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Fool for the 8656A Sig Gen
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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
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