Mesa Mark III producing no sound

[SSheehan925]

I have a late 80's Mesa Mark III purple stripe combo that I run through a 4x12 rectifier cabinet. Haven't gotten around to addressing this problem until recently, but the amp no long produces sound through any of the three channels. Have tried a different combination of guitars and cables and still nothing.

Also plugged my guitar into the effects return jack and it produced a clean tone. However, I recently changed the preamp tubes since this was my suspicion, and still nothing. I had assumed that the power section is fine since sound was produced through FX return. Any idea on what the problem could be?

 

[nor]

Bad input jack?

 

[MrMarkIII]

Replacing all the pre-amp tubes at once is no help, as you could very well have stuck a bad tube in the same place as the tube that was causing the original problem.
Start by replacing V1 (the 12AX7 closest to the input jack) with a 12AX7 *KNOWN* to be good.
If this fixes the problem, then you're done.
If not, proceed to V2.
Leave the 12AX7 *KNOWN* to be good in the V1 position.
Rinse and repeat. One-Tube-At-A-Time.
Otherwise you have no idea which tube is bad.
By the way, as the manual states, V3 is the Lead Channel tube, so usually only causes trouble when you engage the Lead Channel.
No sound from anywhere is most likely V1, because that's where the sound starts.
Also check the effects loop by plugging a cable between the SEND and RETURN jacks. Wiggle it around. These jacks are normally closed when not in use, but can become loose, etc. Do not clean them with anything other than De-Oxit.

 

[McBarry]

Agreed with above and I'd add V2b is after the FX loop, so pretty unlikely V2a is defunct but 2b is OK - even more weight to check out V1.

If it tests OK (either V1 functions in another amp, or a known good tube functions in your amp), the only option is to go inside - it's tech time.

Let us know what transpires - good info for future readers also...
DB

 

[mathlete]

Thought I'd give this thread a bump to see what the ultimate diagnosis was. I've got a purple stripe head that's doing exactly what the OP described in the initial post, and my tubes are checking out to be fine. I can get a clean guitar tone by plugging directly into the return jack, but no other sound comes out.

I assume I'm looking at taking it in to have a tech look at it, but I'm hoping to get at least some idea of what the problem may be.

Thanks!

Jason