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.
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.
My guess would be that it was designed with a single connection to reference safety ground to circuit ground, and that connection has been broken. Look for signs of (a) someone has done repairs or replacements, or (b) mechanical damage, such as a PCB mounted connector separating from the PCB, or having cracked solder joints.
That makes sense, but I have a question.
If it was designed to be wired directly to the earth ground via the chassis, does that mean that the amp would not function if it was plugged into a two prong outlet through an adapter? Or if an outlet had a faulty ground? In either situation it seems like the amp would go crazy again.
That aspect seems weird to me, but maybe it's common. I could just be misinterpreting the evidence.
If it was designed to be wired directly to the earth ground via the chassis, does that mean that the amp would not function if it was plugged into a two prong outlet through an adapter? Or if an outlet had a faulty ground? In either situation it seems like the amp would go crazy again.
That aspect seems weird to me, but maybe it's common. I could just be misinterpreting the evidence.
It could be that the 0V PSU has disconnected from the amp part. Maybe you are just restoring that connection. Connecting to PE would only fix it if some other connected equipment is also connected to PE.
Do you have the schematic?
Do you have the schematic?
It turns out there was a standoff on the bottom of the board that was supposed to connect circuit ground to chassis. Whoever was in there before must've compromised that connection. I repaired it and it works great now.
Does that mean that without a connection to earth ground, this amp doesn't work? Is it inoperable with a 2 prong outlet? If so, is that a common characteristic of bipolar power supplies? Or is this amp just a bit weird? If anybody can shed some light, I'd appreciate it.
Does that mean that without a connection to earth ground, this amp doesn't work? Is it inoperable with a 2 prong outlet? If so, is that a common characteristic of bipolar power supplies? Or is this amp just a bit weird? If anybody can shed some light, I'd appreciate it.
Most likely answer is that your amp uses the chassis to complete the signal grounding of the circuit, for example grounding of individual circuit boards to all the jacks. It will operate OK if you use a 2 prong plug, but you would be defeating the safety design of the amp, which is extra bad in a guitar amp.