Gibson G100A amp Harmonic Multiplier / Preamp / Schematic

[awdman]

I am working on my G100A that is doing well after changing out Q2. well I thought I would check the rest of the unit out to make sure it is working good, If someone could help me out on the Harmonic Multiplier, I am not sure what it does. I changed the photocell because the face looked bad. Turned R6 and it did not change my sound but the bulb gets brighter so I know the circuit is doing something.

2. Since I do not have an oscilloscope I have been using a signal tracer to troubleshoot and that is how I found the bad Q2, I traced out how I think the signal goes, is this correct?

3. The pre-amp is working but my question is I see the input at pin 3 but do not understand where the output gets fed back to the circuit.

See attachment

Thanks,
Anthony

[teemuk]

The harmonic multiplier is just a diode clipper. Whenever signal at it's input exceeds the forward voltage of the diodes (ca. +-600mV), the diodes turn conductive and limit the signal amplitude.

The transistor Q3 controls the luminosity of photocell's lamp L1, which succeedingly controls the resistance of C1. This is series resistance to clipping diodes and effectively controls how much they clip. e.g. infinite resitance=no clipping, zero resistance=full clipping.

Input at pin 3 of IC1 is amplified and the output signal appears at pin 6. It's fed to the spring reverb tank. The output of the reverb is picked up and amplified by Q6. Amplified output signal is taken from its collector, fed to level control trimmer and potentiometer and finally mixed with the dry signal by applying it to the base of Q4. The FET Q7 is just a switch to mute the reverb.

[awdman]

So if the harmonic balancer is not working, I should change the 2 diodes? and how does the preamp work if I do not have the reverb tank hooked up or is it part of the reverb circuit and if it is when I turn p1 down which controls input to ic1 it mutes the whole system?

Thanks,
Anthony

[teemuk]

If the harmonic multiplier doesn't work it could be the diodes but it could as well be the photocell or the transistor Q3 or one of its base circuit's components. You can always take a multimeter and measure what's wrong.

The preamp works even if you don't have the reverb tank hooked up but the reverb feature naturally won't work.

P1 is the volume control. Yes, it mutes the whole system since signal input to both reverb circuit and following preamp stages is taken from its wiper (we can practically ignore capacitor C6). When you turn the control all the way down counter clockwise you effectively ground the wiper and hence mute the signal.