Mark IIA reverb issue

Good evening! On my Mark IIA, when the Master 1 is pulled the reverb works fine... But when pushed in it hums. Also, when there is a patch cable plugged into the reverb foots witch underneath the chassis (and held in my hand) it works when Master 1 is plugged in. So I assume it has something to do with the jack, but it appears to be okay. Any suggestions?

AimSnapFail said:

Good evening! On my Mark IIA, when the Master 1 is pulled the reverb works fine... But when pushed in it hums. Also, when there is a patch cable plugged into the reverb foots witch underneath the chassis (and held in my hand) it works when Master 1 is plugged in. So I assume it has something to do with the jack, but it appears to be okay. Any suggestions?

Click to expand...

The MKIIA factory grounding scheme is a nightmare : at least 14 GND points randomly spreaded on the chassis... Just consider that the GND of the power stage was returned to the HV PSU via a simple parker screw and a spacer. When I discovered that, among other inconsistencies, I even wondered if they knew how to built an amp at MESA at that time...

You may also notice that the reverb driver transformer is placed near the power transformer, that induces hum in the reverb driver transformer. I displaced it at the other side of the chassis.

You can note that the filament circuit is imbalanced by the relay circuit : this generates hum also...

Redesigning the ground scheme of my MKIIA, chasing the hum everywhere... All that was a big job, but it is rewarding in the end : my amp is silent and quiet as I expected. In its original condition, it was a hum and buzz factory, like the MKIIB head that I had too (sold now).

I described all that in a thread called "Boogie MKIIA 60/100 Rev Eq"... http://forum.grailtone.com/viewtopic.php?f=27&t=54614

Good luck !

Try the simple ground jumper cable mod. Search the forum it in here!