to bridge with solder, to connect the header pins to what's left of the pins of the RAM chip ! Might try that...nothing to lose, if that doesn't do it I will just try my chance with newer incarnations of the 6264 chips, and pray that they are still compatible (enough) electrically !...
But at any rate it will slow me down and I am not going to wait to do that, especially since as I said, I hardly believe that corrosion in the acquisition RAM area could (should) brick the scope. So for now I will just test the damaged tracks and pins to see if I can spot a blatant problem/cut trace, and if not... will leave it at that for the moment and rather move on to probing around the CPU area to see if it's running or not, and go from there...
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Do you have a universal programmer to check the remains of your 6264 RAM while out of circuit?
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Nope, no programmer here... though it's very high on my list of gear to get to improve the new lab, of course...
What do you have at the back of your mind ? What would you do if we could look at the RAM contents ?
Also.. I did read here and there that dynamic RAM modules in computers could retain their data for a few seconds once out of power, due to the charge being stored in the capacitors, but for static RAM chips I never heard about it, I assumed the data vanished instantly...
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Wow.... made a lot of progress today... 5AM, time flies when you work on this thing, better get some sleep ...
Alright, so I spent many hours inspecting the board up close, in the corroded area and close vicinity, inspectin gvisually and checking for track continuity, and found many problems, some of which are fixed.
Checked the tracks the guy repaired, they tested fine.
Armed with an A3 format print of the Acquisition schematic (attached), and a highlighter to keep track of what I checked, I started checking methodically each and every pin of the 3 RAM chips, up to where they went on the two acquisition ASIC chips. The address and data buses where daunting... especially since the data bus of the three RAM chips are NOT shared ! They are all independent and go to their very own individual port on the acquisition chip !
Here the findings :
1)
Top right corner of the sheet, R78 pull-up, connected to the three Chip Enable inputs of the RAM chips... tried to measure it, reads open circuits, several Mohms... Tried to find it on the board, to no available, couldn't see it. Checked the parts list to see what value it is, to help me locate it on the board... guess what ? According to the parts list their is no such thing as R78 in this scope ! It jumps straight from R77 to R79... with blank line where R78 would be...
Then I though it's probably a red herring again. I think most likely they just decided not to implement it, probably because it was redundant since that input is probably driven by a totem-pole output somewhere, not an open-collector output, so no need for a resistor. What made this plausible is that they seem accustomed to using unnecessary resistors on inputs... see the OE / Output Enable pins, they are all grounded.. via a resistor again. I bet it would work just fine without that resistor eh..
So I decided to declare this a non-issue, and moved on.
2)
The R/W inputs. They were all connected together, no worries, however the signal didn't make it to U56 nor to the acquisition chip. It was a cut trace but it was hard to spot. On the board nothing looked obvious at all... all that attracted my attention was what looked like tiny dot of black marker/pen... but that dot was right next to the R/W pin of one the RAM chips, and also right next to U56.... so had to dig further just in case. Bingo...at first you don't see any track connecting the two chips, but that's because ...the track happened to be completely hidden / covered by the white silk screen that was exactly where the trace is ! So I scrapped the silk screen to see what's what, and indeed there was a track connecting my two chips, yeah !! :D
Then scrapped the black dot and yep, under it, the track was cut open. Area was too confined to repair the the track, so to get me going I just soldered a bit of mod wire straight onto the pins of the chips as it was the only things that was easily accessible with the iron.
Powered up the scope... still no joy, still brain dead. Oh well, it needed fixing anyway, so that's one less problem in that scope, can only be a good thing ! Also, it demonstrates as I hoped, that the seller was not thorough enough and missed some things. So, it means I CAN dream that the problem is fixable and that he just didn't find it because he simply gave up to soon... I like that ! :)
3)
He removed one the RAM chips... on the bottom side of the board I noticed that one of them did NOT have the pins bents on to the PCB, and that all the solder joints were hand made, and looked like they were done by a cave man should I add ! So I poured a gallon of flux on that chip and reflowed every pin, just for good measure, and to stop my eyes from bleeding... it really didn't look pretty. Other evidence that he removed that chip, is the biog blue Philips electrolytic cap : it's right next to that RAM chip, you do need to remove the cap so you can grab the RAM chip and pull on it. Well, there is a suspicious solder blob on one of the pins of that cap... sure enough, he didn't bother removing the cap, he simply cut on of the legs so he could lift it,then bent it back into place and soldered the cut pin back together again ! I guess it works.... at least this way he didn't damage the pads !
4)
Still on the bottom side of the board, in the close vicinity of that RAM chip, I found a big dent in the board ! A chunk of the fiber glass is missing, a little crater when see up close... right on a track ! Bad luck... I fixed the broken trace.... powered up the scope full of hope.... nope, still brain dead.
5)
After all that, I decided to move on the another part of the Acquisition schematic sheet : the clock circuitry, to get a first shot at probing the CPU clock...
The circuitry is made of a couple oscillator and a couple chips, two custom PAL / programmable chips, 20pin DIP chips. 16V8 IIRC.
There is a 20MHz oscillator that drives the acquisition stuff. I probed the signal on the corresponding pin on the PAL chip, I do get a 20MHz signal in there, all fine. Then I probed for the CPU oscillator, 8MHz.... nowhere to be found ! I probed each and every pin of that PAL, because it's not clear what pin I am supposed to probe since the resolution of the drawing is so poor. Pin 13 maybe, is my best guess. So I flipped the main board so as to be able to probe the oscillator pins directly, no middle man.... and yep, the output of the oscillator is dead as a dodo, a solid flat trace on the scope, it's not even trying ! LOL It's Kaput, RIP.
Replacements are not exactly plentiful... old through hole DIL 14 package, 5Volts... Farnell has zillions of oscillators but only one that would match the package, operating voltage and frequency, and it's on back order because well... just in case you hadn't noticed by now, this repair is driving me nuts. Plus, I am skint and don't want to suffer the minimum order of 15 Euros IIRC.
So, luckily a local shop 40 miles away has one, for the same price as Farnell, under 3 Euros,...except I don't doubt that it will be some unbranded crap but well, if it works, it will do... shipping is only 5 Euros IIRC so that will be 7,50 total versus twice that from Farnell. So I think I will go for that..
6)
Still in the signal path of the CPU clock, I found another break in the path : clock comes out of the first PAL (U54) on pin 13 if I read that right, and is supposed to then enter the other PAL (U53) on pin 1. Well, I don't have continuity there ! :-( Need to investigate....
Those two PAL chips are very, very far away from one another. They are literally on opposing edges of the board, so tracing the... trace, could be "fun", it probably takes a twisty road and switches from one side of the board to the other, multiple times...
So it was quite a fruitful session today, happy camper. The guy really went deep on that one, but butchered it, so it's not quite clear yet if it can be saved. I mean, how can I be sure that I won't miss some of his butchering here and there... I would need a crystal ball to know exactly what he did to that poor scope !
