Marshall VS65R signal path help?

[allante666]

OK, well at least I found the source of the dodgy readings. When I took them in post#25 I had the earth lead clipped to the -ve of C2 as a signal ground. My negative meter lead wasnt clipping on correctly and the 1.7vac I was getting was just stray ac my positive probe was picking up as the other lead wasnt connected!

So, the readings I took in post #42 are correct (meter lead clipped properly to the bracket of PH2 reverb plug) HOWEVER, the IC7 and R1 readings were wrong. The correct readings are (and youre gonna hate me for moving the goalposts this far on!) are:-

IC7 Pin 1 = 227mV ac
IC7 Pin 7= 241mV ac

Either side of R1 = nothing either side.

[allante666]

DrGonz78, yes I do (dont know why I cant see your post here but can see it in the email?)

"Do you have continuity from one side of R16 to R1? Basically checking that there is nothing breaking connection on pin 2 of Con1 from Preamp connection to the pin 2 of Con1 on power amp."

[g1]

So you have signal at IC7, continuity to R1, no signal at either side of R1, then signal again at T4, T9, and T11. Magic.
I don't know if we are measuring noise here or what. T4 you have signal on the emitter, and much less on the collector where it should be.
Without a scope, this is very difficult. I think you are going to need to use AC volts meter measurement, as well as your audio signal tracer. Verify that what you are measuring with your meter is the sound of your test tone. And that it is louder with the higher readings.
For example, T4 base and emitter should sound the same with your tracer, collector should be weaker sounding.

[DrGonz78]

DrGonz78, yes I do (dont know why I cant see your post here but can see it in the email?)

"Do you have continuity from one side of R16 to R1? Basically checking that there is nothing breaking connection on pin 2 of Con1 from Preamp connection to the pin 2 of Con1 on power amp."

After reading my post I deleted it as I felt I might just slow down the thread again, lol. Anyhow I deleted it because it was late and I missed the fact that there are multiple R1's. It was on track except I was thinking you meant R1 on the power amp board part of the schematic. After looking closer I was realized you were probably referring to another R1, the one on preamp side (reverb circuit).

Edit: Actually which R1 were you referring too?

[allante666]

Right, you have a point here about what Im measuring. I just tried again and measured 800mV on pin 2 of the connector CON1 but nothng on either side of R1 and 100mV on base of T4. Then, I switched off the tone generator and I still get the same readings!

[allante666]

Edit: Actually which R1 were you referring too?

Power amp R1

[DrGonz78]

Oh okay so you were referencing R1 on the power board. Did you still measure 800mv at Con1 pin2 preamp side after removing tone generator? And on Con1 pin2 of power amp you measure "Nothing" 0v, even with tone generator off...? So we have continuity from R16 preamp to the R1 connection on power amp board. However, did you test the exact resistance there and not just continuity? Also, can you post voltages of all the legs of IC7?

[allante666]

DrGonz78 I think g1 is correct in that the AC Im measuring is just general noise. Today measuring pin 2 of CON1 (both ends) I get 544 mV ac with tone on or off.

R16 to R1 resistance measures 0.2 Ohm.

IC7 DC Voltages are:-

1=0
2=0
3=0
4=-15 VDC
5=0
6=0
7=370mVDC
8=15 VDC

[allante666]

Any thoughts?

[DrGonz78]

Perhaps measure DC and AC at all the output pins (input too?) for all the IC's going backward from IC7? I am curious of the readings going backwards through the signal chain. It might show us something and it might not.

Edit: Also did you already confirm resistance readings on pin1 Con1 between preamp and power amp boards?

[allante666]

I can do that. If Im going all the way back to IC1 should I put the signal into the guitar input jack rather than the return jack to take the measurements? Yes, resistance between pin1 power amp/preamp board is fine (1 ohm).

