Punch 40 DSM

[Perry Babin]

You can reinstall Q101.

 

[seankane]

The amp plays clean audio on both channels and sounds great aside one problem.

Some reason the bass pot only controls the right channel and does not affect the left channel at all.

C102 maybe?

 

[Perry Babin]

With the bass pot fully clockwise, do you read ~0 ohms between pins 3 and 4 of the preamp board and the RCA shield?

 

[seankane]

No

Pin 3 to rca shield reads 10.6 ohms

pin 4 to rca shield reads 17.5 ohms

 

[Perry Babin]

Well.. that's close enough. There may be a problem on the preamp board. Confirm that pins 3 and 4 show increased resistance when the pot is turned to the minimum position.

Can you post a photo of the preamp board?

 

[seankane]

When the pot is turned to minimum...............

pin 3 reads 0.537 M OHMS

pin 4 reads 0.488 M OHMS

 

[Perry Babin]

OK. I just wanted to make sure those were the right pins.

Photo?

 

[seankane]

Here is the pic of the board.

 

[seankane]

I didnt change the driver board in the channel that the bass pot is not affecting. Could the problem be there?

 

[Perry Babin]

I think you'll have to check continuity between all of the components on the defective channel for that circuit. It goes from there to the larger caps on the right side of the board. Drive signal into only the channel with no bass to find which terminal (3 or 4) is the channel you need to follow).

 

[Perry Babin]

No on #29.

 

[seankane]

I have clean audio on the center 3 pins of the treble pot, why dont I have audio on any of the bass pot pins? Is this normal?

 

[seankane]

If I drive a signal into the right channel which is the working channel I have audio on pin 3 of the input board.

If I drive a signal into the left channel which is defective I have no audio on pin 4 of the input board.

So the problem must be on the driver board?

The left channel produces audio when a speaker is hooked up. If I have audio on the channel why dont I have it on pin 4 of the input board?

 

[Perry Babin]

The connection is broken somewhere. The audio doesn't run through the circuit in one series connected path.

The problem is likely an open connection on one of the traces or a bad solder connection. Either one would be due to leaked electrolyte.

 

[seankane]

I found a very tiny trace missing between C20 and R26 on the input board.

I repaired it and all is working great.

Thanks god I had a schematic for the input board I would have never known that trace was there!!

Thanks again Perry.

 

[Perry Babin]

It would have been a bit quicker repair but I misread the schematic around post #5 and didn't catch it until post #13.

 

[seankane]

Hey no problem Perry I am just glad you could help me. I dont mind the extra time I think it actually helps me learn even more about the amp. Repairing this amp I have learned a few things I didnt know which will help me in future repairs. Thanks again!!!