Hi all, I finally got around to fixing this issue in my MT15. I'm an amateur amp builder, and decided to risk it and just start digging myself. There are 2 x factory defects and 1 x design flaw that cause the hum, all due to the effects loop, and all very easy to fix. If you want you can take these instructions to a tech and they should be able to make the repairs quickly. They are as follows:
- The effects loop jacks are deffective from the factory, have them swapped with good jacks
- The switched tip of the effects loop send jacks isn't jumpered to the tip of the return jack on the PCB, fix this by soldering a piece of jumper wire to jumper the tip
- The ribbon cable for the effects loop PCB isn't shielded and is very close to the power section and bias cable of the amp, replace thhis with a shielded cable grounded at the main PCB end of the run
If you want, while a tech is doing this you could also ask them to shorten and twist the heater wire pairs for the power tubes, as they are pretty ugly from the factory. All up, it's about 15 minutes of work and a few bucks of parts, and it works like a charm. I had to double-check I didn't have my noise gate on when testing after doing the above repairs because the thing is whisper-quiet now.
I hope this helps!
- The effects loop jacks are deffective from the factory, have them swapped with good jacks
- The switched tip of the effects loop send jacks isn't jumpered to the tip of the return jack on the PCB, fix this by soldering a piece of jumper wire to jumper the tip
- The ribbon cable for the effects loop PCB isn't shielded and is very close to the power section and bias cable of the amp, replace thhis with a shielded cable grounded at the main PCB end of the run
If you want, while a tech is doing this you could also ask them to shorten and twist the heater wire pairs for the power tubes, as they are pretty ugly from the factory. All up, it's about 15 minutes of work and a few bucks of parts, and it works like a charm. I had to double-check I didn't have my noise gate on when testing after doing the above repairs because the thing is whisper-quiet now.
I hope this helps!