6:15 AM now, time to get some sleep eh ?! :=\
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....
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Nice steady progress Vince. :-+
The blue resistor ? 2nd from right seems to have a dodgy solder joint:
(https://www.eevblog.com/forum/repair/scope-repair-gould-1604/?action=dlattach;attach=1021728)
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Yep I noticed that too when I looked at my own pictures on the computer screen. With lots of light and a macro shot, it reveals things that go otherwise hardly noticed. Will touch up that joint for sure...
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What do you have at the back of your mind ? What would you do if we could look at the RAM contents ?
Not looking for any data there, only a tool to logic-test the I.C. functionality if you must reuse it, sometimes the electrolyte does creep in through pins.
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Oh, stupid me, testing the chips...yes that sounds like a good idea indeed ! Another motivation for getting a programmer....
Just back from my local shop (in my town)... he had ONE suitable oscillator in stock, so no need to pay shipping and wait 3 days to get it from the other less local shop that's 40 miles away in the next town, great !
Can't wait to install it on the board... stay tuned ! ;D
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4)
Still on the bottom side of the board, in the close vicinity of that RAM chip, I found a big dent in the board ! A chunk of the fiber glass is missing, a little crater when see up close... right on a track ! Bad luck... I fixed the broken trace.... powered up the scope full of hope.... nope, still brain dead.
That looks like crazy bad luck to damage a track accidentally in that location, besides the dent is really clean, did you check what link that was on the schematic?
Just keep in mind that it may have been done willingly by a drill bit on a "factory rework". Are there any added wires glued across this board?
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4)
Still on the bottom side of the board, in the close vicinity of that RAM chip, I found a big dent in the board ! A chunk of the fiber glass is missing, a little crater when see up close... right on a track ! Bad luck... I fixed the broken trace.... powered up the scope full of hope.... nope, still brain dead.
That looks like crazy bad luck to damage a track accidentally in that location, besides the dent is really clean, did you check what link that was on the schematic?
Just keep in mind that it may have been done willingly by a drill bit on a "factory rework". Are there any added wires glued across this board?
Looking at the solder side photo I'm pretty sure those links have been cut away on purpose as a layout correction so should be left as-is.
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Wow !! Track cut on purpose ?! I must say I didn't think f that ! But yes if you say it is "common" practice back in the day.... yes indeed I agree it looks like it was done very cleanly by a drill bit, it did strike me at the time.
Also yes, as you saw on the pictures, in this area of the board there is a ton of factory bodge wires and components !
Hmmm... thanks for this suggestion ! :-+
OK, so I installed the new 8 MHz crystal....
1) I do get my 8MHz signal now, and it does get to the CPU, great. However still brain dead. The clock signal looks... absolutely.... A-TRO-CIOUS ! See pics ! It's not much of a digital signal ! Not even sure how a TTL output could produce such a weird waveform...
I thought maybe the two PAL chips that receive this clock, are defective and load the oscillator output in some weird way... so I pulled both PAL chip in an attempt to unload the oscillator, but I still get exactly the same waveform.
So I thought OK it's non-sense, might a probing issue... so checked for the basics : sufficient bandwidth ? Yes... scope is 100MHz,probe too. Probe is set to x10 of course, so as to get the B/W, and I compensated it, it's fine. B/W limit enabled on scope ? Nope, set to sue full B/W. Channel one defective ? Switched the probe to channel two, still the same...
Need to try a different probe, and a different scope... glad to have 30 scopes in my collection, sure I will find one that will do the job ! :P
The waveform looks like, with some imagination, a mismatched transmission line getting reflections : It looks like a square added to itself with some delay...
2) The output of the other crystal, the 20MHz one, also looks weird, although a different kind of weird ! See for yourself...
It looks "better" in some way... we do have a flat portion at the top, and a decent falling edge. However the leading edge shows massive "dampening" ! :o
3) Talking about bodges, there is a long bodge wire that routes the output of the 8MHz oscillator.... straight to the CPU !!! Yes, it thus bypasses the first PAL chip which according to the schematic is supposed to be the recipient of that clock !
So, I need to suss this out ! Ideally I would want to pull the new oscillator so I can test it outside the scope, with a lab power supply, in a "controlled" environment so to speak, to make sure (or not...) that it works fine. But I fear that would put too much stress on the pads on the board... I mean I have desoldered the old oscillator, put the new one in.. if I pull it again and then put it back in... would be a bit much don't you think... Maybe I can solder a chip socket (oscillator has a standard DIL 14 layout) on the board ?? It's only 8MHz so the extra inductance of the socket won't be that much of a problem I think.
I am hope it's a red herring and just a scope/probe issue, but who knows.
I guess Shaklanokturn might on to something.. I might try removing my repair of that "broken" trace on the bottom, in case it was part of the factory bodge jogs, see if that fixes the waveform....
Also, since the CPU is now getting a clock, though a not very useful /working one, I could not help but start probing the thing, was just too curious ! ;D
I am glad to say that now, it does warm up a little, so it's doing something, whereas before it was dead cold AFAIR.
I probed each and every pin, to see if there was any activity, and the status of all control input pins that maybe cause it to not run. I attached below, the schematic sheet for the CPU, or course. So here it is :
- Adress and Data buses : some lines are high, some low, but there is zero activity on any of them.
- Interrupt input : active low, and all 3 of the m are high, so that's good.
- DMA : it's disabled / hardwired high with a pull-up, but checked anyway just in case... it's high indeed, good.
- R/W output : no activity.
- E and Q : I get a steady 2 MHz square wave, 50% duty cycle, on both these pins. Waveforms looks very clean, though the top of the leading edge / corner, is a bit rounded / dampened, but not the end of the world. Never used a 6809 but according to the datasheet I pulled, these signals are indeed supposed to be identical square waves, though in quadrature, and run at a quarter of the CPU clock ! That's interesting ! 8MHz clock, and I get a 2MHz from these pin, steady ? So, that would mean that even though the waveform of the clock is disgusting, somehow the CPU is happy with it, as it manages to derive E and Q with it none the less ? So.... this means that although I do need to figure out the dirty clock signal, it is NOT my main problem, it does not keep the CPU from working.
- I kept the best for dessert, you will forgive me... the RESET line ! It is..... it is.......... make a bet ! :P
Yes, it's active low, and it IS held LOW ! :D So I have something well defined to investigate / concentrate on now, no shooting in the dark anymore, I like that very much !
So, to summarize :
1) Need to figure out the dirty clock signals : undo the repair I done to that cut trace in case it was done intentionally at the factory. Then try a different probe and scope, in case it's a measurement issue / red herring.
2) Start to investigate why the CPU reset line is held low.
Making progress ! ;D
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The 20 MHz clock looks fit for purpose, not so the 8 MHz. :-//
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That looks like crazy bad luck to damage a track accidentally in that location, besides the dent is really clean, did you check what link that was on the schematic?
