Fool for the 8656A Sig Gen

[Tony_G]

From the schematic, the 5V line carries over to a bunch of filtering capacitors on 8-86. I would start checking those before I started partying on ICs.

Just as an aside that reference mnemonic works like H 14 - Means the signal passes to point H on service sheet 14 - If you see H 15,16,18 then it means point H of those service sheets.

TonyG

[jrharley]

Thanks TG! 
It's hard to estimate just how much time you just saved me. I was chasin' a bogus Rabbit hole. This is such a great example of how invaluable experience is. I thought that small number 14 was something else, not the link to the next node in the chain. According to page 8-23, table 8, "Schematic diagram notes", that number 14 should be BOLD per the example. Yet, sure enough, when I followed to service sheet 14, there it was....... "Big as Ike", as my Dad used to say. "From the +5V supply". And yes, that is a bunch of caps on that line. I'll get to testing and report back. 

Thanks for nudging me back between the white lines!

JRH

[Tony_G]

NP - Happy to help - You'll find some of those numbers being "handwritten" on the schematics - If I had to guess I would say that this manual was produced at the time of moving from drafted schematics to computer-generated so you'll see these various anachronisms around all the schematics (I don't actually know but I'm sure someone over on the HPAK group would).

TonyG

[jrharley]

Hey TG, 8656A update here, hope it's all good -

I did a one night rundown of the caps on string 22H recently. Most of the caps on this line are .01uF ceramic packages. I have assumed that from the appearance of the caps and the fact that the parts list doesn't identify the type, that they're ceramic. They look to be high quality for sure, still shiny. Since each one is associated with an IC pin, I'm gonna' call 'em "Bi-Pass" caps. Not inclined to suspect a ceramic cap, unless it's baked or cracked. That, in addition to their relative circuit significance, has led me to discount any role they may have in drawing down the 5V rail. (Now that I've said that, this will probably come back to bite my 'arse in 6 months!)

There are also a couple of 'lytics in the chain. C1 is 100uF and C8 is 2.2uF. I measured them in circuit at 1KHZ. C1 was pretty much spot on @ 102, with an ESR of .27ohms. C8 was a different story, but I'm not clear if it would matter at all for the disappearing 5V rail. It measured a whacky 102 uF. ESR was so-so at .65ohms. Again, maybe worth a second look, but I can't see this pulling the 5V rail down since it's not dead shorted.

I'm moving on down the chain to look at service sheets 15,16 and 17. Interestingly, things seem to be converging on the sheets that came up as significant in my "bogus rabbit hole" journey.

So that's where I am. More probing around currently underway.

JRH

[bdunham7]

There are also a couple of 'lytics in the chain. C1 is 100uF and C8 is 2.2uF. I measured them in circuit at 1KHZ. C1 was pretty much spot on @ 102, with an ESR of .27ohms. C8 was a different story, but I'm not clear if it would matter at all for the disappearing 5V rail. It measured a whacky 102 uF. ESR was so-so at .65ohms. Again, maybe worth a second look, but I can't see this pulling the 5V rail down since it's not dead shorted.

They're in parallel in circuit, so you're effectively measuring both each time, along with the PCB trace between them that adds a little ESL.

[Tony_G]

As Bdunham7 said, they're in parallel so I would be inclined to remove them from the circuit and test - If you have enough space, you could just lift a lead and then test.

It certainly could be an IC but I'd want to rule out the possibility of it being a cap on that rail.

From memory, I think you said that the main 5V rail stays fine and it is just that pin 2 line that goes low so I would approach it along the lines of checking each cap and if they're all good then start removing the ICs that are related to each cap while monitoring the voltage.

If you've removed all the ICs and still have a problem then you're looking for a different reason. Sort of a longish process but it would allow you to rule out the caps and ICs. Also if you don't have one I can heartily recommend the Hakko FR-301 as it makes desoldering much easier.

