Fender M-80 Chorus Bias/Avalanche

[ilyaa]

hey!

got one of these fuzzy carpet amps on the bench -

R155 and Q12 were blown - had -40 VDC on the left output (yowza!)

i replaced R155, Q12 and Q13, and then did the bias mod for good measure (swapped out R127, 129,154 and 156 with 2K resistors) -

the left side had some funky looking electrolytics so i replaced those, too, as well as Q10 and Q11 (just being thorough!)

now: amp turns on without blowing up, BUT if i measure bias across R165 or R166 i see it starts out okay (around 25 mV) and then starts to creep up, with no sign of stopping. i turn the amp off when it gets above 60 mV because i can tell itll keep going until kaboom.

any thoughts what might be the issue? the so-called avalanche diodes (the BYV 26Ds) measure OK but i have a feeling they are not, in fact, OK. however, i am not that familiar with this bias circuit so i am not sure best way to proceed -

thoughts/advice? thank you!

ilya

[Jazz P Bass]

Well you do have a working amp to compare voltage readings to.
First off, is R 165 cement resistor still good?

[ilyaa]

yeah the power resistors are all good -

all voltages read pretty much exactly the same both sides until the bias starts to crawl up

[ilyaa]

anyone have any thoughts??

[phatt]

Well you mentioned  some obscure bias mod,, and now the bias has a problem.
Hum,,, logic says return bias to stock and see if it remains stable.

I have no idea of this bias mod but be I'd careful with mod claims unless it has been verified by many.
Phil.

[ilyaa]

this bias mod is in a fender service bulletin -

its in the schematic i attached!

and i have reason to believe the original issue (a blown output) was due to the overheating/runaway problems associated with the unmodded bias circuit