First Act amp repair

[jomama]

I'd been happily using this amp for years. It developed some scratchiness in the main On/Off volume pot so I gave it a squirt of alcohol. In the process of turning the pot to clear it, I either went past the stop or the stop adhesive let go. The pot doesn't work now, it spins past the On/Off click point and goes from full volume to ground hum. I'm going to be taking it apart today and was hoping someone could help me make sure I don't make things worse with my repair attempt. I bought a solder station yesterday and have a cheap meter but am otherwise clueless.

[nosaj]

I'd been happily using this amp for years. It developed some scratchiness in the main On/Off volume pot so I gave it a squirt of alcohol. In the process of turning the pot to clear it, I either went past the stop or the stop adhesive let go. The pot doesn't work now, it spins past the On/Off click point and goes from full volume to ground hum. I'm going to be taking it apart today and was hoping someone could help me make sure I don't make things worse with my repair attempt. I bought a solder station yesterday and have a cheap meter but am otherwise clueless.

You may find it needs to be replace, check this link and see if it's within your scope.
https://www.ifixit.com/Guide/How+to+...r+or+pot/67387
Good luck and welcome to the group
nosaj

[jomama]

What a great link, thanks! I've decided to try and repair this. If I screw up, well, that's life. I'm taking the thing apart and will report back on what I find.

[Justin Thomas]

The innards of the pot are probably soft plastic, and the alcohol softened or dissolved it. I wouldn't bother trying to repair the pot, just replace it if possible.

Justin

[J M Fahey]

What a great link, thanks! I've decided to try and repair this. If I screw up, well, that's life. I'm taking the thing apart and will report back on what I find.

Don´t waste time.

1) plain replace it and get your amp working.

2) if you are curious, fine with me, open the old pot and explore it as long as you wish, with a microscope if necessary.
Remember it´s physically *broken* and you lack the tools necessary to repair it.

[jomama]

I got the pot off the board and opened the case. It looks like I crumpled the wiper. It's a B50K pot so I guessed a linear 50 kilohm pot. I ordered 5 and should get them in a few days. I'll keep you guys posted. Thanks for the help

[g1]

Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.

[nosaj]

Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.

Good call, went completely past me on the safety part.
nosaj

[jomama]

Also, if it is the off/on switch, it is probably carrying the line voltage. To repair it is not really safe. It needs to be replaced with an appropriate AC voltage rated switch of same type. Or a regular on/off AC switch could be installed separately from the volume control.

What's this now, did I order the wrong pots? It's a 10W amp so what's the right pot? When the pot was turned down, even before the off position, I could get no sound from it so the 50K value must reduce the voltage to an insignificant level. Please explain the potential danger.

[rjb]

Most guitar amps have an AC power switch and a separate volume control pot.
Yours uses a volume pot with a built-in in power switch, which is more common in consumer products like radios.
Some folks here missed the fact that yours is a switched pot.
For example, that "how to repair a pot" link was for a "regular" pot without a power switch.
The potential danger is that a compromised pot with AC power switch would be more dangerous than a compromised pot that doesn't get anywhere near AC power.

Free associating: I have a little (supposedly) 10W amp that runs on (6)AA batteries or a 12VDC adaptor. Never mind; I see it has a separate power switch.
So anyways, you need to be sure to order a potentiometer with on/off switch.

-rb

[jomama]

I think I understand. If it's an on/off pot and it fails short, the full power feeds through the amp. A pot not rated for the power used is more likely to fail. Does a separate power switch on the amp free me from this safety concern?

[g1]

Yes. A separate power switch would allow you to use a standard type volume pot (no switch).
It may even reduce the hum due to being further away from the volume control.
But safety is a big concern here. If you do not know exactly what you are doing, it is best left to a technician.

[jomama]

I did admit I was new at this stuff but I'm a pretty fast learner. This amp has low voltage caps and a separate power on switch. I'm only replacing a pot on a disconnected circuit board so can you be a little more specific about where the danger is?

[rjb]

The safety concern was that if you have a bad AC power switch (which we assumed was built into the volume pot) you might get arcin' and sparkin' and udder bad tings when you plug the amp into a wall socket.

In the first post, you said you have an "on/off volume pot" with a "click point". If it has a "click point", I assume that it contains a switch. If the switch isn't a power switch, then what are the switch terminals connected to? Maybe the manufacturer used a switched pot just because they had them in stock for other products? Or when you say the pot goes past the "click point" do you mean it turns beyond the original stop point?

I tried to find a manual (or just a picture) online, but there are several 10W models & I haven't found a manual for any of them.
If the pot does have a switch, what did the switch do before you sprayed the innards with alcohol?

-rb

[J M Fahey]

It most certainly is a Power On/Off control ..... what else could it be?
So it carries mains voltage on 2 terminals, and internal 2 switch contacts, one of which rotates under pressure and touches or not the other, it also has a small spring so it snaps one way or the other (that's why it clicks mechanically):
All of those functional parts are small bits of metal pressed into holes punched in a phenolic paper disk, not exactly the most strong or stable assembly technology available.

The point is that switch parts, which carry mains voltage, may stay in their place for a long time if not messed with, but if pot was dissassembled (it was) they may very well become loose and touch pot case, which is connected to chassis, which is connected to end user (you).

So the sensible thing is to replace switched pot with a new one or even better a regular pot plus separate mains switch (safely mounted of course).