Fender Bassman 200

[skip2011]

I'm currently working on a bassman 200 that has a loud pop when turned on and an even louder pop when turned off. I have a diagram for it and every thing voltage wise check except at Q 1 and Q 2. Any suggestions?

[52 Bill]

I'm currently working on a bassman 200 that has a loud pop when turned on and an even louder pop when turned off. I have a diagram for it and every thing voltage wise check except at Q 1 and Q 2. Any suggestions?

Are you talking about Q1 and Q2 of the power amp or the preamp?

In the power amp Q1 and Q2 are part of the turn on delay that will reduce some of this noise. What problems do you have with the voltages?

If you plug a cable into the FX return jack and power on/off the amp is the pop louder, softer or the same?

[skip2011]

Thanks Bill. The popping is about the same. The voltage on the bass of Q1 is supposed to be -45mv but reads .647v. The s0urc voltage is right on the money at -10v.

[52 Bill]

I assume that the 0.6 volts is negative? Not much there to test.

I have no info on the thermistor, so can't help there. I'd probably test C13 as it's part of the timing circuit.

[g1]

How about shorting C14 and see if the pop is still there?

[skip2011]

No the voltage is positive and some thing else puzzles me, the emitter of q1 is supposed to got to ground. I think it is chassis ground but I'm not sure. Kind of afraid to shot it to ground for fear of causing more damage but I think I'm going to try that. Thanks guys for the help and I'll let you know what happens.

[g1]

Shorting C14 simulates turn on mute. When you first turn on the amp, it takes a split second (or whatever) for C14 to charge up as it is like a short when discharged. So no worry about shorting it to test.
If the pop is still there with C14 shorted, it takes the whole mute circuit out of the equation.

As far as Q1, there should be no way for any positive DC to be there.