I'm working on a 140 head that seems to distort too soon (playing a standard strat with passive pickups with amp volume about 1/3 of the way up). I got the 150 service manual and ran some tests using a function generator and scope. The preamp cards seem fine. No distortion, and all specified voltages check out. Following the signal to the power amp section, here is what I found compared to what the service manual says:
Conditions: No load, 1KHz sine wave input
To reach final output of 50Vpp (which is not distorted), the manual says R301 or R302 (depending on which channel you are driving), should be driven with .54Vpp. However, I have to drive them with 1Vpp. This produces the specified voltage of 425mVpp at the base and emitter of Q301.
As for Q302, the manual says the collector should be 0.6Vpp. However, I am seeing 1.4Vpp. Also, the waveform shape isn't quite a sine wave. It looks rounded off - not clipped, but it's rounder than a normal sine wave. What's strange is that the final output at the speaker jack -does- look like a normal sine wave, like it got fixed somehow in the rest of the circuitry.
Also, when I turn on the amp, the final output starts off with a 10V DC offset which slowly decreases over time down to about 2.5VDC. Could this be that C311, the 1000uF electrolytic cap after the output transistors, might be bad & allowing DC to pass?
When I put a 4-ohm load on the output, I see a little bit of crossover distortion on the output waveform, but it still has the normal sine wave shape and is not clipped.