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Old 19th January 2014, 01:08 PM   #1
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Question Ideas to what might cause almost full supply voltage DC offset?

Hey, I'm trying to repair an amplifier that I destroyed. I was replacing the differential amplifier transistor pairs (they were causing high DC offset), when I saw a spark somewhere on the board (I forgot to discharge the main filter capacitors) and now I have 29VDC offset on one channel (supply voltage is +/-33V). It still outputs sound, but the sound is static only (same sound as an old tv with no signal). The junk speaker I had connected started smelling like burnt voice coil (still works though), which is how I detected the high offset. The other channel works fine. I've tried putting the old transistors back but no difference.

When I unplug the amp, the + supply quickly goes down to about 7V, whereas the - supply goes down to -12V, but at a much slower rate.

I've tried many things, but am totally stuck. I cannot understand what is causing this. Got any ideas for me?

Here's a schematic for anyone interested in having a look: jpg.
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Old 19th January 2014, 01:33 PM   #2
tvrgeek is offline tvrgeek  United States
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Joy of a feedback circuit. Clearly something else is blown. There are only 6 more transistors in each side, so take your pick.
I assume this is on one channel only?

To limit further damage, use a 100 Ohm or so load, not a speaker
You might start by collapsing the bias spreader.
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Old 19th January 2014, 01:43 PM   #3
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Yes, only one channel. I use the headphone output for testing, which is in series with a 330R resistor, so that part is covered.

The bias spreader you say? Would that be Q405/406 on the schematic?

I'll check all the transistors next. Already done a couple, but wasn't sure that would be the issue (seen no problems so far).
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Old 19th January 2014, 02:03 PM   #4
Bigun is offline Bigun  Canada
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To get that much dc at the output and to have that rail collapse quickly after power down you must have a lot of current flowing from the +ve rail to ground. By measuring the voltage drop across resistors in the circuit you can discover where this current is flowing - and where it should not be flowing through a blown part. I would suspect the output (and driver) transistor on the +ve side as the first place to look.
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Last edited by Bigun; 19th January 2014 at 02:09 PM.
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Old 19th January 2014, 02:03 PM   #5
cjkpkg is offline cjkpkg  United States
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I would look at the front end 2sa798. Yank it out and check for shorts.
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Old 19th January 2014, 02:42 PM   #6
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Quote:
Originally Posted by cjkpkg View Post
I would look at the front end 2sa798. Yank it out and check for shorts.
You can usually check for shorts in transistors in circuit.
However I have seen weird faults with transistors like hfe =1 where I needed to pull the transistor to check it on a multi meter.

In my experience it is usually an output transistor gone which sometimes takes the driver transistors with it.
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Old 19th January 2014, 02:53 PM   #7
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Originally Posted by cjkpkg View Post
I would look at the front end 2sa798. Yank it out and check for shorts.
That is the one I replaced originally, the producedure that killed the amp (by something shorting), with two KSA992 transistors (wired in correctly). Currently I have put the old 2SA798 back, as a test, but the problem remains the same.

Update on the problem: I have now pulled every singly transistor on the blown side, and none of them are faulty.

And, with all the transistors physically missing - the B+ is still dropping rapidly after power down (whereas the B- goes much slower).

I will try to check some resistors next.

Also note that the phono pre amp is also using the B+ to create its own supply of 29 volts. EDIT: Disconnected. No difference. Tuner's been disconnected as well.

EDIT again: The transistor on the power supply board (Q501) has the intended voltages on the schematic. B = 14.2V C = 28.9V and E = 13.6V. The voltages I get here are: B = 12.1 C = 32.6 E = 11.6. I think this is a bit odd, because B and E are lower than intended, whereas C is higher. The emitter voltage goes to pin 14 on the main amp board (as seen to the left on the schematic). I think it might be important.
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Old 19th January 2014, 03:08 PM   #8
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Check the power supply rectifiers.

Mike
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Old 19th January 2014, 03:21 PM   #9
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Check the power supply rectifiers.

Mike
Already done that too. They consist of two big, dual diodes. One with + - + polarity, and the other with - + -. They appear fine. 0.6V in all the right directions (and no conductivity in the others). I did physically remove them before testing.
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Old 19th January 2014, 03:30 PM   #10
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Have you checked the output transistors? You can start to diode-measure them. NPN: Plus (red wire on your multimeter) at the base and minus (black wire) at the emitter and later the collector. There you should get something and since you have a working channel you can compare.
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