Soundcraft Series 200 - Master PCB Issues

[smilan]

I'm having some issues with the master section on a Soundcarft Series 200 desk.
All of the four meters working good when monitoring groups 1-4.
When I'm switching the "3-4 / MNTR" button to monitor the master mix bus, the lower meters jumps up and stay steady (and doesn't responds to the signal).
When the "3-4 / MNTR" button is down there's a 5.6V dc voltage across the meters.
Pressing the PFL button reduce the DC voltage to 0.7V.

Another thing is there is a -16V dc at tip and the ring of the headphones connector.
This dc offset is normal?

Any ideas how can I sold those issues?

Attached a link to the schematic (this is the best quality I could find)

 

[abbey road d enfer]

Another thing is there is a -16V dc at tip and the ring of the headphones connector.
This dc offset is normal?

Not at all. You must fix this issue first. Probably one of the output transistors is shorted.

 

[smilan]

I think I made a mistake earlier, the dc offset at the headphones jack is 16V positive not negative. I removed and tested TR 7, 8, 9, 10 and all of them tested good with a LCR meter.
Without those transistors the dc offset was still present.
It seems like it coming from the outputs of the TL071 (IC9 and IC10). Without those opamps there is no dc voltge at the headphones jack.

Now as far as testing the JFETs (j111) it seems like my LCR meter is having problem to test them right.
I've measured T1, T2, T5 and T6.
Only one of them tested as a JFET, the others tested as two diodes. (I've tried to test few other J111 that I pulled from broken devices that I have here and all of the J111s tested as two diodes instead as a JFET, so maybe this little LCR meter having problems with testing the J111?)
There's another way to test those JFETs?

 

[Khron]

http://runoffgroove.com/fetzervalve.html#11 - section 11.

 

[abbey road d enfer]

I think I made a mistake earlier, the dc offset at the headphones jack is 16V positive not negative. I removed and tested TR 7, 8, 9, 10 and all of them tested good with a LCR meter.
Without those transistors the dc offset was still present.
It seems like it coming from the outputs of the TL071 (IC9 and IC10). Without those opamps there is no dc voltge at the headphones jack.

Have you checked that the negative rail is correct?

Now as far as testing the JFETs (j111) it seems like my LCR meter is having problem to test them right.

That happens quite often. Ayway I would not put the JFET's as teh most likely culprits, but everything is possible...

 

[smilan]

Have you checked that the negative rail is correct?

Yes, both rails checked correct (around +/- 17V).
The -17V trace that goes to ICs 8-10 was broken.
After fixing it R117 and R118 passing too much current and start to smoke...
I've measured 1.2 Watts (3.4V drop across 9.7 Ohm ) on each of those resistors.
The transistors that installed are 2sc2240 instead of 2sc1681 and BC560 instead of 2SA842.
This can explain why there is too much current passes thru R117 and R118?

 

[abbey road d enfer]

I can't read the schemo well enough, but I think the opamp is drawing too much current (it's the quiescent current in the opamp that biases the transistors).
DID you replace it?

 

[smilan]

I can't read the schemo well enough, but I think the opamp is drawing too much current (it's the quiescent current in teh opamp that biases the transistors).

Yes, the two TL071 drawing too much current,
I've tried few different opamps and even a single TL071 making the resistors to burn.
I've tested again all the BJTs (TR 7, 8, 9, 10) and all of them tested good.

Here's a little bit better copy of the schematic:
https://drive.google.com/file/d/1oj94qW4mcCb3fy-BNDi9hqC9fet3xz-O/view?usp=sharing

 

[abbey road d enfer]

Yes, both rails checked correct (around +/- 17V).
The -17V trace that goes to ICs 8-10 was broken.
After fixing it R117 and R118 passing too much current and start to smoke...
I've measured 1.2 Watts (3.4V drop across 9.7 Ohm ) on each of those resistors.

THere should be about .7 V across the 470r resistors (assuming 1.4 mA through the TL0), which would in turn result in about 0.1V across the emitter resistors, for about 3mA quiescent.
BTW shouldn't they 33r, not 9.7?

 

[smilan]

THere should be about .7 V across the 470r resistors (assuming 1.4 mA through the TL0), which would in turn result in about 0.1V across the emitter resistors, for about 3mA quiescent.
BTW shouldn't they 33r, not 9.7?

I removed TR 7, 8, 9, 10 and everything working good.
No dc offset at the headphones jack, no burning resistors.
The headphones amp working great only with the TL071s.
Also the meters working well while monitoring the mix bus.
I guess all issues has been solved.

The two 10R resistors that bringing the +/- current to IC8-10 are shown here:

 

[abbey road d enfer]

I removed TR 7, 8, 9, 10 and everything working good.
No dc offset at the headphones jack, no burning resistors.
The headphones amp working great only with the TL071s.
Also the meters working well while monitoring the mix bus.
I guess all issues has been solved.

The two 10R resistors that bringing the +/- current to IC8-10 are shown here:

OK, I thought you talked about the emitter resistors.
The h/p amp certainly works with hi-z headphones, but I think it is at pain driving modern (32 ohm) headphones.

 

[smilan]

OK, I thought you talked about the emitter resistors.
The h/p amp certainly works with hi-z headphones, but I think it is at pain driving modern (32 ohm) headphones.

I've tested it with a 32 ohms headphones and there was no problem.

 

[abbey road d enfer]

I've tested it with a 32 ohms headphones and there was no problem.

OK. I don't think you can reach a level that would satisfy a DJ , but if it suits you, OK.