Mixing desk - No output on one side - H/H stereo 16

[FDIM]

Hello,

I have a problem with my HH stereo 16 mixing desk from 1978. I know thats old but it was my first one and I still like it very much.

I have no sound, other than a soft 50hz buzz, comming from the Left master output.

On the headphones output everything is OK : master EQ, Faders.

The Left VU meter acts special :

When nothing wired on the output : needle doesn't move
When I wire an amp : Needle freezes at -10 dB
When I wire an unbalanced jack (should be TRS) VU freezes in the red zone
When I turn the mixer on or off it shows a large peak wile the right channel just moves lightly.

I opend the mixer and took a lot of mesurements on the master section board (just resistance over diodes, resistances, capicitors) where I could compare the left and right channel.
All these mesurements showed equal results. Even from the Faders to the output jack.

I don't know how to find the faillure !

Can anybody help me ?

Thanks a lot,

Frans

[Enzo]

Two things are topping the list. One is simply a bad master fader, the other is a failure in the output drive circuitry. COnsidering the additional clues, I suspect the latter.

Is this thing based upon ICs or discrete transistors? My spider sense is tingling over a DC offset somewhere. That would explain the low hum, the lack of signal passing, and the steady but unwanted VU behavior. Look for the circuit driving that output, and see if some DC is sitting on the signal path.

[FDIM]

Thank you, enzo, for your answer,

I will look at that today !


Also the output drivers are IC based : two Op-Amps 741 per channel, some resistors and a capicitor.
Then just before the actual output some diodes, resistors and capicitors. I thought these were for making the signal balanced.
I could join a picture of the board, but maybe thats of no use as I do'nt have the diagram ?

PS
Please note that I live in France so if you see no answer from me, I'm cerlainly asleep or at work.

[Enzo]

Here is an approach. Assuming you have left adn right master outputs, that means there should be two essentially identical circuits. Look for two sets of similar looking parts layouts. One should be left and one right. WHichever one is working, use it as a reference, then compare voltage readings to the "bad" side. Even without schematics, I can usually identify which ICs are driving the outputs, and so then look at the output pins of the ICs and look for DC there.

[FDIM]

DC mesurement

Thanks a lot , Enzo, you were right about DC offset !

Here's a small diagram of my mesurements.

Diagram Mixing desk.pdf


So, both ICs have -14V DC on there outputs, and R11 and R12 must be cut as there is no current going through (is that right ?).

So now I guess I will have to change those two ICs and maybe R11 and R12 ?
But should I change the same parts on the other channel keep two equal channels ?

And of course : What can be the reason of this problem ?

Im a bit worried about changing the parts and than see them blown again just as I turn on the power. Or is this a "usual" way Op-Amp ICs can blow ?


I have just put the mixing desk away for 2 jears (in a rather cold and moisty place...) and when I took it again the Left channal was KO...

Frans.

[Enzo]

If voltage goes through R11,R12, then they can't be open, right? SInce the two op amps are simply the drive here, ine follow the other. If they are DC coupled, then it is not surprising that DC on one shows up on the other.

Leave teh good channel alone. Replace the two bad ICs and see what results. If that fixes it, fine. If that doesn;t fix it, you are only out a dollar or so, but you know the ICs are OK.

Op amp IC output pin going to power rail is a common enough failure mode.

But before changing anything, verify that those ICs have BOTH power supply voltages present at their powr pins. A missing +15v can leave the output sitting at -15v just as easily as a bad IC can.

[FDIM]

I have checked the power supplies : both positive and negatif 15V are present at the right pins.

I've ordered the parts : TL071s in stead of 741s, i'm curious to hear the difference in noise, as people say that 741s are noisy....
I also ordered sockets so i don't risk to heat them to much when welding, and, if the faillure persists, i'll be able to re-change them quite easely.

Anyway, thanks enzo for your very precise answers, and your generosity !


Frans

[Enzo]

Or 5534.

[FDIM]

I already ordered...

I knew the 5534 is a fine op-amp but didn't know it could go as a pin to pin replacement for 741.

[Vicus]

Someone have the schematic?