HP 3325A 5V supply unregulated

[tonycox01]

Hi,
I'm trying to fix a currently non-working HP 3325A (rev C). I have the manual (unfortunately only available for rev F - but the design changes are not huge in the areas I have been working). I started as per the trouble shooting procedure and I have isolated the power supply board from all other boards and have established that there is a problem there.

The +15/-15V and their sense rails appear to be working fine. Stable voltage within 0.1V of +/-15V

The 5V rail and 5V sense rail are however sitting at 11.49V which is the same as the unregulated, rectified supply from the transformer winding. I've been over just about every component in the 5V regulator circuit and can't find any problems. I've replaced most of the components that could be faulty including the main pass transistor (Q1), the op-amps that regulate it, the filter caps and the SCR/zeners. Nothing I have done seems to bring the 5V rail back into regulation. The 5V supply is referenced from the -15V rail but given this is OK I don't see that the problem would be there  - I have checked the +/-15V circuit main components (cap, transistors) and it all seems to be fine.

It's driving me a bit potty! Can anybody give me any pointers or pearls of wisdom please?

many thanks

Tony



 

[Tony_G]

Something is certainly sick there as a regulated 5v overage should blow the fuse. CR10 and R14 are supposed to control that. I'd start there and try to find out why it is still working.

TonyG

Just noticed this, is the screw referenced in the 'note' in place?

[tonycox01]

Hi Ton,

Thanks for the pointers. First off, I did read the note on the schematic about the earthing screw and checked it is securely in place so I don't think that is the cause. I'll check out CR10/R14 again. But I replace CR14 so I don't think that should be the cause - but at this point I'll try anything!

cheers

Tony

[xavier60]

What are the pin voltages of U3?
Double check R24, R25 and CR15.

[mikerj]

Something is certainly sick there as a regulated 5v overage should blow the fuse. CR10 and R14 are supposed to control that. I'd start there and try to find out why it is still working.

TonyG

Just noticed this, is the screw referenced in the 'note' in place?

CR10 is a crowbar for the unregulated input to the 5v regulator, not the output.  It's also triggered by 16.2v zener so even if it were protecting the 5v rail it wouldn't have triggered.

Start with the basics
What is the voltage on U3 pin 8 w.r.t. 0v?  It should be close to 10v.
What is the voltage on U3 pin 2?  With 11.5v on the output it should be approx 4.8v, if close to 0v suspect a shorted CR15.
What is the voltage on U3 pin 1?  If it is attempting to regulate the output we would expect a voltage close to zero.

[wn1fju]

Maybe I'm just grasping at straws here, but....   I once had a similar overvoltage problem.  Turns out that it was simply an open main filter cap after
the bridge rectifier.  I was looking at voltages with a DC voltmeter which simply will give an average reading (that was wrong).  It wasn't until I looked
at the voltage with an oscilloscope that I saw the massive AC fluctuation.  So if you haven't looked at it with a scope yet, it's worth a try.

[tonycox01]

Hi - thanks for your replies.

The plot thickens :-/  After fixing what seems to have been an earthing problem the +/-15V rails are now way off kilter. The negative rail is sitting at -36.4V. I'm going to recap the main filters and check the 15V op amps and see where that gets me.

thanks

Tony

[Andy Watson]

It sounds like there is some problem with the intergrity of the ground/0V. Are you measuring the voltages with respect to the correct ground ? I've been caught-out a few times by forgetting that circuitry is grounded to the subchassis but the entire subchassis is isolated and floating with respect to the main chassis and the mains ground.

[tonycox01]

Hi Andy,
Thanks for your note. Yes, you are right - the earthing/ground system did have me confused for a while. I realised that I was earthing to the chassis rather than the floating ground of the subassembly (I spotted that it was attached with insulating mounts). When I changed that the +/-15V rails started worked fine and the cooling fan kicked in - which it had not before. The 5V rail (and 5V sense) is still sitting resolutely sitting at an unregulated 10.4V however which is puzzling as all the regulation components check out fine. Currently I'm stumped....

[xavier60]

The 5V rail (and 5V sense) is still sitting resolutely sitting at an unregulated 10.4V however which is puzzling as all the regulation components check out fine. Currently I'm stumped....

We need to see the voltage readings on U3's pins to help with that.

