Fender Prosonic Arcing/Buzz Problem

[Westrek]

I've had a Fender Prosonic head on my bench for awhile trying to figure out the source of a random arcing/buzzing problem. The issue is intermittent and might not happen for many hours of operation and out of the blue, there it is for less than a second duration. I can make it happen by changing the rectifier switch and standby switch. I've been through this amp with a fine tooth comb and can find no bad tubes, solder connections, capacitors, ect. The rectifier switch is a low cost Lorlin plastic rotary switch and I suspect it is the likely suspect. Any others run into this? Video and schematic attached. Thanks, Mike

[Westrek]

[vintagekiki]

Check soldering on SW304d.
When the amp switched to class A, cathode current V5, V6 passes through the R108 and SW304d, and if the contacts inside SW304d dirty, periodical transients occur.
Maybe the solution is to replace the rectifier switch.

[The Dude]

I get that you were trying to show the noise, but I wouldn't use the rectifier switch unless the amp is in standby or off.
Does the amp make this noise only when using switches, or will it produce the noise just sitting there operating?

[drewl]

First of all I'd use a bulb limiter to prevent damage if you aren't using one.

Can you duplicate it by banging on it?
Turn lights off, look for arcing.

Last amp I had that did that had a bad power trans, not that you do.

[nosaj]

I get that you were trying to show the noise, but I wouldn't use the rectifier switch unless the amp is in standby or off.
Does the amp make this noise only when using switches, or will it produce the noise just sitting there operating?

A thousand times do not do this unless you want to kill the amp.!!!

nosaj

[Westrek]

I thought the power transformer could be a possibility. No amount of banging will induce the problem. I’ve checked for visible arcing in a dark room and see nothing. I have had the amp on for 4 hours 2 days in a row with no noise. The third day about 20 minutes in, there it was. Just a burst, less than a second. I thought initially that the standby or power switch could be the culprit. Eliminated them by swapping in new switches. I’ve chopsticked and wire wiggled this thing to death. Thank for your responses.

[nosaj]

Replace the rectifier switch

jrfrond replied to Fender Prosonic Cutting in & Out after several minutes of warm up
in Maintenance, Troubleshooting & Repair
Another problem that I've seen a couple of times in the ProSonic that can cause all kinds of problems is the Rectifier Select rotary switch on the rear of the amp. The contacts burn open and become intermittent.

[g1]

For test purposes you could solder jumpers across the appropriate rectifier switch connections.

[The Dude]

I'm either missing something or at a loss as to why they used a 3 position switch for the rectifier switch. The schematic shows 2 pins of each side of the switch tied together, so there are actually only 2 options. So, can anyone tell me if A) I missed something, or B) It's only 2 options on a 3 position switch. It seems to me it's either tube or SS rectifier and that's it.

[Westrek]

Another short video. Fender no longer supplies the rectifier switch, but it's a Lorlin switch most likely available from the usual suppliers.

[nosaj]

There are 3 settings one is SS and 2 different tube options

• solid state rectifier, class A/B 60w.
• tube rectifier, class A/B 50w.
• tube rectifier, class A 30w.

nosaj

[The Dude]

Oh, I see it now. There's another part of the switch in the output tube cathode circuit that I missed initially. Thanks for the clarification.

[Westrek]

Just an update: I found a Lorlin rectifier switch and installed it today. No change. Any suggestions on diagnosing this problem? Would it be unusual for one of the filter caps to create a problem like this?

[The Dude]

I just skim read the thread. I may have missed it, but I don't see that you tried different tubes. This could be just a tube arcing intermittently.