Marshall ValveState VS100R white noise / buzz problems

[MarshallMallow]

Hi everyone, nice forum you have here. I am an amateur electronics hobbyist who is having problems with a Marshall VS100R that is about 16-17 years old. The amp is working "ok" (everything functions normally). But my main issue with the amp is that is making a Hiss/ white noise on only the clean channel, with volume at 0. There is also a low electrical buzz on all channels. The white noise is sort you would expect if the amp was turned up to high volumes.

Here is an example of how these sounds occur:
With the amp off and all volumes at 0 (Clean channel and OD channels):

1. I plug in my guitar, I turn the amp on (clean channel), immediately there is a low electrical buzzing sound from the speaker (I can also hear this from the transformer area when the chassis is out of the amp cabinet)

2. About 4 seconds after turning on I can hear a "pop" then a lowish but easily audible white noise/hiss sound comes in and builds in volume.

3. If I now press the OD switch (again all volumes @ 0) the hiss disappears and I can just hear the low buzz. Go back to the clean channel and the hiss is back.

4. If I pull the guitar plug out, I can still hear the buzzing from the speaker. The white noise is gone though.

Other observations:
If I plug the guitar into the Effects Return there is no white noise on the clean channel (still buzzing though).

If I use the Headphone jack, I can't hear the Clean Channel white noise (or buzz).


Things I have done so far in an attempt to (unsuccessfully) resolve the problems:

Used contact cleaner on all the jacks/pots/switches.

Fixed multiple cracked solder joints

Replaced the pre amp valve

Replaced/ socketed IC4

I don't know how long the amp has been like this or how much of this stuff is "normal". I could just continue to use the amp and ignore the sounds, if I was playing loudly with a band or just
jamming about...but I am trying to record the Clean Channel and this is what caused me to notice these odd sounds (with the amp at 0 volume, this is a really undesirable noise floor to start with before I even add any other noise from amp volume/guitar etc). The fact that only the clean channel makes this white noise and not the OD channels (especially with the volume off) really causes me to believe something is not right and can be fixed.

Here is the schematic for the VS100R: http://www.amparchives.com/Amp%20Arc...s100r_100w.pdf

Thanks for any guidance or advice.

[olddawg]

Without a scope, tuning these problems down could be a bitch. Have you cleaned the effects send/return jacks and pre in/out jacks? Have you run a variable preamp level signal into the main amp in or effects return jack to isolate the problem to the preamp or the power amp sections. Just off of the top of my head I would be suspecting 16 year old capacitors in the various power supplies. I would start by checking the ripple voltage and symmetry on all of the power supply rails. Again. Much easier to do with an o'scope. But a meter can tell you a lot too.

[MarshallMallow]

Without a scope, tuning these problems down could be a bitch. Have you cleaned the effects send/return jacks and pre in/out jacks? Have you run a variable preamp level signal into the main amp in or effects return jack to isolate the problem to the preamp or the power amp sections. Just off of the top of my head I would be suspecting 16 year old capacitors in the various power supplies. I would start by checking the ripple voltage and symmetry on all of the power supply rails. Again. Much easier to do with an o'scope. But a meter can tell you a lot too.

Thanks for the tips. I don't have a scope unfortunately (no idea how to use one anyway). I have no idea how I would "run a variable preamp level signal into the main amp in or effects return jack to isolate the problem to the preamp or the power amp sections". or how to "check the ripple voltage and symmetry on all of the power supply rails", or even what these things mean. I will try to give the jacks a bit more of a thorough cleaning. I could get replacement caps and change them I suppose.

Perhaps further attempts by myself to fix this problem are a bit too above my head, with my current limited electronics experience. Maybe I should just take it to amp repair tech? Not sure how much they would charge though, this amp is probably not worth any more than €200 at the very most.

[52 Bill]

Welcome to the place. I think that the basic point that OldDawg was trying to make, was to try and isolate the problems to make them easier to locate and fix.

A low level hum could be caused by aging filter caps in the power supply. One test of this is to measure how much ripple or residual ac is riding on the dc power supply lines. If you have a voltmeter that can read ac voltages that are raised by a dc voltage, then to measure this you just set your meter to read ac voltage and read across the main filter caps.

The effects loop jacks is a good point to separate the preamp from the power amp circuits. If you insert a cord into the FX return jack and then connect that cord to a guitar or to an MP3 player, you will be hearing only the circuitry from the FX return jack to the speakers. You already did this test and found that the white noise stops but the hum remains, so the source of the white noise is in the preamp section and the hum is either in the power supply or the power amp circuits.

