Soundcraft radio desk fault

[KevinE]

I'm trying to track down a slightly obscure fault on a Soundcraft Series 10 broadcast desk.

When any PFL button is pressed, the PFL works fine but there's a POP through the left headphone and also the left PPM meter kicks hard over for an instant. It's only the left channel.

The fault can be seen on the PFL 'L' bus that serves the modules. 

AFAIK, pressing a PFL button does 2 things: it sends a  logic signal to the master section which controls the audio routing to whatever's selected to monitor the PFL (pres phones, studio phones, studio monitors, control room monitors,  etc).  And then it places the pre-fade channel signal onto the PFL L&R buses.

The PFL action is working fine, everything is routed to the proper places and L&R audio finds its way through where it should. 

Just a POP !

Anyone had experience on these? I havent....

 

[themadhippy]

Had a similar fault on a soundcraft 200b,it was down to a 10 ohm resistor,buggered if I can remember were it was in the circuit though

[Brian]

1 hour ago, KevinE said:

 

The fault can be seen on the PFL 'L' bus that serves the modules. 

 

 

Aren't they meant to be virtual earth busses? That you can see something on them means a poke around the input to the PFL stage might be instructive.

[bruce]

Do you have a service manual or circuit diagram? 
still a good number of series 10s around in community radio world, I can ask around to see if anyone has one?

[KevinE]

I'm ok on that score thanks! I'm working over tonight and have made progress. I've pulled out all the I/p modules and just used one as a tester - (a mono/mic channel) and it still did it. So, desultorily, I took the the test channel out and put the next one to hand in instead, and that did not cause a pop! So then I fitted the other channels, a mix of mono and stereo line back in, until I had every module PFL'ing without a pop. As you can imagine, when I fitted the one channel I'd been using for testing - it made all the other PFL switches 'pop'.

It's this one mic channel causing the problem.

It has been worked on before. Most of the fusible resistors have been changed; usually a sign of op-amp trouble; however all the op-amps are original. I'm beginning to wonder if an op-amp had failed, maybe gone DC and taken the fusible resistors out. They've been changed by A.N.Other - but the faulty chip is still there.

Mass chip probing and probably changing will come on the Monday, had enough for today. I'll report back for anyone interested!

 

[KevinE]

😃Well, I sorted it this morning. 

Probing the PFL multiplexer IC7 on the faulty module showed the output that feeds  the left channel was not sitting at zero but about 0.5v.   Selecting PFL caused the IC rails to offset by a corresponding amount which then slowly drifted back to 0. Changing the multiplexer IC7 effected a cure.