OK, I removed my repair of this "broken" track. Now the output of the 8MHz crystal looks much better ! Thanks for your suggestion ! :D
See picture below. I still get a huge one full volt of over-shoot and under-shoot, but at least now it looks much more like a square wave, phew !
Overshoot probably due to me using the standard 4 inch long ground strap, maybe...
I tried to figure out what that track was on the schematic, but failed miserably : it just goes fro one via to another one. On one en, the track "stops" under a component as well as under the silk screen, so no way to know where the track buggers off... then on the end, the track goes to a via that leads right under an integrated circuit (which is not socketed so can't removed it) so again can't can't see what's happening...
The 20MHz clock however, still looks just as horrible as before, it did not improve or affect it in any way.
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The 20 MHz clock looks fit for purpose, not so the 8 MHz. :-//
You like the 20MHz clock ? Oh well, I was expecting much better from a TTL output, but well if it's OK to you I will move on to the reset line problem then...
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OK some progress again... I worked on the RESET line problem that's stuck low.
Looked at the CPU schematic sheet, attached. Couldn't be simpler, just an RC network connected straight to the Reset pin on the CPU, 5 minutes job... or maybe not.
Somehow they decided to locate that RC network 2 miles away from the CPU, go figure. Resistor tested OK, diode too, electrolytic cap was not shorted.
I am not too sure about why they put such a big cap in there. 10uF 25V. 25V for a 5V rail... sure you need some derating but 10V would have been enough...
Also, the resistor is only 4,7K ! Couldn't they use a much larger resistor ? That, plus a 10V rating, would have made for a much smaller / cheaper cap, maybe not even a need for a horrible electrolytic cap. Anyway, that's about a 50ms time constant, modulo the 20% tolerance on the cap... I looked at the signal on the scope, single shot at power up, to observe the exponential, just because we can ! ;D Looks good, and indeed about 50ms. So I knew this was working just fine, problem is not there.
Then since the RC network is so far away from the CPU, I rushed to check if I had continuity between the RC node and the CPU reset pin... NOPE !
There is my problem ;D
So I started tracing the reset track on the board. Problems started about a quarter inch after the node : the track doesn't go to the CPU.. it immediately stops by a chip right next to it, and connects on pin 4.. of U33. What the ?! It's U33.... which according to the parts list, does NOT exist.. AGAIN an inconsistency between the board and the manual, I am fed up...
So my Reset signal goes to an unknown chip rather than the CPU, and I can't even rely on the schematic anymore now, great, what to do now ! |O
It's a 74HC132 ... a quad NAND chip with... Shmidt Trigger inputs ! Ah OK... reset line, Schmidt trigger... they probably decided to condition the signal before feeding it to the CPU...though its Reset input is ALREADY a Schmidt Trigger input, so that's redundant, strange. Anyway, maybe that chip is dead, so I probed the output of that gate to see if I got continuity.... no. Hmmm... then I thought oh, it's a NAND, it inverts the signal so they need to make it go through a second gate to recover the correct polarity. So I probed all the pins till I found what other gate it was connected to... then probed the output of that second gate to check continuity with the CPU... no luck, still no continuity. So I started to follow the trace... took an hour, a nightmare... its changes board side so many times that I lost the count of it. Maybe 10 times or so, either using a via, or using a pin on some chip. And it buggers all over the place as well, the Reset signal is not used by just the CPU...
At some point the track hides under a ROM chip.... hopefully socketed so I could remove it to see where the track was going.
Then as I got closer to the CPU, the track is on the component side and goes under one of the Acquisition RAM chips, the one that the guy had removed. No way to know where the track "exits" the chip. I just took a guess... I supposed it was going straight ahead, and noticed that it was going through two pins that looked a bit crusty, not far from the battery that leaked. The same two pins I had already noticed a while back, and why I wanted to removed that chip to check for residual corrosion damage. Well I was right, the break was there ! Right under that chip between those two pins ! The guy went to the trouble of removing that chip... and didn't even fix the broken trace ?! Go figure.
So as a quick fix I soldered a mod wire on the solder side between the CPU and the nearest joint, to bypass that RAM chip, then powered the scope and... the front panel went X-mas tree mode, a real fireworks ! Relays dancing too, sounded good !
So I put the scope together, reconnected the CRT board to the main board, put a few screws here and there to kinda put it back together again, sorta, and powered it up again, and got that :
https://www.youtube.com/watch?v=D6CA8uvCsa4 (https://www.youtube.com/watch?v=D6CA8uvCsa4)
YES !!! IT WORKS !!!! :box:
JOY !!!
Couldn't believe it, that feeling, hard to describe !!! Put so much effort into this repair, so many problems, and just as I was going to give up, it's fixed !!! :D
First power up, no probes attached but already got waveforms on the screen ?!
They were stored in the module attached at the back of the scope. I also know why there is an RTC in that module now : when I went in the menus, I could see the stored waveforms, they all had an individual time stamp on them. So, that module still works just fine and its battery is good !
However of course, the RTC on the main board was reset because of the battery replacement. However I reset the time and date and it remembers it, so the Battery replacement went well 8)
I checked all 4 channels in analog mode, all seems fine bar a horrible and varying offset, depending on what the attenuator is set to. I guess there must be some adjustment for that on the board. In my old Tek scopes there is. I think they call it "DC gain balance" or something like that...
So I played with the thing for an hour, checked every menu. I am new to this GOULD stuff, it's my first one ! ;D
A bit weird but I did not see anywhere in the menus, any self test ! Plenty of them in Tek scopes that like to go wrong, but ZERO self test in the GOULD ?! Can't quite believe it but, well... unless there it is hidden and there is a magic button sequence to play to get access to a service menu...
However when I switch to digital / store mode.... I get the readout and cursors and such, because it's handled by the CPU which now runs fine, but I don't get ANY trace on the screen ! Not even a flat line, no, I mean nothing, not a pixel ! So either I don't know how to operate the scope, or... the acquisition section on the main board is kaput ! Oh no...... :( That's too much for me, the acquisition stuff is too complex, and I am exhausted by this repair.... might come back to it later, once I have recovered some juice / lust, but for now I will just be happy that I managed to bring it back to life despite so many problems and having to deal with the crap that the seller did. Fixing a dead scope is one thing, but fixing a dead scope that has already been visited, taken apart and messed with extensively, is another thing ! >:(
Perseverance paid, it looked hopeless but in the end it was fixed with just a 4+ Euros 8MHz oscillator, not bad ! For a 50 Euros scope, I am not complaining....
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:-//
No YAHOO on the video audio ? :-DD
Nice fix Vince. :)
Now it's time to RTFM I guess. :box: :-DD
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Yeah no sound sorry, I noticed that after the fact, had it turned off somehow ! :o
That said you didn't miss much other than a little bit of fan noise...but I do agree it feels weird when it's 100% silent ! :-[
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So for all you know there is no self test implemented at all, (I'd get hold of a ROM dump if available and check it for text string error messages just to be sure) and digital memory is not running due to other cut tracks around the same RAM IC's.
