ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Re: not fixed


 :
9/4/1999 8:59 AM
Mark Cameron Re: not fixed
I had a Maxon Tube Screamer and the only diff, insted of Ibanez it had Maxon other than that same.  
 
Good news Maxon is coming out with a 808 copy in a MXR style box w/true bypass here in the US.
 
Today
ampage
archive
And now, a word from our sponsors:


 
8/28/1999 5:52 PM
R.G.
Re: fixed!
Just catching back up. If you've checked for solder shorts, rosin, contamination, etc, then it's likely that either  
 
(a) the switch fets are going south or  
 
(b) the gate drive the switch fets is bad.  
 
 
 
There is a diode in series with the gate of each switch fet. The outboard side of the diodes should go from close to ground to close to +9V when you change states. If the voltage swing is too small, the JFET won't turn off entirely, especially if the voltage doesn't go low enough - say under 1V. The JFET most likely to give you what you're describing is the one right after the tone control opamp.  
 
 
 
Actually, I just thought of a third problem you could have. The switch fets rely on having their source and drain pulled up to the 4.5V bias voltage. That way, pulling the gate down to almost ground turns the fet off. If the source and drain are not pulled up high enough, even a good gate signal wouldn't turn them off. Check the DC voltage on the source and drain of the switch fets - should be about 4.5V. Measure the gate drive on the outside of the diode in series with the gate - the gate impedance is so high you can't measure the gate voltage without changing it.  
 
 

<<First Page<PrevPage 3 of 3