Marshall VS65R signal path help?

[Jazz P Bass]

You have to find where the problem Starts.

[allante666]

Yes, and thats where I need help with the signal path. Its all very well putting a tone in from a signal generator but it comes out the other end still as a clean tone. very low volume but clean. Its when I put a guitar type signal in that it comes out totally distorted and broken up.
Im guessing I would be better off inputting a guitar type signal and trying to pick it up at different parts of the signal path to see where it starts to get broken and distorted but im not sure where are the best places to look for it? Can you suggest particular points on the schematic to look?

[g1]

I don't think its a problem with test tone vs guitar signal, rather just a problem of signal levels.
If you don't know what to expect anywhere, you may get signal and think it's ok, but it could be 10 times less than required.
Use your meter on AC range. Set it up so you have 100mV going into the return jack. Post the AC voltages you get at IC7 pin1, pin7, then further into the power amp like T4 base, T4 collector, and at output jack.

[allante666]

Youre right it is indeed a level thing, its just difficult to hear any distortion on the tone but easy on the guitar signal. And yes it probably is 10 times less than it should be. OK, My signal generator is only a cheap and cheerful thing its only got two settings, AF or RF! I can however measure what it puts out in mV so we have a comparison. So measuring the AC voltages at the IC you would use the signal ground (pin 5) not chassis yes?

[g1]

Yes. Use signal ground as the reference for all your readings. Sometimes the connection to chassis gets lost or degraded, especially when things are partially or fully disassembled. Signal ground will still give a good reference.

[allante666]

ok thanks ill do that and get back to you. I think I did check signal ground and chassis ground and there was about 15 ohms between them.

[allante666]

Forgot to ask g1, When I take these measurements, what settings should I have on the amp? Full up, ful down, halfway etc?

[g1]

For the test I suggested in post #18, you're plugging in to the return jack, so no controls should affect it, except fx mix, which should be set to full.
For now, this will just test the power amp section.

[allante666]

You did say that, sorry. was thinking you had said the input jack.

[allante666]

ok, Ive taken the measurements suggested by g1. My generator signal is 120mV and I input it into the fx return. With my meter set to AC The readings I got on IC7 pin 1 was 1.48vac and on pin 7 was 880mVac. Where do I go from here? Do bear in mind that I did swap out the main power amp board for a working one so I ruled all of that out as it had the same results. However, IC7 and associated bits arent on that board they are on the preamp board. In other words everything the other side of CON1 seems to be ok?

[g1]

So when you are set up like that, what do you measure at the speaker out jack?

Maybe the power amp board is ok, but you still don't know if the proper signal is getting through to it via connectors.
If you don't hear a good signal when plugging into the return jack, then the problem (or at least one problem) is after that jack.

[allante666]

At the speaker output all Im getting is 4mV and thats with or without the generator running.

[g1]

Ok, same conditions that gave you 1.5VAC at IC7 in post #25.
What do you measure on either side of R1? How about at T4 base?

[allante666]

With the same setup as in post ##25 I measure 1.78vac on all three of the places you suggested.

[g1]

How about (same conditions) Q4 collector? T9 base? T9 emitter?