May be worth beeping all their pins.
At least you're getting somewhere.
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Google found me this very interesting 4 page topic on the 1604 in some UK forum devoted to vintage radios :
https://www.vintage-radio.net/forum/showthread.php?t=46501 (https://www.vintage-radio.net/forum/showthread.php?t=46501)
It's ancient and closed/archived, I couldn't post to it even if I subscribed to that forum.. but still, 4 pages of interesting stuff.
1)
There is a secret key combination at power up to reset the scope to factory default, because apparently there is some non-volatile memory somewhere that cannot be erased otherwise. Did that. No change... so I guess it was already reset when I got it.
2)
There is a secret Service menu ! Press the '9' key 3 times in a row and the menu shows up ! Sadly it's not much use. Not much in it, and certainly no error logs or the possibility to run self tests.. other than testing the external keypad if you have one.
3)
I gathered the scope does run a self test at power up, but in "silent" mode, and very minimalist/quick, so you don't even notice it. Only way you know that it was performed is that at the end of it, it displays at the bottom of the screen the firmware revision of the scope, and also that of the ROM in the pod you might have installed on the scope. That works for me. I can tell it's very minimalist because I unplugged the large acquisition daughter board (which holds the acquisition stuff for channel 3 and 4. Acquisition for channel 1 and 2 is on the main board) and it powered up just fine, no error message, and works no better or worse than it did before ! So I guess it does a checksum of the ROM on the main board and that's about it, doesn't even bother with testing the acquisition stuff !
4)
Apparently in the UK there are a bunch of these scopes around with leaky batteries...
5)
Didn't think of mentioning it yesterday, as it's only a secondary issue for now, but from a cold start, the trace and readout have some focusing issue, can't get them to be super sharp. Also sometimes the text "wobbles" around and is "noisy" if you look up close.After a few minutes of warming up it gets better and better. Guys in that thread said it's a typical problem, just a few tired caps n the PSU, on the plus and minus 12V rails. So might recap the PSU if I manage to fix the acquisition issue.
6)
GOULD is king as far as inconsistent documentation is concerned, as well as countless undocumented firmware or H/W revisions, some being not even retro-compatible. Basically these scopes are a total mess, and GOULD was on drugs when they designed their scopes.
7)
They shed A LOT of light on the interface " pod " (that's how GOULD calls it) that plugs at the back of my scope. The numbers on it now make sense. A lot of mysteries completely solved now : " 1600 / 160A (105) ". 1600 means 1600 series, matches my 1604 model. " 160A" is the name of the waveform processor "package", ie the pod AND the external keyboard. 160 is the model name for the K/B. "105" refers to the pod itself.
So my 105 pod, is one part of the 160 "waveform processing" package.
How does it work ? When I plug the pod on my scope, I didn't see any measurements functions added in any menu, so I was disappointed and perplex.
But it's there !!! The ROM in the pod holds the code for the measurements functions, but that's it. The scope can then use that code. However the scope has NO provision for adding menu items to let you access these extra features from the front panel. That's where the K/B comes into play ! You plug it to the back of the pod using that RJ11 "telephone" type plug, it's a serial link. I found pictures of that keypad, it's got 26 keys or so (as we can see from its schematic in the service manual). It allows you to remote control the scope in storage mode : you can use the cursors, run/stop/single functions and.... and also select the measurements functions which are implemented in the Pod's ROM !
So if you don't have the K/B you can't use the cool measurement functions. However the K/B is very simple, it just sends a single byte coding the key you pushed, using a serial link. The serial bit stream is not even processed by the pod itself, it only goes THROUGH the pod, without interacting with it... to the main board on the scope. Then the scope goes to the ROM in the pod to fetch the code that it needs to run to perform whatever function you asked for.
Some guy on the forum there, had a keypad and was kind enough to show a picture of it, and make a table of the name of each button, and what byte gets send to the serial port when you push them. Some have just connected a terminal to the pod, to emulate/replace the keypad, and were sending the bytes by hand, and it worked just fine. Just need to add some level shifting if you use a computer (usually outputs 12Volts) to lower it down to TTL levels so as not to fry the main board in the scope...
Others have built there own keypad and programmed some little micro to scan the buttons and send the byte to the pod.
So I if manage to fix the acquisition, I might try to hook up a terminal to the pod to see if I too, can get access to the cool extra stuff... could be fun.
OK so now back to the repair.
Spent some time operating the scope, trying to understand what was going on.
1) In unplugged the pod. Scope lost the date and time ! So that's handled by the RTC in the pod, not on the main board.
2) When I am in storage mode, I can't get a trace but it keeps displaying a stored waveform on all 4 channels. Channel #1 displays a flat line, channel #2 a triangle, channel #3 a square wave, and channel #4 another triangle but at a different frequency. I tried all I could to make these go away... but they just won't. Waveforms are supposed to be stored in the pod, so I removed it. No joy, they still show up. I went into the menus to try to see how to erase them, no joy. Without the pod, all the main board will do is save TWO "reference" traces, two, not 4. Plus, these 2 are empty/deleted, and disabled, set NOT to show up on screen.
Played the magic key combination that resets the scope... still no joy. So I come to the conclusion that these 4 waveforms aren't stored from actual past acquisitions by the user. Instead, they are probably "test patterns", which are hard coded into the Firmware on the main board, which the scope displays when it feels it's right to...possibly precisely because I reset it !
So I have stopped pulling my hair about these mystery waveforms, these ghosts... They will probably go away as soon as the acquisition is fixed and actual data comes in, that will wipe these test patterns from the screen.
So what to do next.... as you said yes, most probable is some more corrosion damage hidden under one or more of these 3 acquisition chips, so need to spend some time in that area. I have already tested each and every pin of each and every of these RAM chips, and fixed what was broken, as detailed in my previous posts. However there is still plenty of of tracks on the acquisition schematic that need checking, other the RAM chips. some glue logic, control lines to the acquisition ASICs. I bet one of these control lines goes under the RAM chips and might be broken... hopefully it's just that and I will stumble on it with enough patience...
But I will need to dismantle the scope all over a gain... it's depressing, I was so happy to have it all back together and alive... only to put it all in pieces again ! :-\
In any case, I also REALLY need to get some work done in the house or else at X-mas I will still be sleeping in the living room !!! Need to work on the bedroom and bathroom. So I will slow down on the repair stuff from now on. Want to do both at the same time though, so I guess I shall alternate between the two activities...say work 2 days on repairs, then 2 days working on the house, then back to repairs etc...
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OK took the scope apart again and worked a bit on it.
Probed a bit more in the acquisition area. Checked that the main acquisition chip (the one that interfaces with the CPU, not the other ACQ chip that handles the RAM data). That chip does get its 20MHz clock, and I see that its R/W line, presumably driven by the CPU, is very active. So I took that as a good sign that the CPU was asking the ACQ ASIC to acquire some data. So it was reasonable to thus expect to see activity on the 3 ACQ RAM chips, no ?
