Quad 34 no output on left channel

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30-Jul-2019 09:20 AM

lool

lool

Posts: 11

Hi, I just bought a Quad 34 of which the left channel doesn't give any sound. After pressing the mono button, both channels work fine with disc input.

After doing some research, it's probably one of the opamps that's faulty. Questions:

- Is IC7 or IC27 the most likely culprit ?

- Does it make sense just to replace 1 IC with e.g. OPA134 or OPA2134 resulting in differences in sound between the left and right channel or should I than just replace all Opamps in both channels ?

The 34 was revised by Quad Revisie in 2014 where they replaced all caps and fixed the balance pot meter.

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31-Jul-2019 03:06 AM

EJP

EJP

Posts: 1677

It is almost certainly not one of the opamps that is faulty. I would be looking for a dry joint or broken capacitor. The fact that it works in mono exonerates everything downstream of the mono switch, so you should look upstream only.
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01-Aug-2019 10:27 AM

lool

lool

Posts: 11

Thx for your comments. I started to do some measuring. All voltages are just fine. I couldn't see a bad joint. If it was a bad joint, than I should at least hear some sound on the channel I guess, be it a bad or noisy sound. Now it looks like the channel is dead.

Measuring the caps on the board is difficult. All elcaps have been replaced a few years ago. Shoudl I check all other caps ? It's not likely they would die.

According to a document in the download section it refers to IC17 as a possible cause.

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02-Aug-2019 02:37 AM

EJP

EJP

Posts: 1677

I meant physically broken. I've seen those square film caps lift right off their leads and disappear.

It could be IC17 or anything to the left of IC22, the common point where the M (mono) logic level is applied between the channels If it was IC17 it might also affect the F1/F2/Slope operation: worth checking if they are out.

My money is still on a dry joint ;-) Have a good look at the six terminals of the volume control, looking for little rings around the centre terminal. If found, remove all solder and redo completely. Ditto the balance control. In fact it mightbe the balance control, in which case you could be in for a bit of fun disassembly.

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05-Aug-2019 12:23 PM

lool

lool

Posts: 11

Thx for your suggestions EJP. Really great input !! It turned out to be RV4, the potmeter for the balance. As the unit was revised a few years earlier, they replaced the right channel by a small potmeter on the print but the left channel was apparently still using the original RV4. After replacing it by wires directly as suggested in your revision document for the 34 everything was just fine.
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