Dual Rectifier not switching channels till it warms up?

[Monsta-Tone]

A friend of mine has a 2 channel Rev. G Rectifier that works perfectly once it warms up.
It is stuck in the Orange channel for a few minutes though, after turning it on.

As you can imagine, this can be an extreme shock! :lol:
I tried it out. Turned it on, couldn't get it to change channels and had cranked the Red channel's Master Volume.
Nothing happened, so I just started jamming in the Orange channel.
After about 5 minutes, the amp switched to Red with the Master dimed...........Holy **** was that loud!!!!!!!!!!!!



Anyway,
Before I dive into the amp, I was wondering if anyone else has had this problem and if they knew what the fix was. It's a freebie repair job and I'd like to keep it as short and sweet as possible.

 

[vick1000]

Clean the footswitch jack. Check that socketed Opamp near the input stage.

 

[JCDenton6]

^ Good info above, maybe also a suspect solder joint?

 

[Monsta-Tone]

Right on, thanks guys!
I plan on opening it up tomorrow morning.

 

[APEMAN]

-

 

[jbird]

If my channel switching was not working, I don't think I would crank the master of the channel I was trying to switch to?

 

[Monsta-Tone]

If my channel switching was not working, I don't think I would crank the master of the channel I was trying to switch to?

:lol: Come on man, live a little! :lol: I had forgotten that I had it up loud.

Can't be a cable issue, there was no footswitch or cable plugged in.

Can't be a wall power issue, as it happened for the previous owner as well.

I'm opening it today. Had too much work this week to get to it any sooner.

I was thinking it was a power supply issue. Maybe bad solder joint, maybe the caps have a hard time building up.
Once it starts working, it works until the amp is turned off. It just takes 5 minutes for it to decide that it wants to switch channels.


Thanks guys! I'll let you know what I find.

 

[Monsta-Tone]

Man.....I'm stumped. It's gotta be something simple, but I'm over looking it......

All readings were taken with no footswitch or footswitch cable plugged in.
All readings were taken when amp is freshly turned on, before channel switching is working properly.

All voltages are correct (with the exception of Pin 6 on the 6N139 chip):
Line Voltage = 120.6
V1 Pin 9 = +3.144
V1 Pins 4 & 5 = -3.144
6N139 Pin 2 = 3.6/1.4
6N139 Pin 3 = 4.45 (when Orange Channel is selected), 0 Red Channel
6N139 Pin 6 = 3.6/.0189 (1 volt higher than the drawing)
6N139 Pin 8 = 5.198 (and falling) Orange Channel - 5.23 Red Channel (not sure what these voltages are supposed to be, they are not on the drawings)
Loop Switch
- Blue wire 3.26
- Blue wire on power cord side 3.26 when Loop Bypassed, -3.44 when Loop is on for any channel

Seems like the power supply is working, at least to the extent that the voltages are somewhat correct.
I am wondering if the 4700uf cap is just not holding a charge when it is freshly turned on, or maybe a bad solder joint under it.
Maybe it's a current issue, not a voltage issue?

I have replaced with no change:
6N139
Q1, Q2, Q3

I know it's something simple, probably why I can't figure it out....Or....too much coffee!
Also noticed that if the Loop switch on the back is set to Bypass, I can barely hear the Red Channel. Channel Master does affect the volume slightly when Loop is bypassed.
Loop LED is very faint when the amp is not switching properly.

 

[vick1000]

Do you have any way to check Q levels on the filter caps? They may be the ones not holding charge. Other than that, maybe an open thermatic resistor somewhere, I don't have the schematic. Once I decided to keep my RevG (built 6/'00), I had the filter caps replaced as a precaution only.

 

[Monsta-Tone]

I ordered new filter caps. They should be here by the end of the week hopefully.
I'll replace them all since I have to lift the board for some of them.

I did find a couple of suspect solder joints and re-flowed them. I also found a resistor or two that look funny. R501 in the switching supply looks burnt on one end. Haven't tested it yet though. I'll probably just replace it.