KB100 blue stripe hum defeat mod

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dak
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KB100 blue stripe hum defeat mod

Post by dak » Thu Mar 30, 2023 10:36 am

This concerns the blue stripe model and is sort of hard to believe. I have an overland line running close to my house, so I get to test the resilience of any amp against external fields involuntarily. Just got myself a KB100 and its base hum level was intolerable to me. Plugging into the power amp input silences it, EQ controls and volume controls don't affect it. So the hum must be sort of a ground loop issue in the final preamp stage.

Taking out the circuit diagram shows that this final preamp stage actually has a ground loop compensation circuit (R130, R131) that should be about on-spot if the reverb is fully on or fully off. Except that R138 is connected to the wrong ground type. You won't notice with a DC measurement: after all, the grounds are connected. It turns out that the PCB faithfully follows the circuit diagram and goes to some effort to route a ground line catching hum from across half the circuit board.
Peavey-KB-100-12-89-Schematic-crop.png
Peavey-KB-100-12-89-Schematic-crop.png (58.45 KiB) Viewed 1987 times
Bummer. Disconnecting R138 from its humming ground line and tapping the right ground from nearby R129 can be done without even removing the PCB and solves the serious hum problem by making the hum compensation work as intended.

Necessary parts for this "mod": a piece of wire and a bit of solder.
P1150462.JPG
P1150462.JPG (1.61 MiB) Viewed 1987 times
Weird that this one slipped through the cracks, but then not everybody has an overland line a few hundred yards from his house (don't ask my opinion about single-coil guitar pickups...).

P.S.: a second scan over the circuit diagram makes it appear that the grounds on the reverb tank input circuit (R109 and R114) should be of the input ground type. The reverb tank itself is a perfect ground separator; so its output circuit ground can be left as-is. I'll probably tackle that one when switching out the respective opamp for an LM4562 (low-noise low-impedance input and solid output).

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