HP 6236A problem... -20 volt range is -31 and not adjustable

[ogdento]

I finally got to looking at a for-parts HP 6236A I picked up and not-surprisingly the thing still mostly works - with the exception of the -20v supply (The 6 and +20v supplies work great... voltage is adjustable and when connected to a load they supply the rated amperage, 2.5A/0.5A)

The -20 supply is a constant -31 volts, and I cannot adjust it... when -20 is selected on the meter, it pegs to full on.  I don't run it for long because I noticed Q4 (driver) gets pretty hot (this has got to be a clue).  I followed the troubleshooting steps in the service manual - without skipping any! - and I'm at a dead-end.

Steps in 5-3 (initial troubleshooting) directed me to 5-6 (-20 troubleshooting).
section 5-6:  High Output Voltage
  step 1: shorting Q13 brings the -20v supply to 0v (and the -20v meter drops to zero as well) so it says to continue to step 2 (i later came back to this and removed and checked Q3, Q13 and CR29 anyway - all OK)
  step 2: TP5 is -0.141, so (a) says check defective Q4. 
             Q4 tested OK in-circuit and out of circuit (I later replaced it with one from another supply - no change).  That seems to be the end of the troubleshooting, but I continued to step 3
  step 3:  TP14 is -0.7, so (a) says check for open CR26 or bad U2. 
             CR26 tested fine, as did U2.  Someone socketed the op-amps at some point so I took out U2, built up a test circuit on a breadboard and verified with my scope that it works as a comparator and an amplifier.  The amp was a bit noisy so I swapped it with U4 but that made no difference and the 6v supply still works great.  (b) says check Z1, which I also did just for kicks.

I then went through section 5-4 to check the bias circuitry and all the voltages are in-spec, i got TP1=7.19, TP2=-6.22, TP3=-12.40.  *TP1 is low, but still within 5%.

I "think" that was the end of the troubleshooting in the manual, so I did some more testing on my own...
- i see that the transformer is putting out 25v volts at the center tap (pin13/15 to pin14) and 50v across 13/15
- i checked every CRxx I could find (in-circuit) and they were OK.
- also pulled out a leg of CR26 and it was OK
- pulled out a leg of C12 and it tested fine at 7uF

Looking at the schematic, I can't see anything else that would cause this -31v behavior.  I know the big caps are often a problem with this old equipment, so I tested C17 and C7 from the +20 supply and they were close to each other - 1712uF and 1585uF respectively (esr on my junk tester showed .18/.16)

I'm at a loss, can I remove any/all of C11, C12, C13, C14, C17, C18, or C19 and run the supply without doing more damage to anything?

Any other ideas?

Thanks for any advice!
-Tom

[dom0]

Q3 has probably failed short. If that one is really ok, check if R34 is ok. If you remove R34, the output should drop to 0.

[pbarton]

"Someone socketed the op-amps at some point" mmmh, that's a bit concerning.
Do the op-amp sockets that someone inserted, have some sort of pin index?
I hope they got the socket oriented correctly!
You may want to check that U2 is oriented correctly within the PCB.

[ogdento]

Hi guys thanks for the replies...

dom0 - R34 is 534 (just within 5%), and I think Q3 is good... I removed it, and using diode check I see Vbe=0.487, Vbc=0.482, Vcb=OL, Vce=OL, Veb=OL, and Vec=OL (to be absolutely clear, my Vxy notation is x gets the positive lead, y gets the negative - so for Vbe, base is positive lead and emitter is negative). It also showed as a good NPN on my transistor tester.

pbarton - good idea... this came from Northrop and Q1-4 all appear to have been replaced at some point, so the socket orientation was the first thing i checked... the pin8 mark on the socket lines up with the pint 8 mark on the board and the tab on the ic can.  I also checked the pins against various points of the schematic and checked for continuity between the socket and the op-amp.  lastly I swapped the opamp with the one from the 6v supply and the 6v still works.


Two things occurred to me this morning:

1.  these supplies use a "full wave center tap" rectifier (rather than a "full wave bridge"), and it's interesting that my -20 supply output is -32vdc, which is awfully close to the rectified voltage of -34vdc I see across C17.  To dom0's point it seems that Q3 is always conducting - but Q3's ce junction isn't shorted according to my test.

2.  in the section 5-6 "-20v troubleshooting" steps, i went down the "high output voltage" branch... but, um, by applying the fancy maths we know that -32 is LESS than -20.  Does the manual consider a -20v supply output of -32v higher or lower than rated??