[tonycox01]

OK, here are some readings - firstly the actual PSU outputs (design / actual)

+15V/+15V
+15V sense/+14.86V
-15V sense/-14.86V
-15V/-15.2V
GND/0V
+5V/+10.6V (same as unregulated)
+5V sense/+10.48V

The U3 Op Amp pins are as follows:

1 +9.35V (2.4V on schematic)
2 +4.3V
3 0V
4 0V
5 +10.65V
6 +10.52V
7 +8.7V
8 +10.0V

Also the base voltage for Q1 is sitting at 10V (8V on schematic)

So something is off. I've pulled the regulator circuit resistors and they all check out almost spot on - also U3 is a new LM358H op amp.

[xavier60]

OK, here are some readings - firstly the actual PSU outputs (design / actual)

+15V/+15V
+15V sense/+14.86V
-15V sense/-14.86V
-15V/-15.2V
GND/0V
+5V/+10.6V (same as unregulated)
+5V sense/+10.48V

The U3 Op Amp pins are as follows:

1 +9.35V (2.4V on schematic)
2 +4.3V
3 0V
4 0V
5 +10.65V
6 +10.52V
7 +8.7V
8 +10.0V

Also the base voltage for Q1 is sitting at 10V (8V on schematic)

So something is off. I've pulled the regulator circuit resistors and they all check out almost spot on - also U3 is a new LM358H op amp.

Pin 7 being +8.7V, should put +8.7V on the Base of Q9 which should not be. Confirm that the Emitter of Q9 connects to ground.
If it does, Q9 is either faulty, the wrong type or fitted incorrectly.
The same likely applies to Q10.

If you have reason to remove Q10,  check that the output goes negative. 0.6V.

[tonycox01]

Hi Xavier60

Thanks for your help - I went back and carefully checked the connections of Q9 and they are correct - its a 2N3904 in place of an original HP part # CP4071 which seems to not be available anywhere as far as I can research - the recommended replacement is a 3904.

It got interesting when I checked Q10 and noticed that although I ordered a MJE240 (as a replacement for the original NPN MJE223) I had not noticed that I have been incorrectly sent a MJE371 which is a PNP transistor! So it seems this may be the root of the problem in the 5V circuit. I'll get it changed out as soon as I have the correct replacement and lets hope that fixes the problem 5V rail...

thanks

Tony

[xavier60]

It got interesting when I checked Q10 and noticed that although I ordered a MJE240 (as a replacement for the original NPN MJE223) I had not noticed that I have been incorrectly sent a MJE371 which is a PNP transistor! So it seems this may be the root of the problem in the 5V circuit. I'll get it changed out as soon as I have the correct replacement and lets hope that fixes the problem 5V rail...

thanks

Tony

That would definitely cause a problem. I have to wonder about the reason for Q10 being replaced originally.
Also I have just noticed that the schematic shows "R18 5.1K", but no symbol. If the resistor is present, then the 8.7V on pin 7 is correct.

[tonycox01]

I think Q10 replacement is down to me and borne out of frustration in the small hours. Seems I may have made my problem worse by doing so.

R18 is a bit of a mystery. I have a Rev C board and only a Rev F schematic/component placement diagram so there is a little variation in some details. R18 is marked and present on the Rev C board and is exactly 5.1k.

The other oddity is the U3 pin 6 connection stopping in mid air after R17. The -15V label suggests it connects to the -15V rail and indeed that is what the trace does on the PCB.

I'll check the U3 voltages again when Q10 is replaced.

thanks
T.

[xavier60]

Be sure to also measure the out voltage while Q10 is removed. It should go to -0.6V.

[tonycox01]

You mean the output of the 5V rail or pin 1 of U3?

[xavier60]

You mean the output of the 5V rail or pin 1 of U3?

The PSU's malfunctioning 5V output.

[tonycox01]

Hi Xavier60,

TLDR; it is fixed :-)

I removed Q10 and indeed the 5V rail fell to -0.6V. Then came the light bulb moment - I looked up the pinout of Q10 and it was reversed. I had removed Q10 before but simply put it back the same way it came out. Since this instrument came to me clearly with a fair bit of prior work done on it - it must have been reversed by the previous repairer who failed to spot the error before finally getting rid of it. I could confirm this because I carefully photograph boards before I do any work on them so I can confirm orientation of diodes and transistors (ironically) etc

I learned several things - pay attention to grounding, don't assume previous repairs are correct - and ask for help :-)

thanks again for your advice - it was very helpful

Tony

[SilverSolder]

That was a pretty tricky one...   a meta-repair?