Marshall 8240 hums, static, little to no volume

[mbrace38]

I've got a 1990's Marshall Valvestate 8240 2x12 combo that gives off a terrible hum and static-type of noise like crazy, the noise increases with any increased adjustment across either channel's gain or volume, and the MV. Additionally, there is little to no instrument signal making it's way to the speakers. If I realllllly crank it up I get a tinge of guitar, but the static noise is so loud it's unusable.

I assume the issue lies in the 'pre-amp section as I've plugged into the FX return and it sounds crystal clear, no static, just clean signal. Upon visual inspection I don't see any bulging, burned or otherwise suspect caps, resistors, or other components. In turning the board over, it's clear someone has been in there as there are some areas with fresher soldier than others. I plan to reflow some spots today, but are there other areas I should be focusing my attention?

I've read the many other threads on these amps and haven't seen any with this type of issue, so I'm not sure what my next move is. I replaced the tube, just to rule that out (even though the clean channel doesn't run through it). I've cleaned all of the pots, jacks, switches and they are quiet when operating. There's a note on the schematic about adjusting RV1 to half rail supply +/-250mV. I've measured across pins 1 & 5 and then 5 & C60, but I'm not 100% on how I should interpret the results. Additionally I've measured voltages on the pins of IC14 and have included those readings as well.

IC14 pins: 1=11.7V, 2=5.8V, 3=4.0V, 4=.78V, 5=0V, 6=6.6V, 7=4.2V, 8=4.1V
Across pins 1-5 = -5.85mV, pin 5-C60 = -399.4mV

https://www.thetubestore.com/lib/the...-Schematic.pdf

So, again, I'm not exactly sure what to do with this data or if/how it applies to my issue. Thanks in advance for any assistance!

MB

[g1]

Between pins 1&5 (IC14) you should have 11.7V, not sure where you got that -5.85mV ? ('Between' meaning one probe to each of 2 points mentioned).
Set the trimmer so between C60 + end and pin 5, you have 5.85V, give or take 250mV.

[mbrace38]

Thank you, I realize I mis-read my sloppily written notes; the voltage between pins 1 & 5 is 11.7V, and I've adjusted RV1 so that the reading between pin 5 & C60+ is 11.7 as well.

This doesn't seem to have rid the amp of the hum/static.

[g1]

No, the pin5 to C60+ should be half of the 11.7V. Not surprised it's not helping the noise but it will determine whether the chorus works right.
Once it's set right, you can carry on troubleshooting. Do you have a scope or signal tracer? I think if you cure the 'low signal' first, you will probably also cure the noise issue.

[mbrace38]

Gah! Yes, of course, half of 11.7V, as noted on the schem.

Yes, I have a signal tracer, no scope.

[g1]

Ok. +/-12V rails must be ok as FX return circuit works.
The only things common to clean and dirt channels are input jack, IC2, and IC5.
Set up for clean and inject a signal at the input jack. See if it makes it to IC2 pin7.

[mbrace38]

Yes, signal at IC2 pin 7, as well as IC5 pin 5.

[g1]

Whatever you are getting at IC5 pin5, it's the same at FX send jack tip?

[mbrace38]

No, the reading is about half at the FX send jack tip as the IC5 pin 5.

[g1]

And with the other setting of switch 4 ?
Also, when you were first testing, did you ever put a patch between send and return jacks?

[mbrace38]

I don't have my notes or the amp right in front of me right now, but I know the 20db cut (sw4) on the FX loop did cut the measuement at the jack tip.

I did try a patch initially while troubleshooting, but it made no impact.

[g1]

Ok. I'm trying to determine if what there is currently at the send jack is what is making the weak signal ("tinge of guitar") at the output, or if it is a good level and not getting through.
How about put the patch cord back in from send to return, set effects pot to full wet and see if it's still all weak and static-ey.

[mbrace38]

So I've tried a few other things (all on the clean channel) to help determine the source of the static/white noise. With a patch in the loop, the noise is no different than without. Again, I plugged the guitar into the return and began turning knobs to see if any of them affected the noise, in the end the gain pot (VR1) was the biggest factor in the noise. This is not the typical high gain white noise though, particularly because it's the cleanest channel setting (I assume). With VR1 at 0 (MV at 2) the guitar is clear as a whistle into the FX return, with the gain pot at 50% the noise all but drowns out the guitar signal.

To address your other suggestions: with SW4 engaged (-20db) the signal at FX send tip dropped by about 80%. Also, the signal did stay true through the loop.

Because there's no dedicated switch to open/close the FX loop, shouldn't it only be engaged if there is something plugged into the send? Otherwise it should be cut out completely? I was under the impression that the jack itself engaged the loop, therefore if the noise is present without the loop engaged, the noise is likely coming from something prior to the loop?

[g1]

No it is not a series loop. That is what is confusing things for troubleshooting purposes. You have to also adjust the FX pot from dry to wet as there is a parallel path to the loop. So if you just want to hear what is at the return jack, you have to set the pot to full wet. If you only want to hear the parallel path, set the pot to full dry.
The parallel path takes off just before the send, at the top of R21. Then it splits and feeds IC11A/B via R86 and R63.