Mark IIC+ channel switch issue

[Ratou]

Hey guys,

Seems like there is an issue with my IIC+ (early DRGX) where it stays stuck on the clean channel, both when using the push/pull pot on the amp and the footswitch. The red light on the footswitch is very very faint when activated (tried with 2 different Mesa footswitches to make sure it wasn't the switch). I've tried replacing all pre-amp tubes one by one but did absolutely no change to the sound. Amps sounds loud and clear which also leads me to think there's no issue with tubes but more with the channel switching itself. Are there any easy checks or fixes I can perform to solve this or identify the issue further? Any help is appreciated! Thank you

 

[Eevil]

Hey guys,

Seems like there is an issue with my IIC+ (early DRGX) where it stays stuck on the clean channel, both when using the push/pull pot on the amp and the footswitch. The red light on the footswitch is very very faint when activated (tried with 2 different Mesa footswitches to make sure it wasn't the switch). I've tried replacing all pre-amp tubes one by one but did absolutely no change to the sound. Amps sounds loud and clear which also leads me to think there's no issue with tubes but more with the channel switching itself. Are there any easy checks or fixes I can perform to solve this or identify the issue further? Any help is appreciated! Thank you

Howdy.

My first bet would be the channel switching LDRs are gone. I had some channel leakage probs in Studio preamp which is closely related to Mark II. The LDRs will fail eventually

 

[Ratou]

Thanks for your reply. Understood - any easy way to check this on my own? Otherwise I guess it means a trip to the amp tech...

 

[Eevil]

Thanks for your reply. Understood - any easy way to check this on my own? Otherwise I guess it means a trip to the amp tech...

Hum. I am not a tech so no (safe) ideas about how to check :shock:

I’d drop it to proper tech who works willingly with Mesa amps.. I’ve heard complaints from every “tech” my amps have been about complexity and how the Plexi is the only right amp in the world :mrgreen: and usually issues are still there :lol:

 

[Boogiebabies]

The last time I saw this issue it was the 680 Ohm 2 watt
resistor and the 470uf 16V power supply capacitor.
They are located at the end (V4) side of the preamp
board under the edge of the power supply board.
Squeeze the tips of the power supply stand offs and move
the power supply out of the way. It should just fold back.
Don’t move it too much or the power transformer leads
will snap. You can visually inspect the 680 Ohm resistor
for damage. They get black from overheating and split
in the center. You can also check it for resistance, but they
can test normal when cold and way off when hot.
Also check the 470uf/16V LDR supply capacitor.
See if it is leaking or if the bottom has mushroomed.
If you have a good meter check it for capacitance.

I hope this will fix it because it’s a nightmare to change LDR 2,3 and 4. You have to take out a bunch of parts to repair them properly. They sit under Master 1, Lead Drive and the Lead Master and it’s seriously tight.

 

[Ratou]

Thanks a lot for the very detailed answer!! I will have a look inside the amp and follow your instructions and revert. Cheers!