Beta Lead A channel barely works but B channel works great..

[ChagaAlaska]

Hello! New to the forum and new to owning a Sunn.

Just received my Beta Lead and the A channel barely barely works. Drive, bass,treb, mid, level all work but it’s just so quite.. I can talk louder than level and main volume cranked for that channel.
B channel works great and is extremely loud. While using both channels I tried to adjust channel A drive and level and could not hear a difference.

The head is plugged into a Marshall 4x12 8ohm cab

The channel isn’t dead so there must be something I can do.

I play in a Funeral Doom band here in Alaska and amp techs are non existing.

Thanks for you time! Any feedback would be greatly appreciated.

[loudthud]

On the back of the amp there are three effects loops. One for each channel and one Master (effects both channels). you want the one for channel A. These jacks should be marked "To Accessory Channel A" and "From Accessory Channel A".  Connect a patch cord or a known working guitar cord from the "To" to the "From". If this restores the function on Channel A, a squirt of contact cleaner (such as DeOxit brand) into the "From" jack should fix the amp. If not, you need someone that can troubleshoot down to the component level with an oscilloscope to repair the amp.

Edit: Changed the jack that might need the squirt of contact cleaner to "From". That's the one with the contact that completes the signal path when no effect is used.

[evilroot]

loudthud's advice is a really solid start.  Given I'm working on a nearly identical unit right now I might be able to help a bit.

Like he said if it's more than contacts things get more complicated.  But I have more questions . . .

Are you comfortable with taking it apart physically?  Can you or do you know someone that can desolder and solder components?  Do you have access to and know how to use (or know someone who does) a quality digital multimeter?

[ChagaAlaska]

Loudthud,

Just tried that and the A channel sounded the same, but I’m still going to try some cleaner.

Evilroot,

Yes I’m ok with opening the amp up. There is rumor of a guy that lives here that used to work in Portland building PA’s. I’ll try and track him down. What do you think I should tell him to look at?

Thanks you two for the quick response. Internet is hard to come by up here.