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Quad 34, lost an output channel

P

Puggie

pfm Member
I have a mid aged (rca/phono sockets but earlier Quad logo) Quad 34 pre-amp. Over night the left hand output disappeared, cranking the volume up high I get a little noise probably crosstalk but that is all. As far as I'm aware it's all original, so probably due a re-cap of the electrolytics anyway. Is the channel out likely to be the CMOS switches gone? Or are there other common issues with these. Other than the output everything else is fine, channel switching, tilt and bass controls etc all behave perfectly.

Any advice before I pull the casing off?
 
Are you sure the fault isn’t down to the amp?
Have you tried to switch channels over?
 
Tried different inputs and the fault stays with the same output. Swap outputs to the power amp and the fault stays with that output of the pre. Not checked the tape outs yet, that might help suggest where the issue is?
 
Ok then, lift the bonnet and follow the signal. Check the balance pot first.
I’ve had one with a bad CMOS switch, one with a bad balance pot.
Also check PCB tracks.
 
Tape out is likely to be in front of the volume control, as it is "fixed" line level, so yes it might limit the options a bit as to where the fault actually is located. If tape out is faulty then something is wrong before the output stage (not that it means nothing is wrong there too).
 
Hi All.
Have you checked the balance pot. Its a known failure mode where the plastic bracket that holds the slider contact onto the resistive element breaks off. Its an easy fix once you identify the problem, just google "QUAD 34 Balance control fail".

LPSPinner
 
chartz said:
#3 Ok then, lift the bonnet and follow the signal. Check the balance pot first.
I’ve had one with a bad CMOS switch, one with a bad balance pot.
Also check PCB tracks.
Click to expand...
I couldn’t agree more dear LPSpinner! :)
About the printed tracks, there can be oxydation, and tracks can be corroded so much as to stopping being conductive. Look for brown ones.
It depends on the PCB rev. fitted though, with later RCA boards being much less problematic I find.
 
Was a failing TL071 op amp, poke it with a finger and it would work for a bit then it slowly fade out. Swapped a new(ish) one in and behaving nicely. Really weird failure mode of the op-amp.
Thinking it may benefit from a recap with the age it is. Is there a preferable/better guide than the DaDa stuff online? I've read of someone not liking the Elna Silmics, I usually go for Panasonic FCs, but they seem in short supply last time I looked on RS. Any love for Nichicons?
 
I have seen IC internal bond wiring failing, but its rare.
Any of the well known brands are good (105C rated). Nichicon are in the top class.
With the shortages these days, just buy what you can get ex-stock
 
Puggie said:
Was a failing TL071 op amp, poke it with a finger and it would work for a bit then it slowly fade out. Swapped a new(ish) one in and behaving nicely. Really weird failure mode of the op-amp.
Thinking it may benefit from a recap with the age it is. Is there a preferable/better guide than the DaDa stuff online? I've read of someone not liking the Elna Silmics, I usually go for Panasonic FCs, but they seem in short supply last time I looked on RS. Any love for Nichicons?
Click to expand...

I went for nichicon muse bipolar, from mouser for the signal coupling caps.
There is little polarising voltage across them and its what caused the originals to fail and leak prematurely.
Use the dada manual to guide you on which coupling caps can be replaced with wire links.
I used panasonic FC for PSU caps.
I also increased the value of input film coupling caps these are a bit small depending on what you use as a source.

And check for corroded tracks under the solder mask, even with no faults.
its much easier to fix before it gets bad enough to show symptoms.
I found a light scrape with a scalpel would flake the coating off corroded tracks but not good ones.
Then I cleaned up with fibreglass pencil and tinned with the aid of a flux pen.
 
davidsrsb said:
I have seen IC internal bond wiring failing, but its rare.
Click to expand...

I've had it happen with a small-signal transistor in a power amp. The connection got intermittent and causesd the output to crackle. Wiggling with a finger or spray showed it. Solder join to track-pad was fine and re-soldering the lead didn't fix it. Had to change the transistor.
 
I think the electrolyte that leaks from the 100uf capacitors finds its way under the solder mask ( maybe at the pads) and the propagates along eating its way through the copper.
The copper is jet black after you flake off the resist.
 
I have seen plenty of electrolytics leak without destroying the copper.
Batteries as in the old Quad tuners are another far more corrosive story
 
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