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Author Topic: Marshall Mosfet Grounding Issue  (Read 183 times)

vfisch

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Marshall Mosfet Grounding Issue
« on: March 19, 2020, 04:33:00 PM »
Hello,


I picked up this solid state Marshall Mosfet 100 guitar amp for $5. It obviously didn't work when I brought it home. It would make sound and seemed to work until I inch the master volume up past three. Then it just squeals and makes robot noises.

After messing around with it, I discovered that If I put a jumper between the circuit ground and the chassis, it would work perfectly. No squealing, no robot noises, just guitar noises.

I checked the voltages across the filter caps, and when the circuit ground is disconnected from the chassis, they go wild. They overshoot the rated values, and are crazy volatile. When the ground is connected the chassis, they are nice and steady. It does use a bipolar power supply. My current hypothesis is that the circuit ground has no reference to hold it at a steady 0 V, so it goes all wild until connected to earth ground? I don't know enough to know anything for certain.

I'm glad I figured this out, but I have no idea why this fixes it. I also have no idea what had been done to it before I got my hands on it. If anybody understands why this happens, and why connecting the circuit ground to the chassis fixes it, please let me know. I want to learn.

Here is the schematic:

https://drtube.com/schematics/marshall/3210-iss7.gif


alessandro_gfdp

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Re: Marshall Mosfet Grounding Issue
« Reply #1 on: March 20, 2020, 04:25:47 AM »
the basic concept is that the whole circuit,preamp and output is referenced to the center tap of the transformer secondary,and probably connected to chassis at some point,maybe the input,or the power amp,sometimes through a loop breaker,that can be a resistor,with a combination of cap and diodes.
In the schematic this arrangement doesn't appear,but i guess there should be some kind of connection.
« Last Edit: March 20, 2020, 05:59:26 AM by alessandro_gfdp »

g1

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Re: Marshall Mosfet Grounding Issue
« Reply #2 on: March 20, 2020, 04:22:15 PM »
Look at the underside of the circuit board for anywhere a board mounting screw stud is supposed to make contact with a ground trace.

joecool85

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Re: Marshall Mosfet Grounding Issue
« Reply #3 on: March 23, 2020, 09:10:50 AM »
Look at the underside of the circuit board for anywhere a board mounting screw stud is supposed to make contact with a ground trace.

This is my guess as well.  I actually added a ground strap (wire with terminal connection on it) so that I have a better ground connection to chassis.  The stud to PCB method works most of the time.  Key word, "most."
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