Fool for the 8656A Sig Gen

[jrharley]

Nope, I do plenty of time wasting inquiry all on my own!

Busy poking around the thing the last couple of days. I eventually got the hint and tested the test points in table I with a scope. (Pin 2, J2 still out of circuit).
The voltages were as outlined previously, but the ripple was all over the shop. Another good data point, thanks!

Importantly I think, it was the worst on the 5V rail. (Worst delta that is, it was actually more or less the same unstable value for all of the test points, varying from 800mV to 1.2V p-p).

I suppose I'm going to be lifting some cap legs soon. Is it possible that the regulator for the 5V rail is just doing its job poorly? The ripple was 1.2V p-p. Yeah, I think the table is saying it should be 0, so not so good. I guess a voltage ripple like that could cause the logic to fritz out downstream? We'll keep at it.

Thanks Tony_G, you're a Wizard! I feel like I'm still making progress.

JRH

[Tony_G]

Good to hear that I didn't cost you any time.

TPs 9,10 & 11 are unregulated and if they have excessive ripple on them then I would check out C17, C18 & C19 - You tend to find ripple in unregulated rails from the main caps and those tend to be large value caps. You might also want to check out A14 C11, C14 & C15 - At least at the start, if all those capacitors test correctly then you can start on the smaller value ones like A10 C15, etc. Dave did a good video on bypass caps if you want to get more background on why there may be multiple bypass caps on a rail.

I'd work to get those TPs into spec first as the other ones are further down the chain (specifically the other 3 TPs from table 1). When you start looking at the regulated TPs then you should probably check the ripple if not in spec then C23, C24 & C25 - I wouldn't bother with A12 U1, U2 & U3 until you can confirm that the unregulated ripple is correct.

Look forward to hearing the next thing you find.

TonyG

BTW It isn't impossible that the diodes doing the rectification are bad but I'd absolutely rule out the capacitors first. As an aside, getting a good LCR meter, like a DER EE DE-5000 is something to add to the toolkit as well but really you can get quite servicable results with some of those very inexpensive eBay devices.

[jrharley]

Really great points to run down Sir! Much appreciated. I'll look at the schematic with those comments in mind tonight. Meantime, I've attached a photo from the project "journal". It's one of the photos taken randomly through the process. I'm testing a small cap in circuit with the LCR meter. 

Report back soon,

JRH

[jrharley]

Brief 8656A update here.

I guess I'm just going to have to do the work.  With this kind of issue in the supply rails, I've got to get a pretty solid test on the caps, bottom line, and that means lifting a leg on C17,18 and 19. I can quickly flay this board open and test these guys, that's not a problem, but here's where my mind is going -

Let's face it, these caps are 40 years old. Good caps originally for sure, and in circuit, they're not raising too much concern. But, yeah, they're 40 years old and there's ripple out of spec. on the rails they smooth.

I'm trying to get this old beast in a place to be a solid contributor on the bench. It seems inevitable that these guys are going to be replaced. Unfortunately, that "thought thread", runs headlong into the form factor problem.

I've got suitable axial replacements in my stash that could go in here. They are about a 3rd the size though. I'm going to have to come up with an appropriate mounting strategy for modern equivalents. The original zap-strap approach from the factory, won't work in the current arrangement for modern caps.

Not saying it can't be neatly done, but I'm just.............sayin'.

So, I'll test 'em, get good data, and strategize about how to get a replacement solution that's appropriate. I have to admit that I found the mounting solution by HP on these guys to be a bit "how ya' doin'" right out of the factory. I'll ponder for a bit.

Thanks again for the feedback.

JRH

[jrharley]

OK,
So all of the big filter caps have been removed from circuit and tested with the trusty DER LCR meter. They are all out of spec to the high side on capacitance value - not horrible, but significantly. If this was an old all American five, I’d still call ‘em good. ESR on every one was strong, about .03 ohms. I’ll take that value. Still, they’re out of spec. I’m unclear on how higher capacitance values would affect this circuit, and the ripple I’m seeing on it. Replacing them, any way you look at it, is going to be a nosebleed. But it’s probably unavoidable in the long run.

