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Too much hum


Richard Klatt (rklatt)
Username: rklatt

Registered: 11-2006
Posted on Monday, November 13, 2006 - 03:35 pm:   

I've been working on a model 50-RD, chassis number 1650-RD-DOM. This is just a head, no speakers. Anyway, a friend of mine bought it on e-bay, and gave it to me to fix up. Originally it had a lot of buzzing and sporadic popping noises. I replaced the tubes, the JE1692 driver transistors, and all the electrolytic caps. I also soldered wires from the ground side of the input and output jacks of the reverb tank to the tank itself. The amp is now very quiet when nothing is plugged into it, even with the volume turned up. But when I plug my guitar into the amp and turn it up even a little bit, there is a lot of 60 Hz hum. I've checked all the ground connections. they seem to be good. On the schematic, it shows that the low side of the input jack is connected to circuit ground, but not chassis ground. In reality, it is connected to chassis ground. Is this right? What else could be causing all the hum? This guitar is quiet in other amps.
Steve Kennedy (admin)
Username: admin

Registered: 03-2002
Posted on Saturday, November 18, 2006 - 06:57 pm:   

If the amp is otherwise functional did you check the settings for the dirty channel (Volume, Gain and Limiter Switch)? If you turn the limiter channel off with the front panel switch or the footswitch it should be effectively grounded out and all you will hear is the clean side of the circuit.

If the 4164 CMOS switch chip used to do the Limiter circuit switching is bad you could be getting a configuration that normally cannot exist in a working amp and this could be responsible for your hum.

Just the first thought that came to mind...