Marshall VS100R hum problem

[avoverdu]

Hello guys, Im new here in the forum. I bought a Marshall Valvastate VS100R with reverb for a few bucks that has a hum issue and I need some help/advise. EVERYTHING ON THE AMP WORKS PERFECTLY the only issue the hum. Here are the symptoms:

1) When I turn it on there is a pop and hum noise follows right after. The hum is there with all the pots on zero and no guitar plugged in.
2) The hum gets a little bit louder when I crank up the volume. This happens on any channel
3) The amp was off since yesterday, when I turn it on today I noticed that at the hum volume increases a tiny bit in time.
4) When Im on the overdrive channel I noticed the treble pot make noises (like the ones you get on a dirty pot) every time you turn it, specially when you turn it from zero to a quarter turn.
5) The frequency of the hum is the same as the vibration of the transformer when current is flowing through it.


These are the things I´ve done with no positive results:
1) I tried 5 different tubes
2) I checked all the joints and resoldered a bunch of them.
3) I replaced the electrolytic filter caps on the preamp section and the ones on the power section as well.

PLEASE HELP LOL!!!

Thanks guys

[The Dude]

The pot noise could be as simple as a dirty or bad pot. Or, there could be DC on the pot. Check with your meter to see if there is DC there.

On the hum, since you've already changed electrolytics and tubes, start on one end or the other and remove tubes to see if you can isolate the hum to a particular stage. In other words, pull V1. Still have hum? Pull V2. Still have hum? Etc.

[avoverdu]

I will check the DC on the pot. This amp has one tube in the preamp and transistors on the power section. This week I will isolate each section of the circuit in order to find which is the faulty one and check every component there.

Thanks for the advise!!!

The pot noise could be as simple as a dirty or bad pot. Or, there could be DC on the pot. Check with your meter to see if there is DC there.

On the hum, since you've already changed electrolytics and tubes, start on one end or the other and remove tubes to see if you can isolate the hum to a particular stage. In other words, pull V1. Still have hum? Pull V2. Still have hum? Etc.

[avoverdu]

I found like 10 caps that were bad (22uF most of them) replaced them all and the hum continues. I removed the tube and the hum remains. The first triode of the tube is a normal gain stage and the second one is a cathode follower that drives the tone stack of the overdrive channel only. This way I'm sure the hum doesn't come from the tube. Everything looks that something is happening on the transistor power section.

Where should I start looking?? Thank you very much

[nickb]

I found like 10 caps that were bad (22uF most of them) replaced them all and the hum continues. I removed the tube and the hum remains. The first triode of the tube is a normal gain stage and the second one is a cathode follower that drives the tone stack of the overdrive channel only. This way I'm sure the hum doesn't come from the tube. Everything looks that something is happening on the transistor power section.

Where should I start looking?? Thank you very much

1) Unplug CON1 on the power amp PCB. What happens to the hum?
2) With all the controls turned down so that you hear the hum only, measure the ACV across the speaker terminals.

[The Dude]

Oh boy. I just realized this is a valvestate amp, so my post #2 makes little sense. (Insert dunce cap emoticon here)

http://www.thetubestore.com/lib/thet...-Schematic.pdf

Start by checking for ripple/AC content on your power supply lines. Look at +LT, -LT, +15V, -15V, etc. Also check that DC levels are what they should be.

[J M Fahey]

Just to check where hum gets in, please do these 3 simple tests:

1) at the power amp board short the Net R1-C2-J111d to ground ... any change in hum?

2) at the preamp board, short across R49 ... any change in hum?

3) at the preamp board, short across R77 ... any change in hum?

[avoverdu]

Thanks Nickb. I have the amp ready, everything on zero, clean channel, no guitar plugged in:

1) I removed CON1 on the power PCB: The hum is reduced a lot but it is still there.