I’m currently thinking that the most “dignified” way to replace these guys, is to re-stuff the originals. I think it could be done by making new cores from radial units rigged in parallel to hit the value. Then I could install the caps back in, just like they came from the factory. I’ll have to do a deep dive on cap dimensions and values, but it’s probably the only cost effective solution. The job will always be obvious to an informed eye, but it just seems to be the least invasive fix.

The million dollar question for me is this: Is this significant to the fault I’m chasing? My gut tells me no. But hey, I’m “Analog Man”, so I may be clueless. Calling all circuit Guru’s ! – should I tack these back in and move on for now? Or should I stop and do the deed to get the big filters into spec?

Any feedback is much appreciated.

JRH

[Tony_G]

I think it comes down to trying to find out why the ripple at the unregulated test points is way out of spec - This really needs to be corrected before you start looking at other things (at least this is how I would approach it YMMV).

I'm assuming here that you have already checked the capacitors on A14 (C11, 14 & 15) as they are in the path from the transformer to the unregulated A10 circuit - I can't remember if you did this so I'm going to go with the fact that you did and they were good.

Now that you've measured the A10 caps and they're all ok (the spec is +75% to -10% so reading high is not necessarily a bad thing) I'd start looking at what else might cause that ripple - For example, those caps (C17,18 & 19) go to a ground point via Note 3 on the schematic so checking that ground is correct would be my next step. If that ground was all good then I'd probably remove J4 to disconnect the circuit further downstream.

If it was still out of spec then I'd probably take a look at C23, 24 & 25 as those appear to be connected via PCB traces to the unregulated test points. If everything was still good but the unregulated voltages still had out-of-spec ripple then I'd look at the various diodes & resistors (with J4 still out) to confirm that everything that is between the unregulated voltage and ground is correct.

Fundamentally, something is causing that ripple and the designers specifically didn't want that there - Whether you address it now or later it does need to be addressed - I'd follow Dave's rule of checking the power rails first.

If all that checks out then it is something way unexpected

TonyG

[jrharley]

I'll take that as a hint to put the big filters back in and keep digging.

I'm embarassed to admit it, but I can't seem to FIND C11,C14, and C15. They're prominent on the schematic for sure, but I'm puzzled as to where those suckers are hiding. The photo of board A14 doesn't show any components on it, and it just looks like a mounting board for the HP IB. I have to be doing something stupid, they're too important. I'll keep looking of course, they have to be here somewhere.....

As always Tony_G, you're interest in my project is extremely helpful, thank you very much!

JRH

[Tony_G]

[Update]
Should have taken a bit more time to think about it - Those caps are in the replaceable parts list with the part number "2425-011-x5u0-102-m" - Looking at DigiKey for a close approximation gives this one https://www.digikey.com/en/products/detail/tusonix-a-subsidiary-of-cts-electronic-components/2425-018-X5U0-102M/4214068 and I would imagine that the ones in your unit will look something like this:



Should have realized that from the schematic where you can see them as feedthrough:



Check A14 or the wiring harness and you should find them.

[Original]
Having not seen the insides of the unit I can't be of much help but there are 2 wiring harnesses connected to A14:

• W19 - Connects to A14 J1
• W16 - Connects to A14 J2 & J3

W19 looks like it connects A14 to both A12 and T1 (and the optional high stability timebase on A16) while W16 seems to be all A14 to A10.

If those harnesses are all plugged in then the caps (assuming that they are in fact discrete components and not some engineering solution using the wires & PCB themselves) have to be on A14.

You might want to try backing into the solution by disconnecting the wiring harnesses for A14 from A10, A12 & T1 then measuring the lines as though they were just caps? They might be doing something custom & tricky.

Look forward to hearing from you - I'm as invested in getting this to work as you are now

TonyG

[jrharley]

A "Feed Through Capacitor"........, who knew? That's a new one on me. Thanks Tony_G!

I think I might have found them.

