Intermittent buzz in reverb

[ksjazzguitar]

Hi,

I have have a problem with my 15-year old Polytone Mini-Brute. It will occasionally develop a buzz. When I switch polarity of the on-switch, it usually goes away. But then after 5-10 minutes it may come back, but if I switch the switch again, it goes away. It also goes away if I turn the reverb down to zero.

Background: I'm the original owner, but there is a lot of where and tear - I was a cruise ship musician for years so it has been shipped several times and it has been in a lot of salt air. I recently replaced the power transistors (with the help of this forum) and also replaced the reverb pan. But the problem existed before I replaced the reverb pan and I'm pretty sure before the transistors.

Any ideas?

Regards,
Kevin

[Enzo]

Every time you flop the switch, the SNAP sends a little sharp vibration through things. So we need to know if it is flipping power on and off briefly is the thing or if the vibration is the thing. Simply done. Next time it develops its buzz, don;t flip the switch, unplug the amp from teh wall and right back in. This will interrupt the power same as a switch, but without any physical vibration inthe amp. Any change? And alternatively, when it happens, leave the switch alone, but ball up your fist and whack the top of the amp. Does whacking the amp cause the buzz to start or stop? If so, that means a loose connection.

Now more specifically, since the reverb seems to be central, pull the rever pan and pull out each cable connector, then push it back in. We want to refresh the contact of the plugs inthe jacks of the reverb pan.

[ksjazzguitar]

...Next time it develops its buzz, don;t flip the switch, unplug the amp from teh wall and right back in....

The buzz stopped.

...And alternatively, when it happens, leave the switch alone, but ball up your fist and whack the top of the amp....

No effect. Other than a brief transient noise (from the reverb I assume), there was no change.

...Now more specifically, since the reverb seems to be central, pull the rever pan and pull out each cable connector, then push it back in. We want to refresh the contact of the plugs inthe jacks of the reverb pan.

I did it again to no effect. With a Polytone you need to disconnect the reverb every time you open up the back, so they have been pulled (and cleaned) several times.


A few new observations: There is a perpetual buzz in the reverb even when functioning "normally." It is so low that I don't even really notice it unless I'm paying attention. Forgive me, but this is the only amp that I've played in the last 15 years, so I don't know if it is normal or not. It has the same "sound" as the problematic buzz, whatever that means. Additionally, I noticed this time (and think that I've noticed it before) that the loud buzz is preceded by a slight ramp up in the background buzz. If we label the loud buzz as 100% and the background buzz as ~3%, then in the 4-5 seconds before the loud buzz, the background buzz ramps up to about 15% before jumping to the full 100% loud buzz. Again, dialing the reverb down to 0 makes it go away (it's proportional to the reverb setting) and swtiching the polarity (and now unplugging it) will make it go away temporarily.

Any ideas?

Regards,
Kevin

[ksjazzguitar]

No one? No suggestions?

[Enzo]

Well, something like this can be hard to fix remotely.

If killing the reverb kills the noise, we can assume the reverb circuit is the source of the noise. WHy power down/up stops it is hard to say. Possibly some unterminated point inthe circuit is allowing itself to charge up to come point. My tests were aimed at determining if the switch and/or its vibration was involved, but apparently it is not. And hitting the amp serves to expose loose connections and cracked solder, apprently none here. So we are left with electrical issues.

One thing to try is to unplug the drive end of the reverb pan. DOes it still do the hum thing? I realize there would no longer be reverb function.

[pdf64]

Sometimes the cables to reverb pans go bad, same as guitar leads. Depending on how the grounding to the transducers and pan chassis was arranged, it's possible that the screen on one of the cables is intermitant open circuit or high resistance.
Usually Enzo's fist of doom would serve to bring that out though.
With Fenders it can be quicker just to sub in a pair of hifi rca / phono cable to eliminate/identify this problem - is that possible on the polytone?