Marshall JCM900 2100 SL-X Preamp Problem

[stereomonostereo]

Hey guys. Working on a Marshall 2100 SL-X and getting no output. Nothing from the effects loops either. The voltages on the power section are all correct, which lead me over to the preamp. When checking V3 i'm getting the following readings:

Pin 6 - 310 VDC
Pin 7 - 175 VDC
Pin 8 - 178 VDC

I did the readings with all tubes in place. When i pull V3 i get the following:

Pin 6 - 347 VDC
Pin 7 - 338 VDC
Pin 8 - 0 VDC

I'm not sure how the voltage is bleeding over to the cathode. It's not the most legible schematic. Anyone have any experience with these? Thaks in advance!

[g1]

One half of V3 is not used. Even with V3 pulled I believe you should still get sound if you put signal into the return jack.

[stereomonostereo]

I'm getting nothing from the return. And all of my phase inverter voltages seem good

[J M Fahey]

Please post your schematic here, even if not *that* readable it's better than nothing.

[stereomonostereo]

[J M Fahey]

Marshall JCM900 2100 SL-X Preamp Problem

Hey guys. Working on a Marshall 2100 SL-X and getting no output. Nothing from the effects loops either. The voltages on the power section are all correct, which lead me over to the preamp. When checking V3 i'm getting the following readings:

Pin 6 - 310 VDC
Pin 7 - 175 VDC
Pin 8 - 178 VDC

Normal. V3b is a cathode follower.
Cathode gets a couple volts above grid voltage.

I did the readings with all tubes in place. When i pull V3 i get the following:

Pin 6 - 347 VDC
Pin 7 - 338 VDC
Pin 8 - 0 VDC

Normal: V3B grid and plate rise to V+ rails.

Remember that part of the preamp is SS, and if that section does not work, you have no sound.
The schematic is cr*p, because it's not an *amplifier* schematic but the PCB .sch showing only what's on board, but not outside nor conectivity.
Absolute lack of respect towards technicians.
The effects loop *SHOULD* live between IC1b (send) and IC1a (return) but I can only guess.
After that signal passes through either IC2a and IC2b , the "Dual Masters".
This also lacks a lot of detail.
Check that you have +/-15V on pins 8/4 on IC1 and IC2.
Also follow the signal path: into > the tube > tone stack > loop Ics > Dual Master ICs > signal path towards > Power Amp.
You might have a bad connector.
Good luck.

[stereomonostereo]

I'm glad it's not just me with the schematic.

I'm getting -1v on pin 4 of both ICs. I'm not able to find where the 15v rail is coming from on the schematic though. It's definitely looking like that's the problem

[g1]

Look at the lower right area of the preamp schematic, coming off BR2.

[stereomonostereo]

Thanks for the insight. After checking the schematic at the rectifiers it seems that BR3 is running really hot. I replaced it, but it's still having the same problem. I thought C8 might be the culprit, but it seems to test out okay.

[J M Fahey]

Please answer whether you still have -1V on all IC pin4's before worrying about Br3.
Anyway, now that you are at it, what voltage do you get across C6?
Does Br3 still get hot if you pull V3?

[stereomonostereo]

I've still got -1v on the ICs, and i'm getting nothing on C6. BR3 still seems to get hot with V3 pulled.

[g1]

BR3 supplies V1,V2, and V3 heaters. Not sure what BR1 is doing.
Try to get your -15VDC sorted first. BR2, R36, and ZD2. Is R36 ok? No short at ZD2? More than -16V at the - pin of BR2? Do IC 1 or 2 feel hot?

[stereomonostereo]

I'm getting -28.5 coming off of R36 and then hitting the - pin of BR2. R35 is giving me 28.5 as well. The ICs don't seem hot

[g1]

The other side of R36 should be -16V. So either R36 is open (or bad solder), or ZD2, IC1, or IC2 are shorted. With power off, measure the resistance across ZD2.

[J M Fahey]

I'm getting -28.5 coming off of R36 and then hitting the - pin of BR2. R35 is giving me 28.5 as well.

-28.5V and 28.5V are *very* different animals, so you can't say "as well".
Please clarify.
Also, as suggested above , measure on both sides of R36 and on both sides of R35.
Thanks.