Polytone Mini Brute II Problem

[rdd82]

Hi all,

I just purchased a mid-90's Polytone Mini Brute II off of Ebay. It was described by a reputable seller as having no issues and the packing/shipping job was first rate, but something very odd is going on. The first time I powered it up and played, the amp sounded phenomenal. The next day I powered it up again, and this time the sound coming out of the amp when playing a note can only be described by me as half the note, half crackly/static-y noise. Furthermore, the volume goes in and out from note to note (playing at same dynamic level). There is no extraneous noise when not playing - just when playing a note.

I tried a different guitar and cable and the problem persisted. I have tried the volume level all the way from just below 0 to 5. EQ levels are all flat - even tried reverb and distortion on and off, but no dice.

The mildly perplexing thing is that the amp begins operating normally again after plugging my headphone into the headphone output, playing a bit, and removing the headphones - however, the problem always returns.

What do you guys think is going on?

Thanks

[Jazz P Bass]

It appears that you may have a dirty or corrupt headphone jack.
If it is an open frame jack you could try to clean it.
If it is an enclosed jack then you will have to replace it.

[rdd82]

Plugging in and removing my headphones is actually what temporarily FIXES the problem. It's when I'm not using the headphones (playing through the amp's speaker) that I hear crackling, static-like noise at "in and out" volume levels.

Very mysterious.

[52 Bill]

Not so mysterious. The jack has contacts that are normally closed when there are no headphones plugged in. These normally closed contacts connect the speaker with the power amp. If they should become oxidized or dirty they will cause the sound of the amp to drop in and out, as well as crackle.

Try spraying a little contact cleaner like DeOxit into the jack and then insert your headphones a few times to work the contacts open and closed. Then see if it is any better.

[rdd82]

Thanks Bill - just picked up some RadioShack brand deoxidizer and will give it a shot! Would it be beneficial to spray some of this stuff into the other jacks - the 2 inputs and the external speaker output?

[rdd82]

Hi Bill,

I'm happy to report that your suggestion worked brilliantly - so far so good at least! Please let me know if it makes sense for me to go ahead and treat the other jacks on my amp (2 instrument inputs, pedal input, extension speaker output, and main output) for the sake of just ensuring that there is not oxidation anywhere else.

Thanks very much for your help.

Ryan

[52 Bill]

While it wouldn't hurt, just remember that a little goes a long way. Unplug the amp, spray as little as possible into a jack and then insert a plug a few times to work it in.

The place where this is really needed is in input jacks, headphone jacks, power amp in jacks and in FX return jacks. Each of these will have normally closed connections that can interrupt or degrade the signal path.

[rdd82]

Bill - thanks again for your help. I treated all of the jacks and everything works/sounds perfectly when plugging the guitar straight into the amp.

Here's the new conundrum - if I plug in my effects processor (Digitech RP355) into the amp (guitar --> rp355 --> amp input because there's no fx loop in this amp) everything works fine; however, when I power everything down, remove the effects processor and go back to plugging straight into the amp, upon powering the amp back up it makes an extra loud pop (louder than usual) and the crackling/static noise when playing returns for a bit. I'll immediately power down, plug/unplug headphones a few times, then power back up and everything is fine again and the problem won't resurface.

It seems as if there must be something going on when switching back from including the effects processor in the signal path to just plugging the guitar directly into the amp. I'm getting a much better sound out of the amp without the processor anyways so this won't be an ongoing issue, but a bit perplexed as to why it happened.

[52 Bill]

I'm not sure, but I doubt that the FX processor has anything to do with the problem.

Try playing straight into the amp for a bit and then shut it off. Wait a few seconds and turn it back on. Do you get a loud pop and static?

Next play the amp for a while and shut it off. This time wait 5 minutes before you turn it back on. What happens this time.

[rdd82]

Hi Bill,

No pop or static when I turn off the amp and then turn on after a few seconds. No pop or static when I wait 5 minutes, either.

That being said, half of the time when I turn the amp OFF the act of flipping the power switch (looks like a Gibson pickup selector switch) I get a loud crackle and pop. Is that something that I need to be concerned about? How can I go about fixing it?

Thanks,

Ryan

[J M Fahey]

No big deal.
When you turn it off, you are chopping the 120V AC sinewave at a random point (there's no way you can synchronize on purpose with something that changes 60 times a second).
If you interrupt it at a low or zero voltage (that's called "zero crossing"), no spark possible; if you do it on a peak: a relatively big one: pop.
Manufacturers often add a capacitor or varistor across switch contacts to kill or reduce this, check the schematic: maybe it originally had some which was lost along the way; in that case replace it.
You need very high quality, UL approved caps there.