Marshall VS100H Hum

[LocoPatrick]

Hello, Everyone,

I am working on my Marshall Valvestate VS100H that I received in 1996. I am having an issue with what seems to be excessive hum from the speakers, even when no guitar or effects are plugged in.

The most helpful post I have found is here: https://music-electronics-forum.com/...0r-hum-problem
I followed a number of steps that were talked about in this post. To summarize the results, only one test changed the output of the hum from the speaker output:

• Remove the tube.
• Set the Power Dimension switch to off.
• Short R23 to ground.

By following these steps, the output hum was reduced to an acceptable level.

Here is a link to the VS100H schematic: https://www.google.com/url?sa=t&rct=...RbRun4PK30tGig

Details of the testing are below. Does anyone have any suggestions on how to reduce hum on the output?


Maintenance to date:

• Replaced large electrolytic capacitors.
  • Power PCBA: C11, C12, C7. Replaced with identical specified components.
  • Preamp PCBA: C7, C8
• Replaced tube. Hum happens no matter which tube is used.

Measurement equipment:

• HP 34401A multimeter
• Tektronix TDS460A oscilloscope powered through isolation transformer

Conditions:

• No guitar plugged in.
• All potentiometers turned to 0.
• Speaker output: cutoff 1/4” mono cable, no load.

Output from speaker terminals is 60mVpp regardless of which channel is selected.
Pressing the “Power Dimension” switch increases amplitude and makes the waveform more sinusoidal.

Transformer:

• The transformer itself gives off a noticeable hum when power to the amp is turned on. Not bad, but audible.

Power PCBA:

• Disconnect CON1.
• Short pins 1 and 2 together. Hum audibly decreases through speakers to satisfactory level.

Output from speaker terminals is 19mVpp.
This indicates to me that the Power Amplifier PCBA is good.


Preamp PCBA:

• Short across R49 (any channel, Power Dimension on or off): NO CHANGE
• Short across R77 (any channel, Power Dimension on or off): NO CHANGE
• Pull tube, short across R23 (any channel):
  • Power Dimension ON: NO CHANGE
  • Power Dimension OFF: Hum audibly decreases through speaker to satisfactory level.
• IC3: Pin 4 with respect to ground: -11.92 VDC. AC coupling shows: 10.8mVpp
• IC3: Pin 8 with respect to ground: +11.89 VDC. AC coupling shows 10.4mVpp. DC voltage slowly declining. Started at about +11.93VDC. Stabilizes at: +11.85VDC
• Disconnect heater wires (brown cables from transformer): NO CHANGE
• Added additional capacitance across C19 and C21 (220uF, 50V): NO CHANGE
• Resistance between tube socket Pin 9 and signal ground: 0.3 Ohms
• DC Voltage tube socket: Pin 3 with respect to signal ground: +127.6 VDC
• DC Voltage tube socket: Pin 8 with respect to signal ground: +0.85 VDC

[Jon Snell]

Don't get caught in the overthinking trap.
Keep it simple.

Insert a lead into the Effects loop return. Have you still got hum?
If so the amplifier board is the cause, if not the stages before the effects loop or signal path is the issue.

Use a loudspeaker and listen, it is easy to read or see ghost images on an oscilloscope. Use your scope right at the end of fault finding.

Break everything down, one bit at a time working backwards from the output stage to the first preamp and listen.

Firstly, what I would do is; What frequency is the hum ... mains or twice mains. Very important!
Twice mains is main smoothing or decoupling so no need to go further.

Mains frequency can be more difficult ... start bu chasing the hum, not your tail. Check ground connections.
CON1 pins 1 & 2 , I read you find it reduces the hum ... it should be quiet with no hum. Sort that first.
Disconnect pre out, effects loop, does it stop. Etc etc etc.
Work backwards and in no time you will find the cause.
Have fun, drink plenty of coffee and take breaks to keep refreshed. It worked for me for over 55 years and it still does.

[LocoPatrick]

Than you for the feedback. I plugged in a 1/4" mono cable into the effects loop return with the two leads shorted. The amp has a adjustable effects loop level. With the level completely turned down, there was no effect on the hum. When I turned the level up, the hum increased. With the level turned up, I unplugged the lead and the hum decreased to what it usually is.

I haven't done anything else with the amp aside from that test.