TonyG

[jrharley]

Hey guys, copy that! Thanks for weighing in bd7!

I wasn’t putting much stock in the in circuit measurements either. Lifting a leg on them wouldn’t be too tough to do and it may come to that, we’ll see. Meanwhile, a little more accurate discovery on the caps in question has come to light:

Once I found the right parts list, (DOH!), the cap descriptions made more sense. The bulk of the capacitors are, in fact, ceramic and they’re labeled so in the description. What I didn’t expect to see, since I’ve never seen any before, is that the other type of caps in this chain are axial Tantalum caps that look like regular axial electrolytics. But the parts list designates them with a “TA” in the description and they do exist. So, there you go, learn something every day.

There are about 20 ceramic caps in this part of the circuit, most are .01uF. I decided to just test the resistance across them all, since I could do that from the top side easily. Maybe I could squeeze a little more information out of the board. All of them seemed to have pretty consistent and reasonable readings in the hundreds of ohms, the bulk of them being around 390 ohms. There are also 2 very small ceramics that are identical and specified as being 22pF. One measured about 700ohms and the other one seemed to measure open. Nothing. So, I cobbed a couple 60pF ceramics together in series and tacked them across the suspect cap “Shango style” to see what effect would occur, if any. Not unexpectedly, no change. At the moment, I’m thinking this is a low level power supply issue so I think I will run down a couple of other lines before sucking on IC pins.

The pins 1 and 3 on the J2 connector where this Rabbit Hole originates, are tied together with pin 2 where it connects, on board A10.  I think I’m going to follow 1 and 3 along for a bit, to see what I can learn.

By the way TG, I found the programming switch you referred to a few comments back. It appears on service sheet 16. This one’s set to 5V, which seems to be correct.

So that’s where I am - this is still a very active crime scene. More discovery later, and thanks again for the input guys, it's very much appreciated!

JRH

[Tony_G]

There are about 20 ceramic caps in this part of the circuit, most are .01uF. I decided to just test the resistance across them all, since I could do that from the top side easily. Maybe I could squeeze a little more information out of the board. All of them seemed to have pretty consistent and reasonable readings in the hundreds of ohms, the bulk of them being around 390 ohms. There are also 2 very small ceramics that are identical and specified as being 22pF. One measured about 700ohms and the other one seemed to measure open.

Just thought I'd check-in and see if Santa brought you a working SigGen.

Out of curiosity, are you measuring those caps using a proper LCR meter or just using the resistance mode on a DMM? They should all be "open" from a DMM resistance measurement and with an LCR meter, you'll see the ESR values. Just wondering.

[jrharley]

Hey TonyG,

Thanks for weighing in!, Happy New Year to you and all the HP fan boys on the EEVBlog forum.
Nope, Santa’s delivered nothin’ but coal lumps so far. But learning has still been happening. On the cap testing, I hear you, I’d considered the same things. My resistance readings were with a DMM, but I also have a decent LCR meter that I’ve been using to test caps as well, both in and out, of circuit. What I learned from just slapping a DMM across those caps was that things were pretty consistent across the board. Call it a “qualitative measurement”. The circuit was definitely influencing the readings, even if I don’t really know how, precisely. It was just a data point, even if a marginally informative one.

First impressions sure can fool you sometimes. I’ve realized that my initial conclusions about the power rails on this guy were a little bit “slap-dash”. This has screamed power problem from the very beginning, I just didn’t look close enough. The bottom line at this point, is that when all connections are hooked up as intended, the +5V rail becomes +1.8V, and it only does that when pin 2 of the J2 connector is connected into circuit. I’ve looked at the string on sheet 14 pretty closely at this point. I’ve tested the tantalum caps out of circuit, (they’re bang on), and really doubt any of the ceramics are the problem, so I’m moving onto sheet 15, following the 5V distribution. I’ve determined that I do not have the 5V at chips U29 and U33 per the schematic. Interestingly, with the wire from J2 disconnected, I got essentially no voltage on those chips, but with it connected, I got low voltage, 1.6V and 1.7V respectively. The interesting part is that's really close to the 1.8V faulty rail reading observed previously on board A10.

