I have an old fender blackface pro reverb on my bench right now. It was brought in for a re-cap and 3-prong power cord. I did all of this, but there is still a loud popcorn-like sound for the first couple seconds after the amp is taken off standby.
The problem seems to be occurring around the phase inverter (when that particular tube is removed, the problem stops) I checked (temporary replaced one at a time) all the caps and resistors surrounding the phase inverter, as well as the phase inverter tube itself, but still with no luck... Its driving me crazy!!!
Little higher than whats on the original fender schematic... What do you think is causing this? the higher line voltage? Do you think its related to the problem I am encountering?
Stopping the noise by pulling the PI tube only tells you it isn't a PA problem. Try replacing the dropping resistors in the doghouse; sometimes these make all sorts of noise. Also could be one or more of the plate loads on the preamps (but these usually don't get quieter after a few seconds). You can isolate by channel by disconnecting the leads feeding the 220K channel mix resistors (first one, then the other).
I did that, and it seems to be coming from (go figure) the vibrato channel. So now I have to check the preamp, vibrato, and reverb circuits... Whats the easiest way to sniff out where this is coming from?
Also, what about those voltages? Any thoughts as to why they are all high?
B+ voltages will all be high if there is no current going through the supply resistors. You do have some voltage drop, which suggests that v1 is probably conducting, but maybe not the other stages (which is why I asked what the plate and cathode voltages were like).
When a tube is conducting, the current in the tube pulls the plate voltage down (because of the load through the plate resistor). Ipso-facto, if a plate (or cathode) resistor isn't connected or is open, then the tube will not conduct etc, or if a tube has lost it's vacuum, it will not conduct etc. Or it could be a leaky/shorted coupling cap affecting the bias of the following stage as well as bypassing the tube? etc - just lots of guesses at this stage without more info)
