Older Dual Rectifier - Channel Switching Problems - Rev. G

Monsta-Tone · Jul 4, 2013

I have an older, Rev. G or so, 2 channel Dual Rectifier in my shop.
It has a channel switching issue.
When the amp is first turned on, it will not switch channels.
After about 5 minutes of warming up, the amp will switch channels just fine.

I am guessing that it is the filter caps for the switching power supply, but was wondering if anyone here has had the same problem.
I don't have a scope anymore, I actually don't work on amps very much anymore (damn day job gets in the way), so any help would be greatly appreciated.
Thanks.

Tone1026 · Jul 10, 2013

Have you tried to contact Mesa?

Authorized Boogie · Jul 10, 2013

Check the switching octo-coupler - 6N139. If its socketed, just grab it with some pliers and give it a good wiggle - then re-seat. It could need replacing, but try scraping the contacts first. Oxidation/corrosion could be the reason, and after the 5 minutes its warmed up enough to make some contact.

If the supply was bad, nothing would be working, no ldr's etc - so there's a fair chance the info above could be it.

Monsta-Tone · Jul 11, 2013

Right on, thanks guys. Don't know why I didn't think of that.

The 6N139 looks brand new still, no oxidation. I scraped the legs anyway, but it didn't do any good.
I'll have to call tomorrow and order one.

Is there anything else that you can think of before I order parts? That way I only order once.

Monsta-Tone · Jul 27, 2013

Well....back to the drawing board. Changing the 6N139 didn't help.

Thanks for the idea though! Got anymore?
I noticed that the majority of the LDR's are the problematic ones.

Basically, the Red channel doesn't work for about 10 minutes. The LED's change when I try to change channels with both the footswitch and the switch on the back.
Red channel passes a tiny amount of signal until the amp warms up, then it screams!

elvis · Jul 27, 2013

I would guess a bad connection. Cold Solder joint, no Solder joint, something that wiggled loose. When it heats up it expands and makes the connection. Try wiggling stuff, or just hit everything with a soldering iron.