So I probed for that. Zero activity on the address and data buses, same on Chip Select pin. At least it's consistent, no chip select means the ASIC doesn't intend to use the RAM, so why would you have activity on the address bus.
So I looked at what provided that chip select signal for the RAM. As you can see from the schematic posted earlier, there is a bit of glue logic there, in the middle of the ACQ chips and RAM chips : a NOR gate U52b, and U56, a quad two input MUX.
The first ASIC doesn't drive the chip select directly. Instead it feeds it to the input of that NOR gate, whose output then drives the second ASIC, the R/W of the RAM, as well as pin #1 of the MUX / U56. Pin #1 selects between the two inputs of the MUXes. Two of the 4 outputs of the MUX feed back to the first ACQ ASIC, and the two other drive... yes, the Chip Select of the RAM chips. So it's all very much intertwined...
Now I probed the NOR gate. Nothing coming out of it, so no Chip Select, normal. Maybe the gate is dead ? Nope.. its two inputs are low, and show zero sign of activity. The first ASIC drives one of the inputs, the other input as you can see is connected to ground via a RC network. Not sure what that's supposed to do... maybe a crude method to delay the signal a bit before the NOR gate transfers it to the second ASIC and RAM chips, no idea.
But whatever... you will notice on the diagram that the output of that NOR gate is precisely where I had a broken track ? Remember, that pervert one, hidden under that "block dot" right next to the output of that gate. So I switched to "if it looks like a coincidence... it probably isn't ! " mode.
So I wondered : what if that broken trace was AGAIN another factory mod, like the cut trace on the bottom side of the board, that was cleanly drilled ?
The "dot" was indeed very circular/clean too, and concave as well. So I thought maybe again they drilled that track on purpose... and decided to cover the hole with a black marker / pen, to keep it from corroding or god knows what, not really important I guess....
I wasn't sure how that would cure my problem, because for all I see, the only thing driving that line, is the gate ! So if I remove my mod wire that bridged the broken track... there would be nothing left to drive that line, it would be floating ! But I thought.. to hell, it's been already demonstrated several times that this schematic just isn't reliable, so maybe on the actual board there is some chip other than that NOR gate, that's there to drive the line. So I decided to remove my mod wire and give it a go !
Glad I did : ACQUISITION NOW WORKS !!!! :box:
So for the second time, big thanks to Shakalnokturn for suggesting that the bottom track could have been a factory mod ! The other track on the top side was too !!!
I was so happy, put the scope together AGAIN... and AGAIN I shouldn't not have done so : I AGAIN have more problems, so AGAIN I will have to take it apart ! GRRR !!!!
What's wrong now ? Well, acquisition works but... only on channel one. I still get the hard-coded " test pattern " on the other 3 channels.
ALSO, I have now lost channels 2 / 3 / 4 in ANALOG mode too, were they used to work before ! Oh no ! :-(
So this scope is very depressing indeed... one step forward and one step back ! Not sure I will ever see the end of it... but well, I guess I am making progress none the less, eh ?!.......................
Of course the first suspect is that the mod wire I removed somehow caused this problem, because in this scope the digital and analog stuff are so closely intertwined, it's a complete mess, nothing is very clear, it's all fuzzy. So I thought oh no, it that mod wire cures the acquisition but makes the analog channels disappear, it's a lost cause... I give up.
But before I give up, lemme put the wire back, to see if the 4 channels come back to life in analog mode : NO THEY DON'T !
GREAT ! Phew ! So the wire didn't cause the problem, it's a new issue altogether, somehow I prefer that !
It was just a coincidence then, just back luck.
So I guess I made some progress after all : found out that the black dot was a factory bodge, re-activated it and now I have acquisition working, albeit only on Channel 1. But if it works on one channel, then 90% of the chain is working and the rest is bound to be a more minor and less complicated issue, I can hope.
The way I am attacking this is : since I lost channel 2/3/4 on both analog and storage modes, a reasonable assumption is to think that there is one single problem, common to both modes. So at first I would rule out any problem with the acquisition subsystem entirely, and focus more on the analog stuff that's common to both modes.
I attached the overall block diagram for the scope, highlighting the signal paths that I suspect most.
As one of you mentioned, most obvious is to check that the signal traces are not simply way off screen. So I checked carefully, thoroughly for that, in both modes of operation.
In analog mode there is no " Beam Finder " button like one is used to see in almost every scope ever made. It's a GOULD after all, it's exotic, so I guess they didn't want to do like the rest of the crowd.
So all I could do was to set all channels to GROUND coupling, trigger set to AUTO of course, then set the attenuators to the least sensitive setting, in this case a whooping 20V (most scopes are 5V usually), then played ad nauseum with the vertical position up/down control switches... no luck. No trace to be seen other than channel #1 that works fine. Tried running the auto calibration routine that's there to get rid of any offset and gain errors in the front end... no joy. Tried the dreaded " Autoset " button, for once in my life... no luck either.
The only hint of an offset/off-screen trace, I found only in storage mode, and only on channel #2 : if I press the "select trace" button, it turns on the cursors and places one of them, which " follows " the trace, on the selected trace. So using that control, I can see that I am able to select channel #2, because the read out tells me the cursor is using it. Also, if I move the cursor along the trace, well an invisible trace of course, I CAN see the bottom half of the cursor sticking out of the top edge of the screen, suggesting that the trace might indeed be there, off screen at the top. Also, the readout shows a couple arrows that point upward, suggesting that the trace might be indeed up there off screen. So that would indeed point to a vertical offset problem, a huge one... but only for channel 2. Channel 3 and 4, no suck luck.
So I think I may indeed have a vertical problem on channel 2, so will investigate that. But it can't explain why channel 3 and 4 are missing, never mind why in analog mode 2 to 4 are missing.
So I think I have a second issue there. Trying to figure out how I could have channel one working fine in both analog and digital, and no trace at all on all 3 other channels... your guess is as good as mine, well better actually, but my own reasonable guess would be.... how does the scope show 4 traces at the same time, with a single electron gun at its disposal ? Yes, if the " beam switching " mechanism fails, it would explain everything...
As we can see on the block diagram, this mechanism is a mess like the rest : it's done in THREE parts ! ... Apparently the scope was designed as a two channel scope originally, so the circuitry for the first two channels is on the main board, then to implement channel 3 and 4 they had to add that large daughter board that sits on top of the main board. It handles part of the stuff for the last 2 channels. Some of the acquisition, and some of the analog stuff.
So there is a switch for channel 1 and 2, then another for 3 and 4, then a third switch that switches between the output of those first two switches ! Lots of switches ! :-//
So I guess I need to take the scope apart yet again (I am kinda getting the hang of it now... ), and start probing those switches...more generally leave the digital part of the scope, move on to the analog part of the board, and follow the signal path from the BNC inputs down to the output of that final switch. Hopefully I am right and I will find something interesting en route... wish me luck :popcorn:
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That RC network was likely a delay circuit for the reset. They put a NAND with Schmitt trigger inputs to get a clean reset release despite the slow rise of the RC voltage.