[pbarton]

I'm looking at the HP 6236A manual located at....
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_6236_6237_Triple_Output_Power_Supply
The -20V Supply Troubleshooting Table 6-6, is on page 5-9.
Try following that first. Let us know your results.

[ogdento]

Hey thanks again, that's the troubleshooting section I started with... I went down the "high output" troubleshooting branch for the -20v supply and got to step 3 (see original post), but I'll go back through the "low output" branch because I'm unclear on what they consider high/low

2.  in the section 5-6 "-20v troubleshooting" steps, i went down the "high output voltage" branch... but, um, by applying the fancy maths we know that -32 is LESS than -20.  Does the manual consider a -20v supply output of -32v higher or lower than rated??

[pbarton]

EDIT: After re-reading your OP, I can see that you have already followed the “-20V Supply Troubleshooting” Table 5-6 or 6-6 on page 5-9.
Note: There is a minor error in the manual. The Table immediately after Table 5-5 should be 5-6. Unfortunately it was printed in error, as Table 6-6 on page 5-9.
In the top right corner of the schematic, two voltages are produced, Va (used by the +20V supply) and Vb (used by the -20V supply).
Can you measure them?

[ogdento]

yeah i was confused by that k04bb pdf at first because the link says "6236A" but the actual doc is for the 6236B - but at least the troubleshooting steps are the same

Q3 is still out, but i measured V_A = -0.602 (at the base of Q1), and V_B = 7.17 (at the collector of Q13)

[jdragoset]

Check Q 15, VR 2, R 60, 61 and 64

[bd139]

I had one of these that did this. The main reference diode had gone pop.

Check the voltage reference supply And rails first before anything else.

[ogdento]

thanks jdragoset and bd139...

jrdragoset - i'll try and test those later tonight... just curious what triggered you to think of it, a low V_A, or something else?  (or is it that my V_A and V_B numbers aren't correct)

bd139 - when you say "main ref diode" are you talking about vr3?  and did any of the other supplies still work?  the 6v and +20 work on my unit

[jdragoset]

Shorting Q 13 makes -20V go to zero (Vb control).
Q 15, VR 2, R 60, R 61 and R 64 combine to bias Q 13 on (toward short), increasing Vb, driving - 20V low.   

[bd139]

thanks jdragoset and bd139...

jrdragoset - i'll try and test those later tonight... just curious what triggered you to think of it, a low V_A, or something else?  (or is it that my V_A and V_B numbers aren't correct)

bd139 - when you say "main ref diode" are you talking about vr3?  and did any of the other supplies still work?  the 6v and +20 work on my unit

Yes. I had to pull it on mine and test. I didn’t actually test the other supplies funnily enough so you’d have to evaluate that.

[pbarton]

You have followed the troubleshooting Tables within the manual correctly. However, the troubleshooting Tables within the manual are just a guide, and are useful for a number simple component failures.
It is possible that the fault is something other than a simple component failure, for example a plated through hole or a fracture in the PCB track.
In which case the troubleshooting Tables aren’t going to be much help.
As you continue to test and eliminate individual electronic components, the possibility that it’s a different type of fault increases.

[ogdento]

Thanks pbarton, I really appreciate the info pointers from you and the others.  I'm getting back into this stuff after a long hiatus and it's excellent to hear from folks who have been there already!

I hope to spend some time this weekend to continue with what you, jdragoset and bd139 have suggested, and I'll post any updates.

[ogdento]

I checked Q15, VR2, R60, 61 and 64... they're all OK.  I'll pull VR3 (and Q14 too for kicks) and test those, but all of my bias voltages check out (from sec 5-4).

At the moment I'm looking again for bad traces or junctions and bad caps. 

Is the U2 "volt comp amp" configured as a non-inverting integrator (with only a cap in the feedback path and resistor to 'com' on non-inv input)?  If so, I'm thinking a bad or leaky C14 would screw it all up so I'll check that too.

[ogdento]

Well, after pulling out and/or testing almost every part in the bias circuit and -20 supply, the problem ended up being a bad C11... I removed it and it tested at only a few microfarads.  Swapped in a 50v/220uF part I had laying around and it is working great now, but the replacement is tiny and looks completely out of place

Ran through the turn-on checkout with a shorting-bar and the 6v supply clamps at 1A, the -20/20 go to about 0.59A - it's supposed to be 0.55 so I might dial that back a touch.  Also ran it up on an electronic load and it looks good.  Now I just have to wait for the replacement caps to arrive.

Thanks for all the suggestions!