Amp crackling sound

[lx585x]

A bad relay contact will act like this. One of the joys of trying to pass low current through a high current relay. Increased volume / voltage will cause the contact to make through the oxide layer, decreased voltage will cause intermittent connection.

Is it possible to open the relay and clean the contacts, or it must be replaced?

I got an idea today - I checked the STK3102III datasheet and there is a possibility of turning it around - so if it is faulty the crackling would be in left channel after rotation. The only difference in pins is pin 7 and 9 (-Vcc and +Vcc) but I could bridge those to the original position, what matters are the channel outputs.

 

[jwilhelm]

Normally the cover can be removed from the relay for cleaning.

 

[Ian Finch]

In reply #20, that's the reverse of the problem the OP describes.

if I lower the volume all the way down there is no crackling

Higher volume level is where the crackling commences. Still, it takes only a couple of dabs of solder and maybe a few mm of wire to bridge the relay contacts temporarily and eliminate that possibility...or not.

 

[jwilhelm]

In reply #20, that's the reverse of the problem the OP describes.

Higher volume level is where the crackling commences. Still, it takes only a couple of dabs of solder and maybe a few mm of wire to bridge the relay contacts temporarily and eliminate that possibility...or not.

OP said it played fine until he lowered the volume.

 

[lx585x]

What I meant by "lowering the volume all the way down" is that there is no permanent crackling/static, it comes with music/signal when volume is increased and at a certain higher volume, distortion goes away.
Sorry if I wasn't clear with my description.

 

[Ian Finch]

OK, so you can try bridging the relay contacts - those that make and break the connections to either +ve speaker terminals, that is.

 

[lx585x]

I decided to open the relay, so took the transformer out, removed the power board and de-soldered the relay.

After opening the contacts were almost completely black, so used some contact cleaner and a really fine scotch-brite.

Reassembled everything back together, powered the amp up and behold - no crackling .
Turned it off to let it cool again, started again and still nothing.
So far it's been working great - more testing to come but it looks like it was the relay!

Thank you again to everyone for helping