unstable right channel

[windscreen]

Hello all,
New to this forum but rediscovering valve gear. To cut a long story short, I aquired an old Sony tape recorded that was mechanically screwed but the stereo amp worked. I want to use it as a playback facility from my Revox A77 (playback head directly wired) It works like a dream but I have instability on the right channel. Now heres the confusing part...when switched to mono the right channel becomes stable. I know that mono is the two inputs joined together so why it should become stable in this way is somewhat confusing. Any advice greatly appreciated.

Gelan

 

[Mooly]

You need to look at how the mono switching is achieved. The L and R channels must be mixed to produce mono.

Maybe the different loading that the mono switching produces causes the amp to behave. Or conversely, is there a problem around the input (around where the mono switch operates).

Another thought, could it be anything to do with the head characteristic or its wiring. Have you got them wired/phased correctly ? Maybe the amp is on the verge of instability with the Revox head.

All very vague I know... it would need a detailed study of the circuit and a probe with a scope to see what is going on.

 

[windscreen]

Thanks for your input Mooly. I forgot to add that even without anything attached the right channel is unstable. The mono switch joins the two amps inputs together thus meaning the two channels are reproducing one source. It seems madness and it's driving me nuts.

Cheers, Gelan

 

[Mooly]

You mean without anything attached to the input ? And then you switch to mono and its OK ?

If so then you need to be looking at input switching. Could the switching be dodgy and the input floating or not ground referenced correctly. Its a bit hard to say for sure without seeing it all.

 

[windscreen]

Sorry Mooly, will try to explain better. The inputs are two fly leads that used to go to the head. Load or no load there is a whistle on the right channel which changes pitch when you finger buzz test it. The buzz from both channels is identical and when in mono the quality from both channels is excellent.

Hope this helps,

 

[Mooly]

Difficult without seeing it.

Do you get any noise or whistles with each set of flying leads shorted together ? In other words, where each winding from the head would be, you apply a short.