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Not sure what you mean ? What reset line ? And it's not a NAND it's a OR gate, and it's a 74F32 which according to the datasheet I got, does not have Schmitt trigger inputs.
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Not sure what you mean ? What reset line ? And it's not a NAND it's a OR gate, and it's a 74F32 which according to the datasheet I got, does not have Schmitt trigger inputs.
I was referring to your Reply #44.
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Ah OK, the CPU reset line ! ;) You didn't quote that old post so I had to assume you were replying to my latest posts.
Yes of course that NAND gate is there to generate a clean reset signal. Not of any use to the CPU though, since it has already a Schmitt trigger reset input. However that reset signal feeds a few more chips as well, not just the CPU. So I assume one of those other chips didn't have a Schmitt input, they had not realized it so they had to modify the board. I don't know...
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It very clearly does appear that, world wide, this Co-Foo-19 does have everyone working with the patience of St. Isaac.
:-+
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Yes, it's not all bad this virus indeed ! Losing lots of money, soon to lose my job most probably, but at least it gives me plenty of time to work inthe house and on this bloody troublesome scope !
OK I spent a few more hours on it. As planned I probed the signal path for analog operation. I fed the probe calibration signal to all 4 channels simultaneously so I could follow the signal on all 4 channels at the same time and compare everything to channel #1 which is the one that works.
I probed at the output of the preamplifier, ie the output of the front end...then looked at the output of the CH1/CH2 switch/MUX, then the one for CH3/4 on that daughter board, which connects to the main board via coaxial cable. All good. The square wave makes it through all 4 channel paths, no problem with offset, noise or signal amplitude. All 4 channel look similar.
Then I looked at the output of the final switch, that aggregate all 4channel, plus the readout information, and then goes straight to the Y amplifier on the CRT board. Well even there, it's good : see screen shot. All 4 channels are there, along with the readout information. At this stage, I can play with the front panel control to modify attenuator setting, vertical position, and it all reacts as expected.
So now at this point, we can say for sure that the signal makes on all 4 channels makes it just fine right up to the last final step, right up to the CRT, and that there is no problem with massive vertical offset that would shoot the traces off the screen.
So that's good to know, making progress...
So now what could cause the traces for channel 2 to 4 not to appear on the screen ?
Updated annotated block diagram attached. In green, I highlighted everything I have probed and that appears to work satisfactorily, ie the ' Y ' analog path, the vertical side of things. That's one of the 3 inputs of the CRT board. Other two of course is the ' X ' horizontal side of things, ie the sweep signal, and then third signal is the ' Z ' side of thing, the intensity control, ie blanking signal. So let's investigate those last two paths...
Two hypothesis at this point, highlighted in yellow :
1) sweep : maybe there is no sweep for channel 2/3/4, and the spot is off the screen on the left or right, invisible.
2) intensity : if sweep turns out fine, then maybe the signal is somehow blanked when comes time to display CH 2/3/4 .
Scenario #2 seems most probable so will start there...
In any case, as shown on the block diagram, both the sweep and blanking signals are controlled by... not the CPU, but by the main ACQ ASIC ! So, we are going back to this area of the board ! I guess that's good in some way, as it means it is now becoming possible that a corroded track is causing the problem. But let's not assume anything... let's probe instead and see what's going on ! 8)
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OK, one step at a time. Just check the X signal, the sweep ramp. Attached is the schematic sheet for the time base. Highlighted is the ramp generator, whose output goes straight to the CRT board. For the life of me I could not locate that chip ! 90% of the component designators on the main board are impossible to read.. because they put them underneath the component. How useful... might as well not put them at all ! :-\
So I resorted to probing the CRT board to board connector directly. Two rows of 16 pins. Here again, no help from the silk screen to tell you which row is which, never mind tell you on what end of the connector is located pin #1 !
So I first probed each and every pin with my DMM to find which pins carried the high-ish voltages from the PSU ( 75V and 185V) so as to avoid getting a nasty shock and/or blowing my scope...
Then I could safely probe all the other pins with the scope. Found only one pin with a ramp signal on it, see below.
We can see clearly a repeating ramp, clean, with a few "glitches" here and there. IF I zoom on it I see it's shorter ramp, of varying length, which appears at "random" places in the normal stream of ramps. I assume these are the ramps for the readout.
So as we can observe, there is a continuous stream of ramps, which is good I think : If the sweep was responsible for the lack of Channel 2/3/4, then I guess we would see just one ramp followed by a blank signal equivalent to the duration of 3 ramps.. but it's not what we have here.
So I think we can say that as far as sweep is concerned, there is no reason for traces 2 to 4 not show up.
So the only thing that can make these traces disappear now, is the blanking signals / intensity control... next on my plate.
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So as we can observe, there is a continuous stream of ramps, which is good I think : .................
I'm not so sure.
While I've never worked on a CRO with readouts those glitches in the ramps would be of some concern to me.
Normally you have a single ramp generator and unless its linearity is perfect....Houston, we have a problem !
Study the sweep gen schematic again 'cause normally there's a expected waveform displayed for reference but maybe not for these Gould's. :-//
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Don't be worried Tautech, I am not.. for a change ! ;D
There HAS to be some ramp for the readout, mixed in with the " regular " ramps, or else how would it be able to display the readout content that's embedded in the vertical / Y signal ?
If you look at the schematic I attached above, in yellow you see U604, the big chip that generates the ramps for the 4 channels. Then if you look to its left we have U609 which precisely generates the ramp for the readout, and mixes it to the main ramp signal from the other chip, to produce the "composite" ramp signal that goes to the CRT board. I wish I could probe these chips individually so you could see the individual ramps BEFORE they get mixed... but I can't because as I said most of the chips on the main board, I failed to locate because the chip designators are not visible. So the only point I could locate easily to check anything really, was directly on the CRT connector, hence only the final, composite signal.
As for linearity, I could not care less.. all that would do is distort a little bit the waveform, but it would not keep the trace from appearing on the screen, nor keep it from stretching fully from the left edge of he screen to the right edge. Same for glitches... basically as long as there is some kind of ramp happening when the chopping control signals switch from one channel to the next, I should get my 4 traces on the screen, no matter how distorted or glitchy or dirty or whatever, but I should still see traces...
As for example wave forms...this is a GOULD scope, not a Tektronix ! ;)
Tek service manuals are marvelous, but one should not take their quality for granted : as soon as you work on a different brand scope, you should be prepared to be disappointed one way or another... :(
I just consider myself lucky that there is ANY kind of service information on this scope at all. Had I not the little that I have got here, the scope would not be repairable at all...
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:)
Yeah I studied the schematic some and most certainly the area around U604 is interesting, quite unlike the ordinary RC sweep gen. :o
Then I spot above it, End of Sweep Detect that of course one suspects is the source of the beam retrace blanking.
