boogie mark iii issue

[jerboyd]

hello, i have a question regarding a boogie mark iii. the amp has been having a problem where after about 20 minutes of use it will all of a sudden make a fairly loud 60 cycle hum noise. the guitar is still slightly audible but it is quiet compared to the hum. it almost sounds like what happens if you take the input cable and touch it.

i have lots of experience working on traynor amps from the 60's and 70's so there are certain circuits in the mesa amps that are unfamiliar to me. when i heard the amp making the noise i thought that it must be an electrolytic leaking ac into the signal path so i checked the filter caps and the bias supply caps and none of them appeared to be damaged or failing. i did however find another cap that looked like it was bloated and the side of it was a little brown from heat due to it's proximity to a resistor. it was a 470uf 16v cap and after looking at the schematic it appears to be a filter for the LDR supply. the schematic makes me think that the channel volumes are controlled by LDR's. is this possible or am i missing something? if they are controlled by LDR's it would make sense that unfiltered power would cause the amp to put out a loud hum.

anyway, i replaced the cap and have gone through a couple rehearsals without any problems but i am curious to know if what i did actually fixed it and why. any insight on this circuit would be really helpful.

thanks

jeremy boyd

 

[Boogiebabies]

Check the two 100 Ohm resistors at the bottom right of the circuit board. These are for the heaters reference to ground. They should read 50 Ohms in parallel. They often get
taken out with power tube failures. They will be between two 39 Ohm resistors ( one on each side) and below four 1N4007 Diodes for the DC rectifier bridge. Just to the left of the first 30uf/500V filter cap.