I did find some notes from a long time ago where I had another amp the same model which exhibited the exact same symptoms. (it said "no to very low distorted output, same with headphone, same with plugging into return"). The cure for that was replace IC7. which was the first thing I did. Dunno if that helps or not!

[DrGonz78]

I can do that. If Im going all the way back to IC1 should I put the signal into the guitar input jack rather than the return jack to take the measurements? Yes, resistance between pin1 power amp/preamp board is fine (1 ohm).

I did find some notes from a long time ago where I had another amp the same model which exhibited the exact same symptoms. (it said "no to very low distorted output, same with headphone, same with plugging into return"). The cure for that was replace IC7. which was the first thing I did. Dunno if that helps or not!

My thought was not to inject signal at all for the first measurements and to see what the measurements were without it. Then try injecting signal at front input and we pretty much have measurements with signal injected at effect return. Mostly wanted to see if we had any other output pins with 370mv DC and AC mv noise without signal injected. Pin7 of IC 7 has a cap that should block DC from going to the power amp stage and we see 370mv DC there, is there DC mv reading on R1 (power amp) side? I think it is odd to see that 370mv DC on pin 7 but perhaps it is normal or it is related to AC mv tricking the meter. Shouldn't Pin 7 of that IC be very very close to 0mv?

[oc disorder]

A fresh look perhaps


It's not that - "Marshalls are very quiet when you turn them on ! " mute fet T16 J111 ????

[allante666]

Right, ok so I have got measurements for all pins of all IC's with and without a signal input. AC and DC.

IC1 No Signal AC Signal No Signal DC Signal
1 0 52mV 0 0
2 0 0 0 0
3 66mV 66mV 0 0
4 0 0 15V 15V
5 0 52mV 0 0
6 0 52mV 0 0
7 0 52mV 0 0
8 0 52mV 0 0
9 0 52mV 0 0
10 0 52mV 0 0
11 0 0 -15V -15V
12 14mV 16mV 0 0
13 0 0 0 0
14 0 0 0 0


IC2
1 216mV 207mV 15V 15V
2 0 0 0 0
3 0 0 0 0
4 0 0 -15V -15V
5 0 0 0 0
6 0 0 0 0
7 0 0 0 0
8 0 0 15V 15V



IC3
1 0 888mV 0 0
2 0 116mV 0 0
3 0 116mV 0 0
4 0 0 -15V -15V
5 0 116mV 0 0
6 0 116mV 0 0
7 0 1.4V 0 0
8 0 0 15V 15V


IC4
1 0 0 15V 15V
2 0 0 0 0
3 0 0 0 0
4 0 0 -15V -15V
5 0 0 0 0
6 0 7mV 0 0
7 0 0 0 0
8 0 0 15V 15V


IC5
1 0 0 15V 15V
2 14mV 16mV 8mV 8mV
3 0 0 8mV 8mV
4 0 0 -15V -15V
5 0 0 17mV 17mV
6 0 0 9mV 9mV
7 0 0 0 0
8 0 0 15V 15V

IC6
1 0 0 -15V -15V
2 0 0 0 0
3 21mV 21mV 0 0
4 0 0 -15V -15V
5 0 0 -15V -15V
6 0 0 0 0
7 0 0 15V 15V
8 0 0 0 0

IC7
1 0 0 0 0
2 0 0 0 0
3 5mV 0 0 0
4 0 0 -15V -15V
5 0 0 0 0
6 0 0 0 0
7 0 0 366mV 366mV
8 0 0 15V 15V


Phew! That took longer to type than it did to measure!

Input was tone generator of 120mV No theres no DC mV either side of R1


WELL THAT DIDNT WORK!! It took all the spaces out that I laboriously typed in! Ok well the four columns are the ac with and without a signal and then DC with and without

Well, I tried editing post but despite i tlooking ok in the edit window it wont save the formatting. Anyone know why?

[allante666]

Nope! I tried that but its not that. Plus its not actually muted it still works, just very low and very broken up and distorted