Hi. I was upgrading an Ampeg b100 solid state amp. I was going to replace some capacitors and resistors. Nothing difficult. I did the replacements and turned the amp on to discover a loud 120 Hz buzz. I was perplexed do to the simple procedure I was performing. I double checked to make sure all correct values were in the correct places with the proper orientation. Checked out good.
Attached is a schematic of the amp.
I was poking around inside the amp while powered up, with a wooden stick, and discovered that part of a transistor stage had generated a electrostatic field. I had replaced C11 and when I touched any part of it I got a much louder buzz like I was touching hot line in to an amplifier. This is with a wooden stick. All parts that are in this junction with the base of Q2 have the same buzz when I touch their lead connected to the junction. Only C11 and C13 buzz when touching the actual body of the part. I switched C11 back to the original part thinking something was wrong with it. No change. Still the same buzz.
I discovered that an electrostatic field was being generated that extended about one inch out from the capacitor. When I would bring my finger into this field the buzz would increase the closer I got to it. This field is picking up the nearby power supply and amplifying the hum.
The volume control did nothing to change the hum so the problem was after the pot R15. Thought maybe one of the parts I replaced were bad so I systematically switched the original parts back one at a time checking for hum each time until all the parts after R15 were back. Problem still there. I pulled C13 out to check its capacitance and it read like 230 pF. Q4 get pretty hot like it needs a heat sink which I don't think is right.
The DC voltages match up with what is on the schematic and the amp does make sound if I plug a bass into it.
Do transistors ever cause anything like this?
Thanks
Scott
Attached is a schematic of the amp.
I was poking around inside the amp while powered up, with a wooden stick, and discovered that part of a transistor stage had generated a electrostatic field. I had replaced C11 and when I touched any part of it I got a much louder buzz like I was touching hot line in to an amplifier. This is with a wooden stick. All parts that are in this junction with the base of Q2 have the same buzz when I touch their lead connected to the junction. Only C11 and C13 buzz when touching the actual body of the part. I switched C11 back to the original part thinking something was wrong with it. No change. Still the same buzz.
I discovered that an electrostatic field was being generated that extended about one inch out from the capacitor. When I would bring my finger into this field the buzz would increase the closer I got to it. This field is picking up the nearby power supply and amplifying the hum.
The volume control did nothing to change the hum so the problem was after the pot R15. Thought maybe one of the parts I replaced were bad so I systematically switched the original parts back one at a time checking for hum each time until all the parts after R15 were back. Problem still there. I pulled C13 out to check its capacitance and it read like 230 pF. Q4 get pretty hot like it needs a heat sink which I don't think is right.
The DC voltages match up with what is on the schematic and the amp does make sound if I plug a bass into it.
Do transistors ever cause anything like this?
Thanks
Scott
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