Hi all,
I'm chasing an issue in one of my Pro Jrs relating to the power supply section. The amp was very reliable until it pooped out on me at a recent gig. I found a fried resistor that was factory installed at half the power rating it should have been. It worked fine at home for some time but in accordance with Murphy's Law, as soon as it went out to a gig it pooped again during sound check.
I'm testing my voltage test points with the amp powered & with all the tubes pulled and the speaker hooked up. According to the schematic I should be reading 319VDC right after the rectifiers, but I'm seeing 342VDC. Not only am I reading that voltage right after the rectifiers (in front of R32, a 2.7k @ 2W) but on the other side of R32 as well where it should read 294VDC. In fact, I'm getting at least 338VDC on the positive side of all 3 electrolytics in my power section.
Is this normal because I'm testing without tubes in it? I'd think those resistors in the power section should still drop the voltage along the voltage path after each one of them.
I guess this is the kind of thing you have to figure out when you're teaching yourself this stuff vs. going to school for it. Any help is appreciated.
This one (of my 2 PJs) is getting very close to a gut & scratch rebuild into an 18w or 5E3 or something with Garolite instead of a PCB. I'd like to give this one more shot at simply repairing it before I start turning it into a parts catalog.
Cheers,
- JJ