Musicman 2475-130 SS driver board troubleshooting.

[hiegdk]

Hey all, I picked up a really cheap Musicman 2x12 combo that needed some work. At first it had a wicked loud hum that seems to have been resolved by replacing all of the electrolytics (minus the two 450V caps in the doghouse which I plan to do later), the 2N6488's (JE1692 on the schematic) and the LM1458. Now the hum is gone but there seems to be an issue with the driver board and I'm not familiar with this type of design at all.

My amp is the first circuit in this doc. the one with the transistor based phase inverter (I guess that's what that is): https://el34world.com/charts/Schematics/files/Musicman/Musicman_2475_130_2275_130.pdf

Current issue is that the amp seems very quiet and distorts very quickly. When following the transistor bias calibration instructions from the Musicman service bulletin (attached) it seems like one of the transistors has no voltage at all at the emitter. I'm able to read 25mV across one of the 3.9 ohm resistors as described, but the other reads 0V regardless of trimpot adjustment. I'm wondering if someone could point me in the right direction for tracking down any possible issues in the driver section that would cause that behaviour - or any suggestions for additional troubleshooting steps.


EDIT: I'm a dummy. Turns out it helps to actually solder the transistor to the board... Works fine now. 

[Latole]

 3.9 ohms resistor are on the emitter of Q1 an Q2 (which are phase inverter ? )

For now, I can't help you