In this respect the schematic is great as it properly breaks everything down into each functional block. :-+
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I found half a page worth (spread over two sheets) of explanation in the manual, about the ramp generator, which confirms what I figured by looking at the schematic. Have a look.
I don't think even old basic scopes used RC combinations to generate ramps, or it would be an exponential not a ramp ;D
Even old scopes use caps charged at a constant current, which does give ramps.
This GOULD scope is no different, as you can read in the pages attached. On the schematics, you can see the bank of timing caps in the bottom right corner of the sheet. This area is tagged " Calibrate Time Base Capacitors " . It revolves around chip U607 which allows to select the desired cap.
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TAUTECH !! I apologize ! :scared:
You had me thinking all night with your RC generated ramps... the constant current charging is so old and wide spread, simple and efficient, that I couldn't possibly believe scopes were using RC networks. I mean for decent / modern scopes, that do have a calibrated and triggered time base. Not talking about pre- WWII stuff of course...
But to be sure, I checked the schematics of an old and really shitty scope, a Hameg type HM 412 from the '70s, that I just fixed and sold the other day. It DOES use RC charging ! :palm:
Then I looked at my old Tektronix tube scopes from the 1950's... they also use RC not constant current ! :-//
So I stand corrected, my apologies ! :-[
I guess if you use only the start / first portion of the exponential, it does approximate to a straight line sufficiently that you can make use of it for a sweep ramp. The specs given for the linearity are not spectacular to be begin with so I guess it's " good enough "...
At least the old Teks had the excuse that tubes were large and space at a premium in the cabinet, so if they could get away with RC and save 2 or 3 tubes by not implementing a current source, it was a reasonable choice I guess...
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OK...I spent a few more hours working on the thing, checking the blanking signals that control the intensity / Z signal to the CRT, the third and last information that goes to the CRT, last hope of finding something wrong.
To cut the story short... I did not find anything wrong, all signals in this area looked as I would expect. None of them had blatant "blank" portions that would correspond to the 3 missing channels / traces on the screen.
So that made no sense. All the signals that go to the CRT should make it display my 4 channels, period.
So I must be missing something, just not competent enough for sure, hey I am not a professional after all, no shame...
So decided to give up on it, having run out of ideas as to why it would not work. Bought if for 50 euros not working at all... at least I improved on it quite a bit, now it powers up, runs, and works in analog and storage mode, albeit on channel #1 only... that's a lot better than the brick it was when I got it. So hopefully I can still sell it for at least what I paid for it !
Proceeded to reassemble it, but wanted to give it one last test anyway just to see, so reassembled it only as partially as possible.. did not even put the front panel assembly back onto the chassis, I just laid it on top of the scope... ribbon cable is just long enough to allow for that, and works just fine ! Hell I should have done that from the beginning, saves time !
Plugged the PSU connector to the main board.... BIG SPARK, BANG, FLASH !!!! Oh no... just because I resorted to give up on it doesn't mean I wanted to kill it for good ! Damn it ! Went to turn the power off.... Ah, power strip was ALREADY OFF hmmm.... maybe the spark came from some big cap on the PSU that got shorted... at any rate the chance of the scope surviving are slim. So turned the power back on... scope still alive and kicking, powers up and runs !!! That's one tough guy !!! :D
Then what do I see... some garbage on the screen, fuzzy and all.... let me adjust focus and intensity a bit... OK... WHAT ?!
I have FOUR traces in analog mode ?! They are BACK ?! Fed the calibrator signal to all 4 inputs at once, I get 4 square waves no problems ! Even better : played with the attenuators : NO MORE random big offsets problem ! Traces don't move around any more as I change range ! :D
I proceed to switch to Digital / Storage mode, anxious.... IT WORKS !!! FOUR traces on the screen, four real traces this time, not " place holders " / test patterns like before !!! IT BLOODY JUST WORKS !!!!!!
Even I, can not believe it, so I took some pictures below to prove it.
Oh boy !!!
Can you believe it ?!
I spent countless hours probing around, studying schematics, pulling hair and scratching head.... but did not actually DO anything to it ... yet it WORKS ! :o
So I guess all that time and effort put into it was not in vain after all... I was just rewarded for my perseverance, there is some justice out there in the end ! :D
So no corrosion problem any more in fact, not a blown tantalum cap, no nothing...
Only explanation is some cold solder joint somewhere, which I must have triggered while working on the board.
That's bad because that means it can stop working randomly at any time. Might not even work after I am done putting it all back together properly, and even if it behaves for so long as to let me take pics of it so I can advertise it online... it might break again during transport to its new owner, and he will be mad at me : "You said it was working perfectly !!! "..... yada yada yada..... :-//
Anyway, even it it lasts only for 5 minutes we shall see, I am none the less overjoyed to see it working, and my hard work rewarded !
At least if it fails again, I know there is nothing wrong so to speak, so no need to chase red herrings and waste time anymore... just a bad connection god knows where.
So I am super happy, I managed to fix what the previous owner failed at ! ;D
So to summarize the repair : was bought "No Power". Found a 5V rail that was way high at 5,40V. Fixed it, was a buggy remote sense feedback line. Then it would power up but not come to life/run. Found the CPU was not running. First found its 8MHz oscillator was kaput, replaced that, still not running. Found that the Reset line got broken in the corroded area due the leaky battery. Bridged that track. Scope now back to life !
...and wasted time by trying to be too thorough and fixing two broken traces that turned out to be factory mods hence did not need " fixing " ! :-//
Thank you Tautech and Shakalnokturn for your support, much appreciated, we won ! :box:
Now here is hoping the bloody thing will still work in 10 minutes once I have put it all back together again... cross all your fingers and those of your cats and dogs, I need as many fingers as you can come up with !....
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:-DD
Vince's new fix method; shock it into operation ! :o
Congrats Vince and what nice sharp traces it has. :-+
Patience and tenacity is generally rewarded.
Hope you make a buck from it as you've certainly earnt it.
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Thanks Tautech.
Trace looks sharper in the flesh, but it's so difficult to take a good / realistic picture of a CRT ! |O
Bags of reflection on the screen, and no amount of playing with ISO settings can reproduce the reality, not enough light so lots of noise/grain, and a bit blurry because I am human so hands shake a bit no matter what. With good / additional lighting, and a tripod to keep the camera steady, and a delayed shutter so I can get away from the scene so that I don't reflect on the screen... would help a lot I am sure.
Anyway, I have just finished putting the scope 100% together..... and it decided to be nice with me : it still works, PHEW !!! :phew:
Took a couple pics, I can put it on-line for sale now ! 8)
Quite happy, turned a 50 Euros brick into a fully working scope with only 5 bucks worth of parts, the CPU oscillator and that's it ! :)
Now on to the next repair, another scope undoubtedly... what's left that I could fix and sell... which one(s) would I survive without...
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For sure I wouldn't have kept the Gould...
It seems a lot of bother (and even more potential bother depending on who buys it and how it behaves) for the price you're selling it.
