Marshall Valvestate VS100 hum

[nrdnlgr]

Hi, I have a Marshall Valvestate VS100 head (schematic attached) with a pretty loud hum at idle. Otherwise the amp sounds great, so I want to get rid of the hum if I can. Please help!

With no input connected, and all volumes at 0, I turn on the amp and it goes from quiet at first but gets steadily louder to a loud hum within a minute or two. When I turn the amp off, there is a loud pop (this doesn't bother me, I just thought it might be relevant to the diagnosis).

The hum is much louder on the overdrive channels, and the hum is at the same volume on both of the overdrive channels. Cranking the volumes does little to the volume of the hum.

If I measure the voltage at the speaker output (with no input and all volumes at 0), I get 140mV AC (with a ~60HZ frequency irregular waveform, image attached) and 15mV DC. If I pull the CON1 connector to the preamp board, the hum goes away. I've swapped tubes and the hum is still there, although it is louder on some tubes than others.

I've checked for the obvious, like loose caps, resistors, etc, with no luck. I'm pretty handy with a soldering iron, and I have a multimeter and cheap oscilloscope, so any help with things to check or other voltage tests I can do would be appreciated.

Thanks!

[nosaj]

i would check the solder joints on the sockets that make more noise. Also where'd you get that little scope?

nosaj

[J M Fahey]

i would check the solder joints on the sockets that make more noise. Also where'd you get that little scope?

nosaj

KILLER pocket scope, a friend has one, it works flawlessly.

Only problem is price

https://www.alibaba.com/product-deta...565845189.html

[nrdnlgr]

I bought the scope on Amazon for like $30 I think.

I did some more testing, and my 15V positive and negative supplies (measured at IC4) are only putting out 12V. Seems like maybe I have a short somewhere that is bringing the voltage down and the extra current draw is causing the hum.

Any suggestions on where to focus my attention are appreciated. The schematic doesn’t really tell me where to check voltages and what I should be seeing.

I’ll start with pulling the boards and checking for shorts on the transistors.

[g1]

Some of the valvestates have a typo showing +/-15V from 12V regulators. The +/-12V you have is probably correct.

Is the hum still there if you remove the tube?

[nrdnlgr]

Interesting. The two voltage regulators that I see are 7812ct, which are indeed 12v.

With the tube pulled, there is still a lot of noise, but it’s different. More of a buzz than a hum, and the frequency is closer to 120hz. With the tube in, there is a deeper octave there that is what I would call a hum.

But with the tube in and CON1 pulled from the power board, the output is dead silent.

[g1]

But with the tube in and CON1 pulled from the power board, the output is dead silent.

That says the power amp is hum free. Reconnect CON1.
Tube V1 gets it's DC supply filtered by C7. Check how much ripple (AC volts) there is across C7.

[nrdnlgr]

Bear with me, I'm not an expert with this stuff, but I think I have a measurement for you. I didn't want to take the board out to measure across the leads of C7, so I measured at the anode side of D2 and both sides of R19 and R22. On both sides of R22, and one side of R19, I have no significant ripple that I can see. It looks pretty flat. However, on one side of R19, I get 0.89 Vpp, 0.24 Vrms at 121 Hz (I would upload the image, but I get an upload quota error).


FWIW, C7 and C8 are the original caps for the amp (I replaced the big filter caps on the power board, and a few smaller caps on the pre-amp board already).