Following other points on sheet 15 where the 5V should be, has led me to the only resistor network on the board, R9. There are 9 - 3.3Kohm resistors in the package. I’m currently doing some investigating of that package and pondering a way to test it. Not sure if I’ll find a diagram anywhere that shows how all those resistors are hooked up inside the package, but it certainly seems like a possible culprit. Great, I suspect that part will be unobtainable in its current form, but we’ll just have to see if it’s got a bum resistor in it somehow. The package has 10 pins.

So, nothing insightful to report unfortunately. This Bodger is still just poking around in discovery mode. More later with any progress....

JRH

[themadhippy]

There are 9 - 3.3Kohm resistors in the package.

Wot like this?

Great, I suspect that part will be unobtainable

https://www.digikey.co.uk/en/products/detail/bourns-inc/4610X-101-332LF/1089196

[jrharley]

themadhippy!
Thanks for the tip!
I'm just now getting into looking at this component, so this is great! Good to know there's some options.

This guy is a good bit snazzier than the HP part, which is pretty "opaque" as far as construction goes. This one's form factor doesn't allow much room for a lot of arrangement possibilities, so I'm guessing they're all in parallel? I'll run it down, I presume digi-key US can supply too.

Thanks again so much for the link and information, it's a big help!

JRH

[themadhippy]

so I'm guessing they're all in parallel?

no,this is the most common arrangement

[Tony_G]

As Themadhippy said, these aren't in parallel, R9 is being used as a pull-up resistor so I wouldn't expect that it would have a problem dragging your rail low.

If you think it might be then remove R9 and see if the rail stays at 5V and that you get +5V to R9 (Pin 1 I would assume but you should confirm on the schematic) - If it does then you more than likely have a problem with one of the ICs that are using R9 as a pull-up.

If that all works (in that removing R9 gives a good rail) then you might want to try using a 3.3K resistor to just provide +5V to each pull-up location (e.g. R9 pin 1 to R9 pin X one at a time) and see if you can identify which IC is dragging it down.

All of this assumes that removing R9 enables the rail to stay at +5V if it doesn't then the problem is somewhere else.

Sorry, I can't be more specific.

TonyG

[jrharley]

 Thanks guys, much appreciated! I'm afraid I'm obnoxiously long on enthusiasm, but woefully short on knowledge, so this is a big help.

Thanks for the diagram themadhippy, that makes sense now. Now I see why they didn't feel like it was necessary to show the package all in one spot on the schematic, among other things.

tonyG, Bodger Alert - I didn't know that there even was such a thing as a "pull-up resistor". Now that I've looked up the concept, I've got a better idea. Thanks for advancing my understanding!
I think at this juncture, I'm inclined to follow your instinct about R9. If it's unlikely that a fault in the unit will cause the fault I'm chasing, then I think I'll put it on the to do list, but keep moving. After all, a digitally naïve, analog assault on this beast, is all I've really got. But I really like your suggestion on how to test R9 and the chips associated with it, I almost want to do it, just to do it. R9 wouldn't be hard to pull. But "eyes on the prize" right? I'll do some more probing tonight.

Always appreciate the feedback folks,

JRH

[Tony_G]

I think that your process of getting the power rails right is the correct first approach to the unit - I'd keep trying to find out why the +5V rail drops to 1.8 (I think you said that) when connected.

Once the rails are right then you can be more "creative" in trying to find the fault.

TonyG

[jrharley]

Thanks Tony_G,

I will stay the course.

On a side note, it occurred to me that I might want to see if there was any output from the unit when powered up. Just curiosity.
I don't currently have any connectors or adapters that will connect to the RF output on this guy. I know I can rig something up, but it got me looking into the connector a little more closely. What is this thing?

