old Kustom preamp bias problem

[Alex R]

Old hands may recognise this issue... but I still have this amp and still have a problem with it, which is that as soon as you turn the volume up at all the signal gets squidged down to nothing and distorted.

I have changed most of the components on this board... I'm obsessive about it...

The voltages I get around Q203 and Q204 aren't right. With the standard values there's very little bias voltage on the base of Q203, and that kills the signal. As soon as I turn up the signal the voltage there goes negative, and that's the end of that.

SO I fiddle around with R219 and R220 till the voltages at idle look right. Same thing - when I turn up the signal the voltages reduce then go negative, and the signal dies.

Not at all sure what's going on. I don't really understand what governs that bias voltage at the base of Q203, and why the presence of signal changes it. I'm sniffing a learning opportunity here - anyone want to play teacher?

[Alex R]

...here's the schematic.

[Enzo]

Offhand, only guesses.

But CEDist, the wholesale division of www.tubesandmore.com, says in their catalog that they have a large number of old Kustom boards for sale at $20 each. This means to me that the retail division would likely sell them as well, and more likley I'd guess $30-40 each. or one of us might be willing to act as middleman. You might contact them and see if this board is available.

But yes, I'd rather solve the puzzle.

Are the +/-8v rails OK? Does the boost switch position make a difference?

I am instantly suspicions of C213. Also C207. You should set R219, R220 back to stock schematic values. Obviously something is wrong, changing the design is not the way to fix it. For that matter C222. Chaned MOST of the parts? Well WHAT? What is new and what is not?

[rf7]

+1.

I've fixed one of these before. Try lifting one side of C207 and C213 to isolate the stage and see if the voltages look better then. Do the transistors measure OK with the diode setting on a DVM? Do all the resistors measure correct? Are those the right transistors in there? If they are equivalent transistors, are you sure the leads are in the right hole in the pcb? Sometimes, two parts are described as equivalents, but their leads may not be oriented the same way coming out of the transistor body.

[Alex R]

I'd rather solve the puzzle too Enzo. But it's nice to know the replacements are there.

I'll get back with the requested details soon.

[Alex R]

OK, returning to this long-running serial...

Enzo, I've replaced Q203 and 204 with 2n4401. All the higher-value caps were tantalum and I've replaced them with electros, polarities as on the schematic. Resistors are back to stock.

Could it be those transistor swaps causing the bias issues?