[RESOLVED] Cry for help ... unexplained offset from op amp (or not?)

[essele]

Hi all,

I know I've kept posting about my Datron repair efforts recently, but I've made great progress with my 4200 and am now back to the original fault (after having to fix at least three others) which was clipped output on the 100V range, I've tracked it down to an unexplained offset at the output of an op amp, which then causes clipping once the sine input signal gets above a certain amplitude.

I've compared this to a working machine and that doesn't have any offset at all.

I'm now banging my head against a brick wall again, so really could do with some pointers.

The schematic is below, and a quick summary of what I am seeing follows:



There is a sine wave input into the gain stage (100V I/P) which is absolutely fine and visible at TP5, this then goes into the main op amp (M8), the output of this op-amp appears to be offset by about +6.4v (this is visible both at pin 11 of the op-amp and also TP4. In fact TP4 sits at +6.4v on this system even with no input, where my working system sits at close to 0v.

The "BIAS OFF" input also appears to be zero, and the output of M10 also appears to be zero. The overall feedback loop from the main power amp (TP3) is also good and not offset at all.

I've switched the op-amp with a known good one, and it made no difference. Also M21 (78L12) is working and I see a stable 12v there.

The obvious place for the offset to be coming from would be the non-inverting input of M8, but this appears not the be the case (at least the output of M10 was zero), or a negative voltage on the "BIAS OFF" input, but that also appears zero (I will recheck these as they seem obvious candidates)

Is there any other way I could be getting an offset at the op-amp output?

Any help will really be appreciated.

[Kleinstein]

The offset seems to be only at the output. An amplifiers output offset is specified for it input side, not the output. The offset at the output is more like a problem of the following power stage, that for some reason needs a different DC level to get into the linear range.
As the amplifier still seems to be somewhat working, my guess would be Q12 that could be broken.
It could also be a problem with R52 (too high).

[essele]

Thanks Kleinstein, that's helpful ... I've already ordered some BC184's and BC214's with a view to replacing these out of desperation anyway. I may pull it now and test it.

I should add that when I first got this device R53 was burnt, and reading about 50ohms ... that would probably support some kind of issue with Q12 as I believe it's part of a current mirror with Q11. I have since replaced R53. I will re-check R52, but I think it was ok.

[essele]

Aaarrrggghh!

I replaced Q11 and Q12 - no change.
I replaced Q10 and Q13 - no change.
I replaced Q9 - no change.

I then replaced Q8 and immediately got a FAIL 7. The gate and drain were shorted together, I had tested it before and it was fine. Like an idiot I put another one in and exactly the same thing happened.

I’ve now put back the original, and I’m getting a fixed DC output on the 100V range, so something else has gone as well, or Q8 has completely given up. The original did test ok, although was different to the replacements.

I’ve also managed to lift a pad from Q8 - sigh.

But it does seem a little interesting given I had a burnt R53 originally, so I’m wondering if the original Q8 just had a different failure mode to the new ones??

So my plan of action now...

1. Double check to see what else is damaged (if anything) as a result of the above.
2. I’m not sure if I checked C26 - I think I did right at the beginning, but worth doing again.
3. Go through each diode and resistor checking them again.

Any other ideas or thoughts?

UPDATE:  C26 was fine. Q13 was toast, totally shorted, all pins. Plus D51 was reading only 0.2v drop either way ... now I like to think I checked that, but fingers crossed I didn't and it's the cause of the problem.

I am going to try the FQP3P50 in place of the MTP2P50 ... they both look pretty much the same as far as I can see, and I can get the 3P50 and Zener for tomorrow! Please shout if you know of any reason why I shouldn't do this. Thanks!

[maginnovision]

If they're not right replace them. Just keep using the good unit for a reference when you're not sure and you'll get it sorted. Hopefully that second unit was a decent price that together they average the cost alright. I'll say one thing, puts me off from getting a 4200. Although my 5200a + custom transformers isn't really as elegant.

[essele]

Ok, I've replaced all the broken bits and it's back to the same clipping behaviour now ... but now I feel even more like a muppet as I noticed that it's clipping at about 32V ... which sounds suspiciously like it's being fed from the +38V supply rather than the +400V supply (you can see D56 provides +38V if +400 isn't present.)

I checked, and indeed that is the case ... there is +400V at Q20 drain, but not at the source.

So it seems like I was looking in the wrong place (again!) ... and it's more likely the driver regulator that's having issues.

I've switched the onto-coupler and that didn't make a difference so I assume that is fine. All the resistors, and all the caps check out. The diodes all seem fine, I replaced the 75v one, but I don't currently have 200v ones to switch those. D54 is fine.

So I think I'm down to either the 200v zeners, Q17, or Q20 ... all of which seem ok with in-circuit testing, and match exactly the working board. I assume Q20 is the most likely to fail.

I think I'm going to have to solder test wires onto the different zeners to check them out.

It feels like progress, but I'm really annoyed that I have replaced all the other bits, and lifted pads pretty much for no reason ... if I had looked more closely at the clipping earlier it should have been obvious.

I wouldn't be put off the 4200, I think it's me that's at fault, not the device!

[essele]

All fixed! How pleased am I?

So I was seeing 24v at the top of the 2 x 200V zeners instead of 400V ... and then 106v at the top of the 75V zener, so that one was definitely OK (and I'd changed it anyway)

But there was something about those two 22uF caps that worried me ... they both checked out perfectly fine, and matched the working unit for both capacitance and ESR, but I thought I'd remove them to make measuring easier ... and ...




... luckily I had a couple of 400V Nichicon 47uF's, so a quick replacement and everything is working...



So ... the actual problems with the system were:

1. Burnt resistor on Power Amp Board
2. Failed electrolytics on Power Amp Board
3. DAC failure on the Sine Board (not sure if I caused this with ESD)
4. Burnt resistor on Current board
5. Blown fuse on Current board
6. A selection of tants on power supply, Sine board, AC board, and Current board.

I'll try to forget about all the other things I changed during troubleshooting!