A few months ago there was a 100 or 200MS/s Gould going for 70€ with one defective channel, I hesitated and figured it wasn't worth the risk. Gould is not a name that sells that easily in France.
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Oh, you saw my ad on LBC for it ? 120 Euros, seems like a fair price given the poor performance of the thing, can't really ask too much for a 20MHz scope can I ! :-\ not to mention as you said, that GOULD is not at all as well known and " admired " as Tektronix !
So I thought I would never manage to get 120 Euros for it, but go figure, only 12 hours after I put the ad on-line, I already have a buyer, and he did not even try to negotiate the price ! :o
Seems like a decent guy, if just because he managed to write a 20 line paragraph with zero spelling error, which is unheard of in this country for the past 20 years. Just for that alone I am willing to trust him ! :-DD
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Gee that was quick, guy doesn't sleep it seems, 2AM here and he was still on-line ! Already transferred the money, deal done ! :D
Couldn't be happier ! I never thought I would manage to sell it for that price, never mind in less than a day ! I guess I am being rewarded for the work I did on this scope... there must be a god of the oscilloscopes out there somewhere ?! ;D
I mean, with the virus induced economic crisis, so many people are losing their job or at least suffering major income losses, that I expected it would take weeks or months to sell this scope, and that I would at any rate never get the asking price. I am very surprised indeed. I guess there must still be people out there who don't suffer financially from the virus... lucky them !
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I got a Gould 1604 for repair. So far the repair was easy, an exchange of 3 electrolytic capacitors and 1 burnt resistor. I have not put it all together again, but so far it looks good: the supply is back to normal and traces are visible.
There is one question remaining: how important are the batteries for the memory bachup. As expected the batteiries are old and are starting to leak - so they must go. At least no damage to the baords so far.
Are the batteries really needed and if so, what would be a suitable replacement ?
As far as the calibration instructions go, there are no calibration constants stored in the battery backuped RAM.
There may be NiMh batteries, but they don't like the constant tickle charge very much and may not last very long.
Super-capacitors may be an option (maybe with faster charging - which could be a plus for a more infrequent use), but maybe with a limited old time.
Holding the settings for 1 day or so would already be not so bad.
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Wow can't help you much, I sold this scope a year ago ! :-//
I have long forgotten the details of it. Just read my thread maybe.
From what I remember both the batteries on the motherboard and the one in the external pod looked like standard Varta batteries, should be pretty straightforward to replace them...
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Put is back together and it works for 3 out of 4 channels. Channel2 has a problem (huge offset and no reaction to trimmer) with the input amplifier, but I leave it like this. The actual input uses a small ceramic hybrid with what looks like a chip on boad for the 2 input fets. The part on the main board uses 2 hard to get transistor arrays. So a repair would be very hard.
It is odd they used an extra hybrid for a 20 MHz input BW.
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Yes, they are really weird scopes... very complex for no good reason because very low specs at the end of the day.
A comparable Tek scope of the day was much better designed simpler and better specs and features. The Gould just doesn't make any technical sense...
I honestly was kinda disgusted after repairing mine, so much so I doubt I will ever buy another one for repair.. they are just not worth the trouble, IMHO...
I bought mine purely out of the curiosity factor, because I thought they looked "cool"... but the coolness factor soon vanished as IW as working on it... :--
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Hi !
I managed, including with your help, to bring to life a DSO1604 oscilloscope. I can't understand how they measure the volts, respectively the frequency !!! I don't have interfaces with it ... and neither does that remote control !!!
Any advice is welcome!
Thanks.
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Completely off topic, in my computer operating days in the 80s, our oldest computer was a Gould/SEL, haven’t heard the name in years.
We had 6 of the old vacuum tape drives on it.
Had a few good simple games on it like the text adventure colossal cave and a chess program.
What was great about playing chess was that all 6 drives would pause while it computed its next move.
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Hi !
I managed, including with your help, to bring to life a DSO1604 oscilloscope. I can't understand how they measure the volts, respectively the frequency !!! I don't have interfaces with it ... and neither does that remote control !!!
Any advice is welcome!
Thanks.
Sorry for the late reply... Gmail somehow decided to send all my EEVBlog e-mail notifications... to the SPAM folder ! :wtf:
I don't have this scope anymore so can only go by memory but... I don't think you need any interface/pod to get the voltage cursor measurements, should be a basic feature. I don't recall having any difficulty getting the cursors to work, I just fiddled with the various knobs and buttons until it worked.... it was intuitive enough, don't recall struggling at all.
At any rate just go download the user and service manual to know all about this scope, they are out there on the web, had no problem finding them. So just click click click..... ;)
I would attach the user manual here but the PDF file is a bit over 4MB in size and the maximum file size allowed for attachment is precisely 4MB... so won't work.
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So I will stop worrying about that rail for now, and rather concentrate on the +5V digital rail which most definitely is not correct at 5,40 Volts !
Manual says 5,20 Maximum allowed, so we are way off the scale here. Hopefully not bad enough that it would cause permanent damage to all those 74 series chips ?! Hmmm... maybe it's tool late and they are all dead... will need to pull a datasheet for one of them and remind me of the maximum tolerable voltage that they can safely handle before permanent damage occurs...
Hello Vince,
sorry for digging this out but I have an addition to the 5.4 V topic. I have two 1504 scopes in repair at the moment and both 5V rails were precisely at 5.4V. After a bit of reverse engineering I found out that 5.4V is indeed the voltage Gould designed the feedback circuit of the L296 regulator to target. In the service manual there are two resistors listed in the parts list of the PSU that are indeed located on the mainboard. They are part of the feedback circuit I documented here (https://www.vintage-radio.net/forum/showpost.php?p=1439403&postcount=11) and theoretical(from the parts list) and measured resistor values for R18(680 Ohm), R30(4.7 kOhm) and R31(470 Ohm) match perfectly to result in an output of 5.4V (the L296 feedback pin needs to be at 5.1V). This also matches very well with the L296 reference circuit implementations. So, Gould most likely forgot to update the PSU output specifications, but I can only speculate why they raised the voltage to 5.4V. Maybe for stability reasons.
Best regards
Denis
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Thanks a lot for your detective work and taking the time to tell me about it ! :-+
Most useful because.... I happen to have ANOTHER 1604 to fix ! Yes, I was stupid enough to buy another of these things... :palm:
Have not yet looked at it but shall start working on it in the coming weeks or months I think... thanks to you I won't be wasting time on that power rail and will rather concentrate on the most likely problem : again, corrosion on the main board due to the battery leaking :palm:
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Regarding that corrosion: My two units of course also got crystal blooming batteries. Fortunately, they did not drop their evil excrements onto the mainboard... or so I thought. Someone must have put the scope verticaly, so that it stood on its back side and while doing that it must have dropped a certain amount of the evil crystal mucus onto the legs of the three row plugin connectors on the back of the instrument. It ate up the nickel plating to the bare copper on one leg. :rant: Be sure to not only check near the battery for corrosion, but everywhere else too.