Mark III owner - troubleshooting previous owner mods

[sahlomonic]

I've had my black dot Mark III for about a year now. I got this in trade for a 6505+. I was told that it had been modded a little bit, as in R2 having a gain *reduction* and what appears to be an attempt at the R2 volume mod. I see on the PCB there are some burn marks, missing components, and it appears some other caps have been replaced for ones with different values.

It sounds incredible on the Lead channel, but for being 30 years old it is a bit temperamental and isn't really consistent on how it sounds. Anyway, could I get some guidance to what might actually have been modified on this amp and what can be done to get it where it should be if needed? I do want a correct R2 volume mod (the knob does work, but to me it's more of a gain control than a volume control), and I eventually do the III+ mod (which I have searched for on this forum already).

I outlined the previous owners R2 mod, which is inconsistent from what I've read on this forum.

 

[BlackBoxy]

hi,

as you can see here http://www.classictubeamps.com/schematics/Mesa/Mesa%20Boogie%20MKIII%20schematic.pdf

nothing looks value changed on low suply circuit, the burned resistor made a lil black tain around but was replaced with lil modified circuit (course all burned)

the only thing to do on this gear would be changing big blue caps on power high tension, date on it is 1985, Mesa parts are exact drop in remplacement !

i wouldn't touch film and ceramic or mika caps (orange etc...) because its life time part!

bring it to a tech to restauration works or some mods !

P,

 

[McBarry]

Hi, couple observation..
screen resistors all changes on power sockets,
couple series resistors across rear jacks - def don't look original,
that bias section work would cause me to lift the preamp board to check underneath and repair if needed. Yes, a pain, but a definitive repair is now possible.
Also I'd be changing both adjacent electro's - they would've gotten bloody hot and replacement is cheap insurance..(the 220/63 and the purple - bias from memory..)..
Soldering on the ..52B.. diode next to new resistor looks a little uneven - reflow prolly suffice..
The (presumably) R2 pot looks a little cheapish - worth a squirt of cleaner, or maybe replace for quality.

Re "temperamental".... tons of possibilities...
If U can narrow the symptoms AND curcumstances when they occur then a more helpful direction might be gained for any advice..

All the best, Dave B