Mark !! (A) Problems - Noisy Master Volume & EQ Not Working

[StevieB]

I'm the original owner of an early serial number Mark II (A).

I would appreciate some help troubleshooting a few issues that have just developed.

(1) The amp is very quiet when the Master Volume is at 2 1/2 or lower (even when Volume 1 is at 10. However when the Master Volume is turned to 3 or higher there is a loud hum. The loud hum is present even when Volume 1 is lowered to 4 or even less. I have flipped the ground switch and also changed from 60 watt @ 4 ohm to 100 watt @ 8 ohm for my single EVM 12 speaker, but the loud hum is still present. The ONLY time the loud hum stops is when I pull or kick in the Lead Drive switch. Weird! Any ideas!

(2) My built in EQ is not working. It doesn't work when I turn it on via the switch on the front panel or when I use the dual foot switch (EQ/Gain). Any ideas on what the problem might be?

Thanks in advance.

 

[dodger916]

Assuming you have not tried anything, the simplest and easiest way to start is with tubes. I had a similar problem with volume in my Mark IV and it turned out to be a dead power tube. Replace tubes with appropriate replacements. Also try tapping each preamp tube with something non-conductive, like a chop stick or pencil while the amp is on and not in standby. A bad preamp tube will sound bad through the speaker. You can also try replacing each preamp tube with one known to be good and see what happens. These might be long shots, but it's something you can easily do.

 

[Old BF Shred]

Makes me wonder if a power supply cap(s) is failing. Have you ever replaced any of them? The originals would be pretty old by now!

Good luck!