Kay M710 Spring Reverb repair

[belzrebuth]

Hi all,
A friend gave me a Kay M710 Spring Reverb in bad physical condition and it's not working properly.
A previous repair attempt probably burned a Ge transistor (Q10) and the guy replaced it with a 2n3906 as-is (at least he got the pinout correct!).
The rest of the circuit seems okay when trying to trace it along the schematic and I get a signal on the output but I hear a constant loud buzz and very little of the actual effect passes through.
The signal before it hits the effect is badly distorted.
I don't know what the culprit is as of yet but I guess the first stage should be properly biased and it's not since it was meant for a germanium transistor.
Can I somehow modify that part to actually work with a 2n3906?
I've tested all the other transistors with a transistor tester and they seem to work okay so since this transistor is missing and it's not that easy or quick to find a replacement could anyone help into modifying the circuit a bit to make it work with a silicon pnp?!

[t1d]

Please give us the part number for the failed transistor. I can not read it on the schematic. (Maybe post a larger schematic.)

I am thinking that a germanium replacement, or substitute, can still be sourced. There are boutique supply houses, for just this sort of thing. Tubes and such... If you want to maintain that vintage sound, you are going to want to steer clear of modern adaptations, as much as possible.

Noise can get into vintage audio gear in lots of ways... Certain wire pairs must be twisted... Wire routing paths must be done in a certain way - like running them close down on the chassis... Ground loops... Etc. Make sure the chassis earth ground connection is solid - no rust under the washer, etc. and that the earth ground wire is 100% all the way to the end of the wall plug. If it is an old/original cable, just replace it.

HTH.

[floobydust]

The schematic shows no bias resistor for the reverb-line driver transistor 2N408 20V 70mA 150mW hFE 65.
Without it, I would expect lots of distortion. I don't know if leakage current provided it (bias) from C4 or the Ge transistor itself. The original sound of this reverb might have been a bit "dirty"? I don't see a control for wet/dry.
But a 2N3906 is fine, I would say it will just need to be biased into Class A. It depends on the coil DC resistance if this will be a problem, i.e. the transistor idling current say 10mA through the coil.
Ideally there should be bias resistors and a coupling cap to the reverb coil, parts I would add unless it upsets the original grit.

For it's age, all the electrolytic capacitors will be dead, dried out C8 and C5 would cause hum and buzz.