Any clues as to what this connector is, would be appreciated. It's larger than the SMA ham radio stuff I see. The parts list isn't a lot of help as to what this thing is called. I gotta' believe it's at least available, and I'm just clueless. This connector to BNC female would be sweet.

Updates forthcoming.......

JRH

[Swainster]

Hi,

You need a 'N' Male to BNC Female connector to convert the output to BNC. Or better to switch to type N connectors on your cables, and just use N to BNC cables for BNC stuff. If you start working microwave band stuff (300MHz upwards) then N connectors will be pretty much standard for test equipment.

I've been following along with interest as I have a couple of 8656Bs, one of which is working but the other is incomplete. Someone has asked about buying the working one so my interest in getting the non-working one up and running is much higher (it's missing the power transformer and GPIB connector, but according to the previous owner everything else is OK).

[Tony_G]

...but according to the previous owner everything else is OK...

Heard that before Much liked "Pulled from a working environment"...

That said, I think the unit should work without the GPIB board (for clarity I mean A13 and not the main processor board where all the actual GPIB smarts are but you'd need to check the schematic/service sheet) but I'm actually not sure - it certainly won't work without the power supply.

As for the adapters - These probably suck in terms of RF quality but in terms of getting you something that you can look at they should work OKish:

https://smile.amazon.com/Adapter-Connector-Extension-Antennas-Female/dp/B091C5W4HM/

As Swainster said, the output on this unit is a N-Type connector - The ones on HP gear are typically good up to 18GHz (in "as new" condition). The actual connector spec is only good to 12GHz I think from memory but HP did some higher specs up to 26.5GHz but this unit should only have a standard 18GHz one.

There is a whole bunch of information on how to confirm that the connector is in spec but seeing that the 8656A only goes up to 1GHz you're probably fine with whatever the connector is right now.

TonyG

[jrharley]

Thanks guys, BIG time saver! Appreciate it.

I'll source some cheap versions to aid in my testing and order the $30 versions for when I'm testing microwave antennas for a fighter jet. Seriously though guys, I really appreciate this dialogue, it gives me hope that I just might be able to get this piece of equipment up and running again.

Thanks for weighing in swainster! You're in a good place with 8656B's from what I understand, they've got a better rep than the A version. I'm glad you're following the conversation.

Tony-G - Thanks for the info and insight. It seems there's quite a science to RF connectors!
I've had my eye on a suspicious Triac in the power rail that I want to test. (Q3 on the A10 board). Think I'll pull it this weekend and see what I can learn. Continuity testing of it, in circuit. has yielded confusing results.

The Brute Force tech team stumbles on.............

JRH

[jrharley]

8656A update here:

OK, so I pulled the "TRIAC", Q3 from the A10 board, which is more precisely an SCR, and proceeded to beat on it for a bit. Just for grins, I powered the unit up without the device in circuit. No change. Although I don't fully understand how the thing would act in the circuit, it's part of a "Crowbar" arrangement intended to protect the attenuator I think, so it's not entirely surprising that it's condition, in or out of circuit, doesn't change anything here.

As for it's condition, I'm basically getting "open lead" no matter how I measure it. I tried turning it on with a 1.5V AA battery thru a 7ohm resistor, but it didn't seem to stick. No resistance could be measured across the legs after pushing a little current through it. But it's also possible that I've pushed a bit too much current through it. The battery was putting out about 200 mA, if my poorly timed measurement of the battery set up was correct. In a head slapping episode of realization, I decided to check the data sheet and it says the maximum triggering current for the device is 15mA. DOH! I may have screwed the pooch here. Hard for this Bodger to know at this point. The SCR, aka Thyristor, is an RCA 2N3528. The TO-8 package for any substitute for this device seems to be hard to find, based on a brief search, but maybe you folks know of a good substitute. There are other form factors that could sub available, but that would be tacky. Who knows though, the thing could be fine.

