PM1000 Channel Gain Switch Problem

[brendandwyer]

So i'm attaching the schematic for the PM1k input channel for reference.

I'm racking two channels and on the second channel, there is only one gain switch position that passes audio.  I'm terrible at understanding gain switch / multi deck stuff so i'm stuck.

I should add that i have recapped both channels, and done the Neve eq mod with WIMA capacitors.

Can anyone suggest a course of action?  I would trace the signal and have tried, but because i don't understand the switch i don't really understand the path.

Thanks,

 

[Khron]

So, which position DOES pass audio, then?

 

[brendandwyer]

+4, -2, -8, -14, -20 all pass audio.  Nothing after that

 

[brendandwyer]

I sorted it out.  One leg of R6 had a busted pad, but the real culprit was my oversight in populating C8 10uF / 35v cap.

Thank you for your help though

 

[Khron]

Well, there's not a helluva lot to understand about those switches, apart from the thing that all the wipers move at the same time, all together.

If the first six positions all pass audio, but the other six don't, let's look at the schematic you posted.

In the first two "layers" that are right at the input (S2a & S2b), the last seven positions are connected together, so that can't be it.

Third "layer" (S2c) is a "maybe". First six positions are all connected together, 7 and 8 are commoned and go through R21 (56k) and so on. But that pair and the remaining four don't really have anything in common, right?

Which takes us to the fourth and last "layer" (S2d). The +4 to -20 all bypass the preamp, and they're just a padded-down version of the input signal (the "bottom" terminal of R28 goes to ground). On the other hand, the last six switch positions DO all have something in common this time - the signal goes through the preamp.

An oscilloscope (or worst case, a signal tracer of some sort) would come in real handy here.

Coupling cap(s) crapped out (C1 or C8)? Or some of the silicon gave up the ghost?

[Later edit]

Well, there you go then

 

[brendandwyer]

Thank you for that, its helped me start to understand at least a little bit.  I didn't know that +4 through -20 is just a pad.

 

[Khron]

No probs. For a while, i kinda wondered why that signal up top was labeled "Preamp (bypass)", but once i saw that the "bottom" of R28 went to ground and that signal line came from before the preamp...

https://en.wikipedia.org/wiki/Voltage_divider (just made up of three resistors, not just two)

Thank you for that, its helped me start to understand at least a little bit.  I didn't know that +4 through -20 is just a pad.