Here's the full post from the "Mesa Boogie Mark lll Owners" FB Group:I modified it a bit, less sharp trebly-ness as most blue stripe amps are, replaced all electrolytes just because they were 30years+ of age......nothing fancy.
The main problem for mushy bottom is the adjustment of the pre-gain tone stack when playing in the rhythm and lead channel. There is an easy mod you can do to can fix this. Solder a resistor in series with a Vactrol opto isolator over the bass potentiometer - Vactrols became obsolete, but you can take the Tesla version, which is still available - choose the value of the resistor by measuring the value of the bass potentiometer at setting "3" (approx. 27k) - then take a 33k which gives you 29k parallel to the original bass potentiometer value of 250k. Connect the vactrol to the channel switching circuit, so it will lower the bass knob value range when you switch to the lead channel.....with this mod you can still enjoy the full and deep low flavor in the clean channel, and a less mushy and defined low end in the lead channel.
There were also a few small mods on the overdrive stage.....the 4th triode stage (or 2nd OD gain stage ) uses a 330k plate and 3.3k cathode resistor bypassed with a 220nF capacitor I also added a 250pF treble cut capacitor between plate and cathode. To cut down the treble a bit more, I rework the nfb line in the poweramp section.....there are a few other mods in there to improve the dynamic behavior, so i can turn down the volume on my guitar in the lead channel to almost clean without volume loss.
https://www.facebook.com/photo.php?fbid ... mO-CWUC1Vg
Demo of his modded Mark III Purple Stripe:
https://www.youtube.com/watch?v=reHVukN ... s_Z7AyT-Rg
I have a Simulclass Red Stripe. It would be helpful if anyone is able to give more detailed info on these mods. I've tried contacting the OP on FB but he's not responded. Thanks.