I'm going to have to re-group, secure board A10 first, and then go to surgery in the back of the unit. See photo attached.

Always appreciate the dialog!

JRH

[Tony_G]

In your PDF - You're pointing at A13 I think - A14 might be the board to the upper right in that image but it isn't the board with the GPIB connector on it.

TonyG

[Tony_G]

BTW Did you check the ground connection for A10? You should probably check for A14 as well when you find those feedthrough capacitors.

TonyG

[jrharley]

Hey Tony_G, you're right. Once I cracked it open again and took a closer look, it became clear where A14 was. I was indeed pointing to A13, my bad.

Now I'm working out the best way to get access to the A14 board assembly. It's mounted through a copper shield that is attached to the back of the compartment. Some disassembly is in order here. This could take some doing, those caps are new to me as a form, so I'm being a bit cautious in the tear down. It almost seems like they are a modern version of a "gimmick capacitor" that one can sometimes encounter in really old gear. Hard to see how they could fail, but hey, leave no stone unturned! I think with hemostats and some long tweezers, I can extract this thing, but we'll see.

I'll look at all ground points for sure. I think A10's ground is via screws to the frame per the schematic notes, and I've periodically checked ground continuity when testing, but I'll keep it top of mind as I go about things. The ground wire under the screw that holds the A14 board and shield assembly looks real solid. Don't think anyone's ever removed it.

Thanks for the feedback!

JRH

[Tony_G]

It might be easier for you to just use the ends of the jumpers/wiring harness rather than try to extract the board/assembly.

TonyG

[jrharley]

Too late! I'm in like Flynn! Couldn't resist.

Since I've got no idea what I'm doing really, I have to learn by examination. I hear what you're saying about the testing points, but I gotta' say, that access to the contacts was not great while assembled, and removing the connectors in place looked risky too. I decided I'm going to air this corner out a bit, clean things up, and educate myself about it. It seems to make sense to take a fair amount of time just seeing what I can see with this part of the circuit and improving things where I can. No other fool should ever be compelled to come back in here!

I may end up pulling board A14 out completely, for a closer examination. I've never seen a set up like this. The caps are "invisible". Very curious to me.

See the attached photo for a view inside the A14/A12 area.

Thanks Tony_ G!

JRH

[Tony_G]

Would be interested in seeing photos from various angles of the A14 board if you do remove it.

TonyG

[jrharley]

Of course! The crime is unfolding.

I'll be looking at what I've uncovered with respect to the schematic tonight, and sticking a meter or two across the pins of the board to see what more I can learn.

Once it was free of its tethers, the A14 board was easy to remove. Photos attached.

JRH

[Tony_G]

Interesting - The caps have to be somewhere This is really turning into a mystery...

The parts list for the A14 board shows the connectors being part of the board:



And the caps themselves are listed as standalone parts:



Coming back and looking at the schematic:



 It shows a 1000pF cap between the ground and J1 P15 & J2 P7 - The note says that the chassis ground is provided by the mechanical connection of the D-Type to the frame - Try measuring between those points to see what is there.

My only other thought is that they are in the wiring harness somewhere so you might want to follow that and see what happens to it between A14 and A10.

TonyG

[Swainster]

Hi guys, I'm still following along. I definitely agree with the idea that the caps are inside the potted D-sub connector.

BTW, I swapped the rear panel from my working 8656b to the non working one and it powered up without issue. Well except for the frequency being pretty far out and 3-6dB down from the front panel setting. So I guess the seller was on the level about it just missing its power transformer.

From your pics, the mechanical arrangement of the rear panel of the 8656a is slightly different to the 8656b, but I seem to think that the overall power supply circuit is pretty similar. Would you like any points measured for comparison?

[Tony_G]

I definitely agree with the idea that the caps are inside the potted D-sub connector.

I think you're absolutely right - I asked on another forum and that was the feedback.

I'm also wondering about the rectifier diodes as a source for the ripple seeing that the caps are all good.