Something that you stated earlier was that with headphones the white noise and buzz stops. This is odd as the headphones are powered by the main power amp, only at a reduced level. Have you cleaned the headphone jack as well?

You also said that you have replaced IC4. Why did you do this?

[MarshallMallow]

Welcome to the place.

Thanks for the welcome!

I think that the basic point that OldDawg was trying to make, was to try and isolate the problems to make them easier to locate and fix.

A low level hum could be caused by aging filter caps in the power supply. One test of this is to measure how much ripple or residual ac is riding on the dc power supply lines. If you have a voltmeter that can read ac voltages that are raised by a dc voltage, then to measure this you just set your meter to read ac voltage and read across the main filter caps.

Kind out of my current level of knowledge again here. I set my multimeter to AC, I get that, but I'm afraid I don't know how to "read AC voltage across the main filter caps", or interpret the results, sorry . I do know this is an area to treat with a lot of respect and I would not muck about here unless I was 100% clear on what to do. I'm guessing the amp has to be on to do this test too? ( sorry, stupid question probably, but stupid questions are normally the ones not asked)

The effects loop jacks is a good point to separate the preamp from the power amp circuits. If you insert a cord into the FX return jack and then connect that cord to a guitar or to an MP3 player, you will be hearing only the circuitry from the FX return jack to the speakers. You already did this test and found that the white noise stops but the hum remains, so the source of the white noise is in the preamp section and the hum is either in the power supply or the power amp circuits.

Ok, this is good to know.

Something that you stated earlier was that with headphones the white noise and buzz stops. This is odd as the headphones are powered by the main power amp, only at a reduced level. Have you cleaned the headphone jack as well?

It's possible the buzz is still there on the headphones, just really really low. I am almost 100% sure the white noise is not there though. To clean the jacks I sprayed contact cleaner into them and inserted and removed a 1/4 inch plug several times. I could clean more with a q tip. Do you have any other suggestions for effective cleaning (particularly the switching ground part?)

You also said that you have replaced IC4. Why did you do this?

Some one who had a similar problem on another forum suggested to do this to try to fix the white noise.

Thanks for the assistance.

[52 Bill]

Kind out of my current level of knowledge again here. I set my multimeter to AC, I get that, but I'm afraid I don't know how to "read AC voltage across the main filter caps", or interpret the results, sorry .

Nothing to be sorry about, but lingo here does help to get a better understanding of a poster's technical skills.

I assume that you understand that the amp runs on dc voltage. This dc voltage is created from the ac voltage that comes from the wall socket. During the conversion process, the ac is converted to pulsed dc which is then filtered by the filter caps into what is closer to pure dc. If the filter caps lose effectiveness, there is a higher than normal amount of ac voltage still present with the dc voltages. This residual ac is heard as hum in the audio signal.

The two main filter caps in your amp are C11 and C12. If you were to measure the dc voltage "across" each of these caps (red meter lead to the positive end of the cap and black meter lead to the negative end of the cap), I would guess that you would read about 45 volts dc. If you then set your meter to read ac voltage, you will be reading the ac "ripple" (the residual ac) on the dc lines. And yes, the amp needs to be turned on to perform these tests.

If you feel unsure or unsafe about performing live voltage tests on an amp, then the safest thing to do would be to refer servicing to a qualified tech.

As for replacing things based upon other people's experiences, I don't recommend it, but it is your choice to do so. There are only two ICs in the normal channel that are not also used by the distortion channel, IC4 and IC2. You could try replacing IC2 and see if that clears up the white noise problem, but remember that you're taking a shot in the dark.

[olddawg]

Have you tried simply running a short instrument cable between the effects send/return jacks and the pre-out/ main in jacks. That many times takes the internal switching out of the equation. Generally, spraying some Caig Deoxit on a plug and working it in and out of jackets works ok for simple jacks like instrument input jacks that have simple tip and sleeve contact surfaces. But if they are switching jacks you have to get to the switch contacts. That can be problematic, especially in sealed plastic jacks.

[MarshallMallow]

Thanks very much guys (nice explanation 52 bill). I'll have a look later at some of those things and report back.

Really appreciate the help!

