SState Marshall 5212 Hiss/static after warm-up

[retrorod]

Hey all, I need help in finding the source of low-level hiss and static in a solid-state Marshall 2X12 combo.
http://schems.com/manu/marshall/50w_5210.pdf

This noise develops after 30-45 minutes of run time. It goes away with the Master volume down. It quiets down with a jack inserted into the SEND Jack or halfway into the PHONE Jack on the rear panel.

After turning it off and back on the next day.....it is dead quiet until warmed up for 30- 45 minutes or so.

I have cleaned the jacks and even replaced the Send jack. No changes.

Any trouble shooting ideas and suggestions? Thanks in advance, Rod

[52 Bill]

You have isolated the problem to the preamp, so continue to try and isolate the problem even further. Is the noise controlled or changed by any of the volume or tone controls? Does it matter which channel is selected?

Before the noise starts, take a few dc voltage readings on the two chips IC1 and IC2, especially the output pins 1 and 7. Once the noise starts read the voltages again and see if there are any changes to the readings.

[Enzo]

This is what freeze spray was made for. Once it warms up and makes noise, we spray freezer on the board and if it stops the noise, we have a plan. Let it warm up again, then we freeze small areas one at a time to isolate the noisy part. It will most likely be a semiconductor - diode, transistor, IC.

[retrorod]

Thanks! I will give these a try and report back!

[drewl]

I had the 1x12 version.

As mentioned, one of the opamps is probably the cause, see if any feel warm.
Is it on both channels?
That might help isolate which stage its coming from.
You could find it with an oscilloscope if you can get your hands on one.

[retrorod]

I just took some DC voltages from IC 1 & 2 upon power up and not making noise....as follows-
IC1
1 -.412
2 .001
3 0.00
4 -16.57
5 -.007
6 -.007
7 -.003
8 +16.58

the voltages on IC2 were nearly indentical with the exception of 1 being .004

neither of these IC's get hot to the touch or were effected by freeze spray while the noise was active last night.
I will check IC 3&4 tonite!

[52 Bill]

IC3 and IC4 are in circuit after the master volume control and if the noise goes away when the master is turned down, then they will not be the cause of the noise.

What about the question that was asked about the channels. Does it happen with either channel selected or just one?

And once the noise starts are there any changes to the voltages on IC1 and IC2?

[retrorod]

I wanted to follow up and answer the questions. The noise remains on either channel and is unaffected by any controls other than master volume. Is safe to say that the voltage varies and drifts on #1 leg of IC1 with time and noise.
I have had some success ( quiets down) with the freeze spray in the area of IC1....After reading this thread about a 5210 (symptoms identical)
http://music-electronics-forum.com/t20034/

I am suspect of C12 but will have to do some more checking. What do you think?

Rod

[52 Bill]

I am suspect of C12 but will have to do some more checking. What do you think?

C12 is only in the audio path of the clean channel, if the noise is on both channels, then C12 alone can't be the problem.

IC1 and C26 are more likely candidates.

Edit: Upon further review, as the clean channel signal is turned off before it gets to C12, it is still in circuit when the channels are switched. So yeah, it could be the problem.

[retrorod]

Thanks Bill!
This IS encouraging. I have to order parts....