[HAM] Vibrato Problem

[David Anderson]

Mark,

For comparison, I checked the voltages at the line box capacitors on  
mine.

I got varying DC voltages, but no more than in the +/- 1 millivolt  
range in either C or V positions. That would be accounted for by  
small shifts in the preamp's power supply as a result of varying line  
voltage.

If you are finding +/- 0.5 VOLTS, not millivolts, something is wrong.  
Is there a DC voltage at the vibrato line drive wire (brown) preamp  
terminal?

If yes, there might be something wrong with the preamp's vibrato  
input circuit that is causing the circuit to oscillate when presented  
with the higher load of the vibrato line in Chorus settings.

I recall you said screen and plate resistors were replaced. Is it  
possible that a mistake was made, creating a  low frequency  
oscillation at the cathode of the 12BH7?

It would be interesting to put an oscilloscope on the vibrato line  
drive wire and/or the 12BH7 cathode and work backwards from there.

David

On Apr 13, 2008, at 11:10 PM, Mark Laesch wrote:

> David,
>
> I have not checked this. My check indicated that when in any of the  
> the C
> positions,  varying DC voltages in the negative range .554 to -.595  
> volts on
> the top of the capacitors. When in the V position,  zero volts DC.   
> Does
> this indicate C13 is bad?
>
> Regards,
>
> Mark