Quote:
The fuse and the fuse holder are both a comfortable distance away from CR1A...
Just throwing this one out there. Could one or more of the rectifiers (DR1A-DR1D) be toasted from the previously backwards install of the first two? And if they are toast, could they be tripping the fuse?
Still reading approximately 26.5V at the power supply
-Chris
EDIT: Tom - All TO-92s, MOSFETS and QB1 are installed according to layout
No, I do not believe toasted rectifiers could cause a short. Rectifiers are very simple devices: either the current goes through them, or it doesn't. If it doesn't, then the heat will build until something blows. In your case, it was the fuse. So, honestly - I would not suspect the rectifiers. Besides, you measured 34.5VDC coming from them, so they're working just fine.
26.5VAC from the walwart is also good - just about right without much of a load. So, I'd say we can maybe dismiss any issues with the power supply. The tube circuit is very simple - only a few resistors, the tube sockets, and the CCS transistors and resistors - plus the heaters and resistor. IOW, it's very unlikely
anything in the tube circuit could cause this kind of short.
That's why my last post was focusing on the buffer. If you have a short in the buffer, it can do all of what you've experienced - high current, blown fuse, etc., etc. In my experience, something like this is usually found to be one of the power output transistors or MOSFETs (depending on which type of MAX it is) that are shorting out - maybe one of them is swapped with the other kind, or someone put them in backwards, etc., etc.
Jake's idea is a good one and worth trying just to completely eliminate the power supply as an issue. It's fairly simple - just melt the pad to one of the leads of the polyfuse - PF1 - and remove that lead. That will completely isolate the power supply from the rest of the PCB. If we can measure and adjust to something around 27VDC, then we can completely eliminate the power supply as any cause of the problem. To check this: if you remove one of the PF1 leads, simply touch your V+ DMM probe to the inside pad of PF1 (toward the VREG heat sink) and the other probe in GND. If you can measure something in the 20-30VDC range and then adjust it to ~27VDC with the trimmer, then the power supply section can be completely dismissed and any source of the problem.