Fender Vibrolux Reverb Channel Power Loss & Distortion

[Pastortom]

I just swapped for a Vibrolux Reverb (1971) ....all circuitry is working OK, nice reverb & tremolo, and the normal channel has nice clean power with lots of headroom.  BUT, the vibrato channel (channel 2) has an obvious power problem....and when I turn it up, after midway it loses MORE volume, with lots of distortion....distortion (overdrive) comes in at a low volume......definitely a sick circuit somewhere.


At first, I thought it was an obvious pre-amp tube problem.....so I switched V1 and V2 to see if it made a difference....it didn't.  I also changed V4, but to no avail.   The vibrato channel, when cold, will come on and you almost think it's acting fine set on about 2 or 3.....but it noticeably loses some volume after a few minutes...and if I turn it UP, it overdrives and loses more power as I go up above 5.  The vibrato circuit seems to have no effect on the problem....it works fine, and the problem persists whether or not it's activated.  Same with the reverb.  Works nicely, and doesn't change when turned on or off.


It doesn't seem to be a tube problem, at least not at the pre-amp stage.  If anyone has an idea here, I'd appreciate it.  It's on the bench right now, and I'm gonna freshen the solder joints on that entire circuit......BUT, seems like more than a cold joint.   

[vibrolax]

Just some guesses for you, before you start shotgunning for cold solder joints:
1) Plate coupling capacitor leaking DC, messing up the biasing of succeeding stage
2) Check the plate voltages and cathode voltages in reverb channel.
3) (Power off) check the resistance of the plate resistors.  Don't know if you can check the cathode resistors with the cathode bypass caps in place.  Cathode bypass cap may be shorted.
4) Maybe a preamp tube pin isn't making good contact.
5) Check the resistance of the volume pot throughout its range.

Do you have an oscilloscope scope?  That might make it easier, but it's probably not strictly necessary?

Jon

[EL34]

I always did a full Fender service on Fenders first and when I was done, it had eliminated the problems.


My Fender service guide here is where I would start if you have not done that already. :)

http://el34world.com/schematics.htm#Servicing fender amps