2) -AC voltage across the speaker (8ohm) with the CON1 connected= 0.2VAC
- AC voltage across the speaker (8ohm) with the CON1 disconnected= 0.0003VAC

If it works here are the VDC readings too: with the CON1 connected= -0.015VAC
with the CON1 disconnected= -0.015VAC

1) Unplug CON1 on the power amp PCB. What happens to the hum?
2) With all the controls turned down so that you hear the hum only, measure the ACV across the speaker terminals.

[nickb]

Thanks Nickb. I have the amp ready, everything on zero, clean channel, no guitar plugged in:

1) I removed CON1 on the power PCB: The hum is reduced a lot but it is still there.

2) -AC voltage across the speaker (8ohm) with the CON1 connected= 0.2VAC
- AC voltage across the speaker (8ohm) with the CON1 disconnected= 0.0003VAC

If it works here are the VDC readings too: with the CON1 connected= -0.015VAC
with the CON1 disconnected= -0.015VAC

Good job! So it's coming from the preamp. Now we do what Juan suggested and see which part of the the preamp it's coming from. This is what he said:

2) at the preamp board, short across R49 ... any change in hum?

3) at the preamp board, short across R77 ... any change in hum?

[avoverdu]

OK, these are the results:

1) at power amp, if short at the point that connects R1 with J111 (R16 in the schematic) at the pin d, which according to my tester is the center pin= the hum is reduced a lot but still there.

2) at the preamp board if I short R49= hum is reduced but is still there

3) at the preamp board, if I short R77 hum is reduced even more. I think the reduction on 1 and 2 are about the same but number 3 is almost dead quiet BUT THIS BEHAVIOR IS JUST FOR THE OVERDRIVE CHANNEL, ON THE CLEAN CHANNEL THIS DOESN'T DO ANYTHING TO THE HUM


Thanks in advance

Just to check where hum gets in, please do these 3 simple tests:

1) at the power amp board short the Net R1-C2-J111d to ground ... any change in hum?

2) at the preamp board, short across R49 ... any change in hum?

3) at the preamp board, short across R77 ... any change in hum?

[nickb]

It seems like it coming in around the OD tone stack as when you pulled the tube earlier the hum was still there.

Pull the tube and short out R23. What happens to the hum?

Also check the ACV on pins 4 and 8 of IC3. Hint: you will, unless you have a really good meter, need to use a small capacitor 0.01uf to 0.1uF in series with one of the meter leads to do this.

[avoverdu]

Here we go, I pulled the tube, clean channel, R23 shorted= it sounds like the low octave of the hum is filtered out.

About the ACV I have a 0.1uF cap, do you want me to read between pin 4 and 8, or between 4 to ground and 8 to ground??? Let me know

It seems like it coming in around the OD tone stack as when you pulled the tube earlier the hum was still there.

Pull the tube and short out R23. What happens to the hum?

Also check the ACV on pins 4 and 8 of IC3. Hint: you will, unless you have a really good meter, need to use a small capacitor 0.01uf to 0.1uF in series with one of the meter leads to do this.

[nickb]

Always with respect to ground unless stated otherwise. There was actually a slight problem with the R49/R77 measurement in that only one was valid as it depends on the which channel is selected. Since R77 made the big difference ( i assume about the same as when we pulled CON1) I'm looking at that path and assuming that channel was selected.

[avoverdu]

Ok in that case:

R77 shorted on clean channel: it does nothing
R77 shorted on overdrive channel: reduces the hum

R49 shorted on clean channel: it does nothing
R49 shorted on overdrive channel: reduces the hum

let me know, as soon as I measure the voltages on IC3 ill let you know. Thanks man I appreciate it

Always with respect to ground unless stated otherwise. There was actually a slight problem with the R49/R77 measurement in that only one was valid as it depends on the which channel is selected. Since R77 made the big difference ( i assume about the same as when we pulled CON1) I'm looking at that path and assuming that channel was selected.

[nickb]

Please go ahead and measure the ACV on IC3 pins 4 & 8.

PS: Measure the DCV at the same time ( don't use the series cap)