Quad 34

[Cotic101]

Hello Vintage Radio,

I have a Quad 34 that I recently sent for repair (one channel not working) and to have an upgrade kit fitted. The unit was returned with the kit installed; so all the caps have been replaced but one channel is still not working. My repair specialist says it was working when he had it so it must be my cables, despite trying 5 different cables and two different systems, the faulty channel refuses to work. I have read an old thread (now closed) which suggests D32 may be the problem and I am thinking of changing D31 whilst I am at it and possibly R102, R103.
However, the codes that I have found for these parts are probably from Maplin who are no longer trading. Can anyone tell me which are the correct components for the positions D31, D32, R102, R103, if I order from Mouser or Digikey.

Thanks.

[mickm3for]

Hi the parts d31 d32 r102 r103 are in the power supply and common to both left and right chanels Mick

[Mooly]

Do you have any test equipment such as a scope?

A dead channel should be relatively easy to pin down with a few careful measurements.

[Cotic101]

Originally Posted by mickm3for

Hi the parts d31 d32 r102 r103 are in the power supply and common to both left and right chanels Mick

Thanks for the reply Mick, If they are common to both L&R channels then I guess that is not going to solve my mute L hand channel. Can anyone point me in the right direction to fault find this problem? I am fairly handy with the soldering iron but very inexperienced at finding out where the problem is.

Originally Posted by Mooly

Do you have any test equipment such as a scope?

I don't have a scope but may be able to borrow one (could take a little while).
In the meantime, I could do some MM tests but need it to explained like you are talking to a 5year old.
Thanks, Mooly from Lancashire (my neck of the woods originally).

[mickm3for]

Hi this will help

[Cotic101]

Thanks, Mickm.

[Mooly]

Originally Posted by Cotic101

I don't have a scope but may be able to borrow one (could take a little while).
In the meantime, I could do some MM tests but need it to explained like you are talking to a 5year old.
Thanks, Mooly from Lancashire (my neck of the woods originally).

You can still get creative with just a meter and some lateral thinking...

Firstly you need some 'safe' way of listening to the amp output, 'safe' in the sense that its not a good idea to have a faulty preamp connected directly to a power amp as anything could happen while investigating the fault. I'll leave that up to you but one possible option is to wire headphones to the preamp output. The output should drive low impedance phones enough to hear what is going on and the series resistance already at the output is a good safeguard.

First check the supplies to those final output opamps, checking supplies is the first rule in faultfinding. Plus 9 volts on pin 7 and minus 9 volts on pin 4. Absolute values don't matter at this stage.

Pin 6 of each should be at zero volts.

Do you get audio when 'Mono' is selected?

If yes then we are looking further toward the front end.

If no then short out pins 1 and 2 of IC22, just use a screwdriver blade. Still no audio? In that case we are looking at the output stage.

[Craig Sawyers]

The quirky thing about the 34 is that the supplies are not symmetric. +8.6 -9.4. Expecting them to be +/-9V will cause some confusion. Allegedly the odd choice of rails is because the asymmetric clipping behaviour of the the opamps used. So the choice of rail voltages is to ensure that clipping is symmetric.

Craig

[Mooly]

Thanks Craig, your absolutely correct and Quad did that little trick just to avoid negative reviews as far as I understand. I was just trying to keep it simple

[Cotic101]

I am going to order a scope (it is about time I got one) so I shall get back to you once it has arrived and I have had a little practice with how it works.

[Mooly]

A 'scope is probably the single most useful piece of test gear you can own. Although even a simple and cheap USB one would show line level audio signals I would probably recommend either a decent traditional analogue CRT based dual beam type or a well specified modern digital one. Even a simple 20MHz analogue 'scope would show with ease complex signals that would evade a seemingly higher specced digital type.

[ForsbackaKalle]

My 50cents goes to the crappy balance slider who can cause this kind of problem. Try to bypass it and see if you get the channel working.

[AC/HL]

I repaired a muted one for a friend, and it was C69. Both channels of course, but if you suspect muting it may be worth lifting the muting transistor from the dead channel.