Crackly amp channel help

[Dylan]

This is really getting annoying, the clean channel on my amp is strangely intermittent. When you adjust the volume you have to tap the volume pot a bit to get it to full volume (it starts off slightly muted sounding, then when tapped fully opens and sounds as it should). I thought the problem here was a dud pot, so I took the amp in to have the pot replaced with a suitable spec Marshall pot. I got the amp back the other day, and while the pot has been replaced and now feels like it should, the problem still remains and I have to tap it still to get it to sound properly. What else could possibly be wrong with the amp? This only happens on one channel so I guess it's localised to that pot.

[niftydog]

dude, sounds like you got dudded by the repair guy. Are you sure he replaced the pot?!?

It could be dry joints, it might be ANOTHER pot, it could be the input jack, cracked tracks... man, the list is endless really. It could be a lot of things.

Just open it up and make sure he actually did change the pot! Look for fresh soldering (more shiny than the old stuff)

Take it back and say "I asked for this crackle to be fixed, and it has not been fixed, now fix it properly please!"

[Dylan]

I'm pretty sure the pot has been changed (before when turned it basically felt like it was just turning in your hand) but it hasn't helped. I've looked for other problems but that seems to be the only one. I'm going to take it back on Tuesday (not open Monday) and get them to fix it and actually test it this time. I guess there's a chance the replaced pot was a dud. Removing the circuit board is a pain in the ass on this amp and I can't be bothered with it. Going to leave it up to them to sort (and will mark the old pot before I send it in to check it is changed to a good one this time).

[niftydog]

just don't say anything about "changing such and such a pot".

Just say "my amp is crackling, please fix it" and let them sort out what's wrong.

If you ask them to change the pot, they'll change the pot and charge you for it. But that doesn't necessarily mean that the problem is fixed.

[Dylan]

I took it in to them the last time and said, "my amp is cutting out and it's probably about time it needs a service". I also pointed out to them how the pot was affecting the sound and they said that they would diagnose any problems on the amp and repair them, I really do doubt whether they did an awful lot. I don't even think they tested it at any point. I did mention the pot needed replacing, but I also mentioned that the main problem was the intermittent volume and that's what I brought in for them to find out and fix (which they didn't). I'll see what they say come Tuesday.

[niftydog]

well, they didn't do the job that you paid for. Make a noise, jump up and down, and do it in front of some of their customers and they'll fix it properly this time for no charge...

or at least, they SHOULD if they have any scruples at all.

[R.G.]

The Tube Amp FAQ and the Tube Amp Debugging Page at GEO have some helpful hints for self diagnosis. It also shows some ways to localize the problem that you could do.

Your amp is probably intermittent for one of a few reasons. Some top ones to check that would have been missed by an amp tech replacing the pot are
(1) DC on the pot caused by a leaky coupling cap
(2) bad ground connection caused by loose pot or jack nuts
(3) an array of other loose and intermittent connections including but not limited to the bad solder joints, cracked traces, cracked components, bad tube sockets etc.

The tech should redo the fix if what you specified was "fix it" not "replace the pot".

[Dylan]

I'm fairly confident the problem lies in the "new" pot supposedly fitted, I took my multimeter out and didn't find any breaks in the solder tracks in significant places. Also, because the problem only happens on one channel I don't think it is anything related to the power supply section of the circuit.