Scope Repair : GOULD 1604

[Vince]

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 
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 ! 


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 

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 ! 

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 !!! 

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 !!! 

[Vince]

Edited my first message to talk about the expansion/interface stuff at the back, here are the related piccies...

[Vince]

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 ! 

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 !!! 

[Vince]

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 !   
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 !!! 
How is that even possible ! 

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 ! 

[shakalnokturn]

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.

[Vince]

Hi Chacal Nocturne, nice to see you're still around !

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 ! 

[shakalnokturn]

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.

[Vince]

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 !    I was a bit surprised I must admit...

For sale on LBC :

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 ! 
... hopefully not yet another red herring...

[Vince]

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 !!! 


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 !!!  "   

[Vince]

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... 

[Vince]

....

There you go ! The 25th attempt worked, here are the piccies at last !

Never despair...

[tautech]

Hi Vince....following along.....
You need a bigger bench ! 

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.

[Vince]

Hi Tautech ! Nice to see another familiar face on here ! 

Yeah bigger and proper bench is on its way, as I said it's a matter of a few weeks tops ! 

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 !?    I think the server is well confused...

[tautech]

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.

[Vince]

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 !    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 ! 

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...

[Vince]

...


Schematics for the front panel are attached at the very bottom, after all the pics.

[shakalnokturn]

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.

[Vince]

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.

[Vince]

OK I bit the bullet ! See pics... the scope just had a bad day ! 

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 ! 

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 !!!     

[Vince]

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 !!!

That's excellent news indeed !  So it's definitely worth spending some time on this board, it might actually be fixable !   

[Vince]

...

[tautech]

Might be time to invest in a desoldering iron Vince ?
Removing a 40 pin DIP should be no problem. 

[Vince]

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 ! 

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...

[tautech]

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.

[shakalnokturn]

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.