SD12-2 Mixer Channel Distorting

[RishiRishiRishi]

Hi,

I'm currently repairing a Allen and Heath SD12-2 Mixer and have managed to fix a lot of issues through replacing all electrolytic caps, ICs and transistors across the different boards.

I've got one channel that was distorting when monitoring the direct out of that channel strip, but does not distort when monitoring it through the main output, which makes me think it's distorting within the direct out amplifier section (schematic attached).

So far, what I've done on this channel is replace:
all electrolytic caps (have checked orientation and all seem correct)
I've replaced all transistors (BC307 for BC557B, BC109 for 2N3904 and 2SC732 for KSC1845)
Replaced the 741 IC (have done this three times now)

Following replacement of said parts the distortion still persists on the direct out, but again not on the main output, and so there has been no improvement.

The base on the replaced 2N3904's within the line amplifier section before the direct out is reading ~16V, which on a working channel is around 11.8V. Also the output on the 741 is reading 24V (edit: now reading 12V), which on a working channel is 12V. There doesn't appear to be a short between pins 6 and 7 on the 741 (checked with continuity on DMM).

I'm not sure what specifically is wrong with this - I measured the 1N4148 diodes on the board with the DMM in diode mode, and DC voltage and they're comparable to a working channel.

If anyone has any ideas as to what's wrong, that would be hugely appreciated.

Many thanks!

 

[Khron]

Also the output on the 741 is reading 24V, which on a working channel is 12V. There doesn't appear to be a short between pins 6 and 7 on the 741 (checked with continuity on DMM).

What about voltage on its pin 3?

 

[RishiRishiRishi]

What about voltage on its pin 3?

Pin 2 and 3 are measuring 12V, and pin 6 is now measuring 12V too, and pin 7 at 24V

 

[Ike Zimbel]

I would take a close look at the two 470k resistors that form the voltage divider going to Pin-3 and make sure that they are making proper contact, in particular make sure that the lower one is actually making contact with the 0v bus. If that is floating, I believe you will see 24v on the output. Odd that the direct out distorts but not the mix bus though...

 

[RishiRishiRishi]

I would take a close look at the two 470k resistors that form the voltage divider going to Pin-3 and make sure that they are making proper contact, in particular make sure that the lower one is actually making contact with the 0v bus. If that is floating, I believe you will see 24v on the output. Odd that the direct out distorts but not the mix bus though...

Thanks, I’ll check them both. The output (pin 6) is now reading 12V. Makes me think that the issue lies within the direct out amp section rather than where it’s tapped for the main out. Was thinking possibly the diodes, but if they’re measuring ok and similar to working channels, I guess it rules them out?

 

[RishiRishiRishi]

Checked the resistors on and they're both connected ok - the direct out jack is measuring ~15V whereas a good channel is measuring ~8V