Well, unfortunately, my fingers are still crossed, but no joy so far...
Everything appeared to be complete and in order, so a couple of days back I figured it was time for the flame test. Using a variac and a dim bulb tester, I tried bringing it up to voltage, but the 250w bulb refused to dim. Poking around, I found that the 16awg power cord that I had installed was apparently shorted. Don't know where that cord came from, but the hot leg read as open and I'm guessing it was internally shorted since the bulb was at full brightness. Since I tend to never throw anything away, this was just some random cord out of a bin of stuff and appeared to be relatively new. Anyway, it was odd that it was faulty, but dummy me for not checking it first.
So, new cord, DBT dims, and I set the bias. So far, so good. Things seem stable, so I hook up some speakers and a preamp and get nothing.
Start checking voltages to the boards, and all look good until I get to the buffer board, and find that I had completely overlooked the power leads from the transformer. Not ashamed to admit that this was a complete dumbass vapor lock on my part.
Guess some times you just can't see the forest for the trees.
Cobbled together some new leads to the buffer board, powered it up, and still no joy. Started checking fuses and found that number 602 was blown, one of the 38v inputs from the transformer. Installed a new fuse and it blows again. No fireworks or magic smoke, just opens up. Coincidentally, this leg corresponds to the snipped off leg of the 1S1850 rectifier on the board. This modification was the same as the buffer board from the parts carcass, and also the same as was noted by our member Hyperion in post #15 here:
http://audiokarma.org/forums/index.php?threads/ba-5000-for-repair.545595/
and also an earlier thread of mine:
http://audiokarma.org/forums/index.php?threads/ba5000-buffer-board-variations.594679/
This was the board as found, with the cut trace corresponding to the cut leg of the rectifier:
and some of the ancillary components of the underside:
I thought I was pretty meticulous in matching what I found, but maybe not. I've pulled the board (kind of hurts to dismantle again, but oh well) and will start going through the work I've done, and start trying figure out the circuit as modified. My comprehension gets challenged past a certain point, so I'm posting a copy of the pertinent part of the schematic :
and as soon as I can, a copy indicating the mods that were on mine in the hopes that someone can suggest what might be the problem.
And Hyperion, if you're reading this, I am a little unclear from your postings regarding the buffer board mods that you encountered whether you rebuilt as you found it or as per the schematic. I figured that since these mods had appeared on multiple boards that they were a factory mod and that it should be rebuilt as found...
That's the story so far- a bit of a setback, but at least other than the buffer board quandary the rest of the unit has kept its magic smoke inside and the bias dialed right in.
A couple of other questions to throw out there...
I am seeing a fairly high Dc offset, around 60 mv, and if that holds once the buffer board is up and running, how concerned should I be? I've read that some pretty high offsets can be the case in designs that don't allow for it to be adjusted, but I'm not sure what to expect from this amp.
Also, regarding the the 15A "midget" fuses in the speaker outputs and the mains input, should the mains be a "normal" blow and the speakers be "fast"? Between the mixture of fuses in this and the parts unit, there seem to be some of the original Toyo fuses, but 2 different kinds. The mains fuse was marked with a number that I found a reference to as "normal," but the speaker fuses have no identification other than voltage, amperage, and brand. Any thoughts on this?
Thanks in advance for any suggestions-