Unloaded PS voltage reading weirdness?

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

The voltage readings on schematics are (unless it tells you otherwise) the normal voltage with the power supplies loaded at the proper impedence. When you disconnect elements of a circuit, such as removing the tubes it may change the voltage you see at a specific point in the circuit. If I could review the schematic I could better tell you how pulling the tubes will effect the circuit, but I would expect power supply voltages in a tube ampt to be a little higher with the load removed.

Schematic is right here.

Cheers,
- JJ

Hey JJ - don't concern yerself too much with unloaded PS voltages - and every modern Fender product we've been inside lately has higher than spec voltages when they're running properly anyway - I don't know why Fender bother publishing those figures... they're useless... you know I'm gonna suggest the garolite route but good luck with getting it sorted... :D

Quote:

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you know I'm gonna suggest the garolite route

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Yeah, I know. :laughing:

I've fixed it once and since I have 2 of these puppies plus a perfectly good HRDx I think I'm just going to gut it for a new build. I just thought I'd be doing the first hardwire conversion on the HRDx, not one of the PJs!
:rip

In fact, I was just doing a mock-up for a layout a little while ago using the Dreamtone PJ layout minus the tube rec and using the original power caps instead of cap cans. Weird thing about this particular amp is that the schematic shows four 22uf caps in the power section but both my PJs only have a pair of 22uf and a single 47uf. Took me a while to figure out why there's no C16 in either one of my Pro Juniors. Then it hit me ... paralleling sums capacitance. Since 2 x22 = 44 and at 20% tolerance 47 is quite literally "Close Enough For Rock & Roll", there is no C16. :arhhh:

Yer well - it'd help if they made 'em they way they're drawn - with a bit o' luck you might have the choice of makin' a 20W 5E3 outta that HRDx when she dies - but I reckon you've got the right attitude towards it all - and anyway - once you get these things right you just don't worry about 'em anymore... TJ's really into the copper caps but Ted insists on using FedEx (who I refuse to use for a very good reason that I've already had a whine about here) so I make my own version which by and large work fine and gives you a reasonable facsimile of the feel of a tube recto in terms of how the amp responds to your string attack even if it doesn't - IMO - make it all that 'brown' sounding... it's better than a straight silicone rectifier for sure... good luck with it any road :D