Exploding Pot

[lawson-stone]

On one of my several old amps, one of the pots literally seems to explode. I'm not talking about scratchy. I mean at any volume, you turn that pot and it's like a thunder-clap explosion.

It has done this since I acquired the amp, so I don't know the history on it. But I'm wondering if this is more than dirty contacts, maybe I need to replace the whole pot?

[Little Jay]

I suspect a loose connection of one of the wires attached to the pot, that would make a loud bang.

[Franz 1997]

I had the same thing on one amp. Replacement solved it - it was probably an intermittent break in the carbon track. Switch cleaner didn't work.

[Archie]

Pheww I was worried my joint was gonna blow up for a second there

[helios]

Pheww I was worried my joint was gonna blow up for a second there

I'm in WA state, and out here pot is exploding (not that I would know from personal experience)!

[citizenk74]

You hafta take the seeds out...so I've heard....

[jazzbow]

You hafta take the seeds out...so I've heard....

Lol.

I thought Red Rock Burns was a Psychedelic Guitar player from Colorado!

[lawson-stone]

You guys are not going to believe this, but the humor you're finding in my title was absolutely not in my mind when I wrote it. I'm sitting here spitting coffee out I'm laughing, it just didn't occur to me "exploding pot" could mean something other than very badly shorting out potentiometers.

I have lived way, way too clean a life, and at 61 it's too late to change!

[Klatu]

[icr]

Significant DC potential to ground on the pot can cause that. Most common cause in guitar amps is a coupling cap that leaks DC.

You might find something like this. Picture of a capacitor I put in my Gretsch amplifier when I had re-built it in 2005.
I had just finished restoring my EH-125 and the Gretsch was looking a little shabby, so I recently had the chassis out just for cleaning. Might be hard to tell but the capacitor is discolored and there is a hole in it. The capacitor is rated at 600v but the the plate voltage for this capacitor was only 200v. This is one reason why I only replace capacitors that fail and don't like removing functioning capacitors from any device. Frequently the original components are better. For example the original capacitor in this amp lasted 40 years. This one lasted only 10 years.

[RonD]

Pheww I was worried my joint was gonna blow up for a second there

I ruined more than one tie-dyed t-shirt in the late 60's...
Just sayin...

[SamBooka]

Significant DC potential to ground on the pot can cause that. Most common cause in guitar amps is a coupling cap that leaks DC. .

that was the first thing that came to my mind as well. Scratchy pots are one thing but OPs description doesn't sound like that.

[Greentone]

Sounds like a ground issue. Happens a lot with input jacks on old amps when the chassis nut comes loose.

[GNAPPI]

I bought an early SF super reverb recently that the volume went from zero to 10 without touching it. Sprayed it out with pot cleaner, ran it up and down with the amp off a few dozen times, re-sprayed, problem gone.

[Little Jay]

I still suspect a broken solder joint somewhere around the pot.

[AlohaJoe]

I have lived way, way too clean a life, and at 61 it's too late to change!

Oh no my friend, t's never too late...

[lawson-stone]

You all have given me some good leads. This is a Yamaha G100 210. It has killer tone, and I like the controls, but channel 2's parametric EQ pot that controls the "Q" is the culprit. There is some damage on another knob that makes me think it might have fallen over some time in the past. Anyhow, since it's not my primary amp, I think I'll take a chance and open it up to see what's in there and whether it looks like something I can fix.

[icr]

Ok that is a transistor amp, yes? So not likely a leaking coupling capacitor. Try deoxit and see if that helps.

[lawson-stone]

Yes it's a transistor amp. I have the service manual for it, but of course those are written for people who pretty much know already what they're doing. I think I am up to taking things apart, having a look, and spraying cleaning stuff where needed. Some soldering probably I could manage. Channel 1 is fine, so worst case would be to put it all back together and live without the richer EQ of channel 2, but I'd like to have the full functions if possible.

Will report back!

[Little Jay]

Those pots are probably soldered directly to the print. I can imagine if the amp fell over or something hit the knobs a fracture or crack in some soldering connections might have occured. The slightest movement of the fractured parts (like turning the knob) could be audible even giving a loud pop or bang.

It's very hard to detect a crack like that, I would just reflow every joint around the pot. Of course clean and spray the pot as well while you're in there.

[GNAPPI]

LJ, Some of the time you're right, but just re-flowing the solder assumed the solder broke free from the land. In a case of a cool solder joint that broke free this will fix it.

More often on an impact the land around the pin cracks necessitating a repair to the printed circuit board itself. The larger the device (like a tube or cap) the more likely the PCB land itself is cracked. Also the most violent strikes to small parts will also likely crack the PCB. Reflowing near a crack without a PCB repair will not be a permanent fix (even if it does temporarily work) because the resist etch will "resist" a jump of solder across the crack.

The image below illustrates it.

1. Scrape off the resist etch coating for a distance as long as possible no more than say 1/4" (Image 2)
2a. Reflow solder over the exposed land area (Not the best repair) (Image 3)
2b. Lie a thin piece of copper solder wick on the land and flow solder onto it and the PCB land. (also Image 3)

[Little Jay]

Yes, if the PCB is broken, then there's a bigger problem.