M5201 op amps in Marshall 8080

[pa911]

I have decided to take on another project, the revival of a non functioning 8080. The first and major problem found was that the previous owner had replaced the 5201's with tl072's. My question is a technical/theoretical one. My understanding of the 5201 is that the switch is binary, either on or off. So why then did they use different switching voltages for channel selection and reverb switching--12.5v and 7.5v respectively? Looking at another Marshall schematic for the vs65 shows it switches at 12.5v also.

[x-pro]

Activation of the 12.5V channel is delayed. Perhaps even smoothly, so as not to cause a click. This smoothness is provided by C42 in the collector of TR1.
I assume that this value of 12.5V is selected at the switching boundary. It is to reduce the impact of transients when switching this channel on/off (Shock noise level)-page 7.
https://panda-bg.com/datasheet/1346-...MITSUBISHI.pdf

[pa911]

Thanks, that makes sense.
Can I assume that switch noise on reverb is less detectable since IC7b is only running as a buffer and IC7a seems to do nothing.

[x-pro]

The reverb circuit is included in parallel in the signal line. Summing takes place on IC8A. The reverb does not break the signal line, it is only swept into it. So the turn-on noise is masked by the useful signal.
The other gain channels, have breaker circuits on controlled IC3 and IC5.

[pa911]

Could you answer one more question on this amp for me?
It's regarding the clean and overdrive signal paths. From the schematic, it looks like the input signal is split right after the coupling capacitor C1 into the clean and overdrive channels. Both signals are then fed into the two halves of IC1. The clean channel then goes through optional clipping through IC2, then through a tone stack, then onto IC5(5201). The overdrive signal goes through sw3 onto different gain states at IC3(5201), and from there onto the12ax7, through a tone stack , through IC4, then IC5. Control of IC3 and IC5 are tied.
So when IC3 and IC5 are on, it looks like the overdrive signal is mixed with the clean signal and sent on to IC6 then IC8.
But when IC3 and IC5 are off, How does the clean signal get through?
I also don't understand what it means for the break in the trace after R4 and between the two halves of IC7.

[g1]

But when IC3 and IC5 are off, How does the clean signal get through?
I also don't understand what it means for the break in the trace after R4 and between the two halves of IC7.

The 5201 IC's are not 'on/off' but either A or B, depending on the voltage at the control pin (see datasheet in post #2). It's easiest to see at IC3 where both sides are close together. When A is on, B is off, and vice-versa.
The break in the line coming off R4 looks like it's just a drawing error, the break should be connected.

[pa911]

Thanks, this clarifies the schematic. And now to tackle the amp.