Emperor-TK
Well-known member
My friend just gave me his silverface Champ to look at, since he was getting shocks while playing. Any advice on what to do would be appreciated, here's what I found so far:
1. I measured 120V AC on the chassis with the ground capacitor on the AC input connected, and 95VAC with it disconnected.
2. There is a three prong cord, but the ground leg is broken, so the chassis is not grounded. Hot and neutral lines feed the power transformer, neutral does not contact the chassis (only through the previously mentioned capacitor). There is no virtual center tap (with 100R resistors) on this amp.
I looked and probed, but I can't seem to see how the AC is getting into the chassis. Could it be because there is no earth reference to the power transformer secondary, which only references the chassis, which then floats up at 120VAC relative to the rest of the world? I'm afraid if I just put on a new three prong cord, I might be shorting line voltage to ground.
Also, what's the capacitor on the AC input doing, filtering line noise to ground? Should I just snip it out? I remember in a previous electronics safety thread that the general consensus was these could be dangerous. I know these questions tie into that thread, but I still don't completely understand how a chassis gets hot.
Thanks,
Chris
1. I measured 120V AC on the chassis with the ground capacitor on the AC input connected, and 95VAC with it disconnected.
2. There is a three prong cord, but the ground leg is broken, so the chassis is not grounded. Hot and neutral lines feed the power transformer, neutral does not contact the chassis (only through the previously mentioned capacitor). There is no virtual center tap (with 100R resistors) on this amp.
I looked and probed, but I can't seem to see how the AC is getting into the chassis. Could it be because there is no earth reference to the power transformer secondary, which only references the chassis, which then floats up at 120VAC relative to the rest of the world? I'm afraid if I just put on a new three prong cord, I might be shorting line voltage to ground.
Also, what's the capacitor on the AC input doing, filtering line noise to ground? Should I just snip it out? I remember in a previous electronics safety thread that the general consensus was these could be dangerous. I know these questions tie into that thread, but I still don't completely understand how a chassis gets hot.
Thanks,
Chris