Fixing Dukane 2A75 Mixer

[templemark]

Hello.

I need help fixing a Dukane 2A75 mixer. This is a 5 channel preamp mixer.

It has 5 preamp cards, 2 Line Amplifier cards and a power supply (plus a few smaller front and back panel mounted boards)

It seems something is wrong with the power supply and this may have damaged one of the Line Amp boards. The power supply rails were putting out 32-37V instead of 23V. Possibly the regulator was not working correctly as when I turned the output switch to On it dropped from 37 to 32V.

(Since i didn't think it was broken) I didn't see this fault until running it for about a minute until R210 and R211 (top middle of schematic) on one of the Line Amp boards started smoking. Thankfully the other board seems fine and since it is an exact copy it has been helpful for troubleshooting. On the Line Amp card that is faulty I am getting about 8V across  R210 and R211 (after replacing them) instead of on the working card of less then half a volt so something clearly wrong there.

So i disconnected the main power supply and hooked it up to a  +-15V test power supply for troubleshooting the rest of the circuits to see if they are ok.

Here is my question:

It shows across Q201 (top middle of schematic), you should measure +0.5V. I am getting -2V or so. What does this mean? How could this be pulled down to a negative voltage?

Why am I getting 8V across R210 and R211 on one of the Line Amp boards while only less than half a volt on the other.

I checked all the simple diodes and they are ok. 

What can i check next?

 

[PRR]

Let's get a non-cross-eye close-up plan on the thread.

> when I turned the output switch to On it dropped from 37 to 32V.

C3 (2000u 6V chassis mount) is bad. (If it were good, DC voltages would not change.)

> the Line Amp card that is faulty

Clearly the "lower", main, card. The upper one could not smoke the 10r resistors before the 220r resistors.

> I am getting about 8V across R210 and R211 instead of on the working card of less then half a volt.

Then almost certainly CR204 and CR205 are NOT working (despite "I checked all the simple diodes").

Short test-points 10 and 13 with a wire. If all else is well, the card should pull into DC bias, pin 9 "near" ground (+/-2V), R210 R211 dropping essentially zero volts and stone-cold. This may not pass small audio well, but we gotta get the DC conditions fixed-up first.

If shorting TP10 TP13 does not calm things down, then one or more (probably more) of Q204,5,6,7 has gone toast. Any modern type of correct polarity, similar case, and correct/corrected pinout will work. May be best to omit Q206 Q207 for a while... it will work without them (not to full output and load).

EDIT: remove Q206 for now. You had 72V across this nominal 40V part. If it failed short C-E, this would match many symptoms.

I suspect R217 is drawn wrong. It should run to V-, not V+. Compare with preamp topology.

 

[templemark]

Let's get a non-cross-eye close-up plan on the thread.

I attempted my best to crop the 2 pages together. See attached.

C3 (2000u 6V chassis mount) is bad. (If it were good, DC voltages would not change.)

I'll try and swap this out. Although I did measure it outside of the circuit with my Fluke 87 meter and it reads 2828uF.

Clearly the "lower", main, card. The upper one could not smoke the 10r resistors before the 220r resistors.

Yes this is correct.

Then almost certainly CR204 and CR205 are NOT working (despite "I checked all the simple diodes").

I was thinking the same thing and upon checking these again yes. CR204 and CR205 are failed.

They are 1N4817 1.5 Amp diodes. I have a string of 1N4004 1 amp diode. Can i use these in for initial testing?

 

[PRR]

Yes.