TonyG

[jrharley]

Wow, 'yall! Terrific feedback. Lot's to run down, so it's going to take a bit, but I really appreciate the input. Thanks for weighing in swainster, good to know I can reach out for a cross check on the power supply measurements.

I've managed to test the A14 caps and they all read low. I thought it was interesting that the parts spec's said that they could be low by 20%, but not high at all. A couple are a bit too low, but otherwise this looks good to me. I couldn't help but then look to the hidden board A12. Looking at the schematic, these caps on A12 are all associated directly with the regulators, which I've taken as good so far. All the caps on the board are Tants. So far, they haven't measured well in circuit.

The immediate task at hand here is clear for me though. I've got to modestly restore this area before moving on. No sense in breaking into this thing just to leave it worse than I found it. I'll secure things a little better, clean the area as much as I can with Alcohol, and clean all the connectors. More study is needed in here. This stuff does take time, but it's very peaceful for this old Bodger. Bear with me as I stabilize things.

I've attached a photo of the seldom seen A12 board.

JRH

[Tony_G]

That poor SigGen has had a rough life...

I wonder if it may have been dropped at some point and the chassis has twisted a bit - These things weight at 2/3rds of a hernia so it wouldn't surprise me.

JRH - The GNDs for A12 are all direct chassis connections so while you're cleaning that up you might want to check that it is well grounded - I suspect just putting nuts and washers onto the posts will do that but worth checking in passing I think.

TonyG

[jrharley]

I know right? It's amazing to me that a piece of equipment that cost as much as a new car back in the day, could be abused in so many ways. I don't think the chassis has flexed though. It's a solid sand casting I think, very rigid Aluminum. Which is not to say that it hasn't been dropped. It weighs 37 pounds.
Anyway, the compartment is cleaning up nicely. I've swabbed all the grunge out of it, cleaned all the connectors and pins, and treated them with a light brushing of D-100. I've also discovered how the A12 board is mounted and it makes sense with the schematic diagram now. The "nutless" screws through the A12 board don't need nuts. Closer examination revealed that the holes in the board have a threaded insert in them and the screws are directly coming from the TO-3 package regulators on the other side of the chassis wall. The A12 board is directly coupled to the regulator mounting screws. Changing any caps on A12 would mean pulling those regulators. Right now, I'm thinking I don't need to change any. Once I disconnected the A12 board from the circuit, I got decent capacitance readings on them all. No visual signs of distress either. Grounding all seems solid, a single grounding point at the copper shield mounting bracket binds the ground to the chassis well. The bottom of the shield bracket which covers the A12 board, is uncomfortably close to the top of A12 when installed. I may glue a square fish paper sheet to the bottom of the shield bracket for good measure, we'll see.

I think I'll button this up and move on with some of the testing ideas that have been offered up next. I'll send a photo of the compartment right before button up. Thanks for everything TG!

JRH

[jrharley]

HP Sig Gen fans,
We're moving on to the next dead end here!
Back cavity containing the A14 and A12 boards is back together. See attached photos.

 At this point, I think we've eliminated some variables, so all good there. Still, the original issue remains. The good news is, no harm done, the unit powers up exactly as before. Ready to follow up on the backlog of great testing suggestions!

I'll post up any new developments.

JRH

[jrharley]

whooops, looks like a couple photos didn't attach.......

[Swainster]

Just to check, my understanding is that the current status is:
1) 5V supply is ok without the MCU board connected
2) 5V supply goes down to 1.8V when MCU board is connected
3) It is unknown if the fault is due to excess load from the MCU board or a problem with the supply.

If this is correct then perhaps a few comparisons will help - I made some current measurements of the 5V supply on my 8656B, measured at the 6A fuse:

• Plugged in but 'powered off' -> 64mA
• Powered on in default state -> 3.425A

Measuring just the MCU board current (hopefully I identified the correct pin on J2 as it's a bit different on the 'B' version), powered on -> 761mA

Lastly, when the MCU board in disconnected from the 5V supply and I power up the instrument, then the front panel displays all '0's and the total 5V current is 2.807A