[J M Fahey]

You don't have a problem.
The slight hiss you hear is because it's alive.
You hear it appear after turn on because there is a slightly delayed mute circuit.
The buzz you hear is mostly mechanical (transformer) .
Only silent amps are those turned off, either by a power switch or a blown fuse.
You find it objectionable because you are in a silent room.

[MarshallMallow]

I assume that you understand that the amp runs on dc voltage. This dc voltage is created from the ac voltage that comes from the wall socket. During the conversion process, the ac is converted to pulsed dc which is then filtered by the filter caps into what is closer to pure dc. If the filter caps lose effectiveness, there is a higher than normal amount of ac voltage still present with the dc voltages. This residual ac is heard as hum in the audio signal.

The two main filter caps in your amp are C11 and C12. If you were to measure the dc voltage "across" each of these caps (red meter lead to the positive end of the cap and black meter lead to the negative end of the cap), I would guess that you would read about 45 volts dc. If you then set your meter to read ac voltage, you will be reading the ac "ripple" (the residual ac) on the dc lines. And yes, the amp needs to be turned on to perform these tests.

If you feel unsure or unsafe about performing live voltage tests on an amp, then the safest thing to do would be to refer servicing to a qualified tech.

Back again! Managed to do this test. Took me some time as I just wanted to get a better (safer) multimeter and some safe test clips before doing this.

These are the results:

C11
volts DC:038.1 (last digit fluctuating)
Volts AC:0.069

C12
volts DC:037.8 (last digit fluctuating)
Volts AC:0.069

What do you think?

Thanks for any assistance.


(edit: how did my new post appear above older posts? Weird, never saw that on a forum before, confusing! )

[52 Bill]

C11
volts DC:038.1 (last digit fluctuating)
Volts AC:0.069

C12
volts DC:037.8 (last digit fluctuating)
Volts AC:0.069

Those readings seem perfectly normal.

If you plug a guitar in and turn down the guitar's volume control what happens to the noise/buzz?

Does the noise/buzz happen regardless of location? Plugged into a different outlet, or at another location?

Have you checked to see that all of the jacks, pc boards, transformers, etc. are all tightened down?

Just trying to eliminate the basic stuff first.

[MarshallMallow]

Those readings seem perfectly normal.

damn

If you plug a guitar in and turn down the guitar's volume control what happens to the noise/buzz?

The buzzing is there all the time. It doesn't seem to matter if there is a guitar plugged in or the volumes are up or down on ether the guitar or the amp, as soon as the amp is turned on the buzzing is there. The white noise on the clean channel only appears when I plug into the input jack (due to the grounding switch input jack). Again, once plugged in though, no volumes affect it; guitar or amp.

Does the noise/buzz happen regardless of location? Plugged into a different outlet, or at another location?

Tried some different rooms/ outlets in this building, no difference. Can't easily try at another location atm.

Have you checked to see that all of the jacks, pc boards, transformers, etc. are all tightened down?

Just trying to eliminate the basic stuff first.

I have had the amp disassembled many times now, removing and reinstalling different parts. Never after having done any of these dis-assemblies and reassemblies has either of the noise problems changed at all in characteristic, so I would be fairly confident everything is well secured. Gently bumping the amp has zero affect on the sounds also.

------------------

In an attempt to fix the Clean channel white noise, I would be interested in replacing IC2 (Mitsubishi M5201), but it's a hard component to get my hands on. The only thing I can get that seems close to it is JRC 2121 (ideally it should be a 2120). Would this do the job as a replacement? I saw discussion about this IC here: http://music-electronics-forum.com/t12447/

M5201: http://www.audiolabga.com/pdf/M5201.pdf
JRC 2121: http://www.njr.com/semicon/PDF/NJM2121_E.pdf
JRC 2120: http://www.njr.com/semicon/PDF/NJM2120_E.pdf

Thanks again.

[g1]

2120 or 2121 will both work fine as replacements.
As far as the audio (op amp) parts of these chips go, 2120 is equivalent of 4558, 2121 is equiv. of 4560.
So, if you are trying to make some improvements with regard to noise & performance, 2121 is probably more of an upgrade.

[Deadear]

The 5201 chips are out there. I bought three on flee bay about a year ago. $5 each ouch.http://www.ebay.ca/itm/SWITCHING-OPE...UAAOxymcdRf9mN

[MarshallMallow]

Yeah, I managed to find one myself for 5.90 Euro while I was looking for some thing unrelated (an OC44 transistor for building my own Dallas Rangemaster). Still waiting for it to arrive.