leigh
Well-known member
Hey y'all,
I've been refurbing a Harmon Kardon all tube mini PA. Haven't gotten to recapping the PS yet, just cleaning up the audio path (taking out old crusty pots, unnecessary coupling caps, etc). I wired up a 1/4" jack for a DI and tried playing bass though it. It worked, but I was noticing that if I slid my fingers down the strings, there was a crackling noise like static electricity coming out the amp.
Suspicious of a ground loop problem, I measured the chassis voltage to ground, AND IT SHOWS A HUNDRED AND TWENTY SOME ODD VOLTS!
I immediately unplugged everything and thanked my lucky stars I didn't fuck myself up.
Here's the technical picture so far. No schem to be found for this beast, sorry:
1. The power cord is 2 prong. I got a 3 prong to replace it with, but I'm wondering now if I ground the chassis if I'll just be blowing fuses.
2. The power supply does use a big transformer. It's not one of those scary old transformerless deals where the chassis can sit at line voltage by design.
3. The PS rectification is done with only 2 diodes, not 4. On one of them, the lead to ground is quite blackened, I'm wondering if maybe the diode is leaking to ground. With the unit unplugged, a meter reading shows a 0.5 V diode drop, seemingly OK, but maybe when the real juice is applied there's a breakdown. ??
4. The PS filter caps consist of 4 in a multi-cap can, and 1 other one that's got a cardboard casing. Could these be leaking line voltage to the chassis?
5. There's no continuity measured between either side of the power cord and the chassis.
Thanks for any tips for dealing with this. I'd be happy to run some further tests if that'll help provide a clearer picture.
Leigh
I've been refurbing a Harmon Kardon all tube mini PA. Haven't gotten to recapping the PS yet, just cleaning up the audio path (taking out old crusty pots, unnecessary coupling caps, etc). I wired up a 1/4" jack for a DI and tried playing bass though it. It worked, but I was noticing that if I slid my fingers down the strings, there was a crackling noise like static electricity coming out the amp.
Suspicious of a ground loop problem, I measured the chassis voltage to ground, AND IT SHOWS A HUNDRED AND TWENTY SOME ODD VOLTS!
I immediately unplugged everything and thanked my lucky stars I didn't fuck myself up.
Here's the technical picture so far. No schem to be found for this beast, sorry:
1. The power cord is 2 prong. I got a 3 prong to replace it with, but I'm wondering now if I ground the chassis if I'll just be blowing fuses.
2. The power supply does use a big transformer. It's not one of those scary old transformerless deals where the chassis can sit at line voltage by design.
3. The PS rectification is done with only 2 diodes, not 4. On one of them, the lead to ground is quite blackened, I'm wondering if maybe the diode is leaking to ground. With the unit unplugged, a meter reading shows a 0.5 V diode drop, seemingly OK, but maybe when the real juice is applied there's a breakdown. ??
4. The PS filter caps consist of 4 in a multi-cap can, and 1 other one that's got a cardboard casing. Could these be leaking line voltage to the chassis?
5. There's no continuity measured between either side of the power cord and the chassis.
Thanks for any tips for dealing with this. I'd be happy to run some further tests if that'll help provide a clearer picture.
Leigh