In any event, I am pretty certain at this point that it's not a factor in the fault I'm chasing. I put it back in.

Meanwhile, the weekend has revealed that my B&K 1430 3 channel power amp, seems to have lost a channel. Since I've got to have tunes, I need to detour for a while to try and get that guy up and running again.

Appreciate the interest in my efforts folks, and any feedback is welcomed.


JRH

[Tony_G]

I believe that the 2N3528 can be replaced with an NTE5455 but you should compare the datasheets to make sure.

Now that Q3 is part of the 5V crowbar circuit and not specifically related to the attenuator (from my quick reading of the schematic/service sheet) - If it is being activated then it should cause a fuse to blow.

I would recommend that you get a tester - I use one of these https://www.amazon.com/Atlas-ADVANCED-SEMICONDUCTOR-ANALYZER-Tracing/dp/B00O0BMNX2 but you can really use any of these testers (including the ones off eBay).

Let us know how it goes.

[jrharley]

Thanks for the cross reference Tony_G! I'll check out the NTE number and store it for future reference should I need it.

Very good point about the SCR action! I'd already checked the fuses associated with it just for curiosities sake, and they're good. The circuit makes more sense now.

When still hot off the board, I dropped the SCR into a cheap, classic, fish8840 tester just to see what I could see. It seemed to indicate that I had 2 diodes. The tester is supposed to test thyristors, but you never know for sure - it's a cheap tester, operator error, solar flares.......

 Oh, for a suite of those Peak testers.......... Nice kits.

The B&K power amp issue turned out to be just gummed up output level controls. I love it when that happens. So now I'm back to the HP for more terrifying technical explorations.

As always, I really appreciate the feedback.

JRH

[Tony_G]

Good to hear - Good luck with the unit - Look forward to hearing what you next discover.

Just to refresh my memory, you checked the voltages per service sheet 22 and did so by removing all the boards not used in SS22? This would tell you that the rails themselves are good (though it is possible that once a rail is loaded down it slumps in voltage) and let you add a board back in at a time to see if that board causes the rails to droop.

Again, look forward to hearing the progress.

TonyG

[jrharley]

Hey Tony_G,
I see where you’re going with that. Good point.
I haven’t systematically isolated all the other boards, but I did isolate the Audio/Power Supply section on board A10 by disconnecting connectors J2 and J4 entirely. I tested most of the components within the isolated assembly, like the SCR Q3, and I’m reasonably convinced the draw down doesn’t originate in that portion. You can see that there are test points for all three main rails on either end of the rails. I tested the voltage at these points, (running the unit off the 120V mains), once with the #2 wire from connector J2 out, and again, with it in the circuit. First number is with the J2 wire out of circuit.

Testing the pins in test socket J6:
 
 “Left side”
TP 11: 21.8V, 22V
TP 9 :  12V, 13V
TP 10 : -23.4V, -23.3V

“Right side”
TP 13 : 14.9V, 15V
TP 3 : 5.4V, 1.8V
TP 2 : -14.7V, -14.7V

Under each of the voltage call outs for each rail on the “Left side”, there are small “hand written” voltages. These voltages, whatever they are supposed to represent, are much closer to my measured voltages than the rails are designated to be. A different “state” perhaps. No idea at this point, but find it hard to believe it’s coincidence. I need to re-visit the theory of operation info if I can find it on this section.

I’ll study the schematic and see if isolating other boards provides any clues. Thanks!

Onward…….

JRH

[Tony_G]

Sorry bit of a brain fart on my behalf - I was imagining that there was a motherboard but these units didn't have that.

Apologies if this caused lost time.

Did you get a chance to put a scope on those test points? Some of them have very tight ripple tolerances (eg TP13 should only have a 10mV ripple on it with the DC value between 13V-17V)?

Detail is in Table 1 on SS 22.

TonyG