Need help fixing a punch 4040 dsm amplifier with a scratchy/distorted channel.

[thebestofall007]

Hello all, I am fixing a punch 4040 dsm amplifier I got for free that had a shorted output transistor that caused the left rear channel to output rail DC. I have replaced all the buz11 output transistors and it solved the DC problem, but now the audio in that channel is scratchy and distorted at all volume levels. I've tried playing with the gain, treble, bass, phase, and input switches to see if any of those were causing it (like if they need a touch of deoxit for example), but no go.


What causes this and what parts would I need to replace next?

 

[Perry Babin]

Did you check all of the SMD resistors on the main board for this channel?

Did you check the source resistors for that channel?

If you have a scope, post a photo of the distorted waveform.

 

[thebestofall007]

Which ones are the source resistors?



Do you happen to have a schematic of this amplifier by chance? That way I can know what to look for.

 

[Perry Babin]

The source resistors are the large 0.1 ohm resistors connected to the source legs of the FETs.

 

[thebestofall007]

the smds looked fine, but those source resistors, r315 and r322, look like they have gotten hot and the bands are hard to read. From measuring, r322's resistance is infinite.


I think I found the culprit.

 

[Perry Babin]

If those burned, the protection circuit may have been damaged.

 

[thebestofall007]

To troubleshoot the scratchy audio problem, I took a source resistor off another channel and put it on the left rear channel and the channel I took the resistor off of now has the scratchy audio. Gotta order some .1 ohm resistors now.


The DC problem in the left rear channel resurfaced again, unfortunately. I had the amp go into protect at one point.

 

[Perry Babin]

After replacing the 0,1 ohm resistors, did it ever play normally on that channel?

 

[thebestofall007]

Yessir, it does now.


I found out why I think that DC issue happened: I had a voltmeter hooked up to the bad channel and wiggled the driver board for the channel and the DC went away, indicating a broken connection. To fix that, I reflowed and added new solder to the board's connections, and that fixed it. That issue might have also blown the source resistors and transistors for that channel as well (correct me if I'm wrong).



I now have to order some 0.1 ohm resistors and all the channels will be sounding good. Thanks a lot for your help, Perry.

 

[Perry Babin]

If it had one bad connection on the driver board, it could have had more which could have caused the outputs to fail. The failure of the outputs is likely what caused the resistors to burn.

 

[thebestofall007]

What kind of resistors are the source resistors on this amp: wirewound, ceramic, metal film, carbon...? I do know they are 0.1 ohm 5% tolerance 2 watt as per the schematic.

 

[Perry Babin]

Metal film or metal oxide film.

I think these were used in some amps.
https://datasheet.octopart.com/MOS3C221J-KOA-datasheet-13176473.pdf

In amps where there are parallel outputs in a channel, I recommend 1% parts in Rockford amps but this amp has no parallel outputs.