233D Transmitter Restoration Project
Iâ€™m trying to add the warm glow of the 6 872As in the HV rectifier but Iâ€™m running into a problem. In spite of the fact that I took pictures, made drawings and labeled wires when I tore the chassis down Iâ€™m having trouble getting the thing fully back together. I was careful when I pulled the wiring harness to make sure it would go back in the same as it came out. Problem was that one of the filament leads didnâ€™t quite get to the terminal that remained after I had reattached all the transformer terminals to labeled wire. Besides coming up short, it was also clearly labeled with "2". Problem is that the only "2" is on the primary side and this was clearly not going there. I sat it aside thinking that my photos and drawings would help later. Much later (yesterday) I started getting ready to put the chassis in the rack to make sure the rack power was turning on the filaments. When I started tracing wires, I got a bit of a shock; not electrical, but a surprise. The schematic shows all of the filament windings have center taps; nine terminals per transformer. But we only have 6 terminals per transformer. The plates of three of the tubes go to ground and the center taps of those tubes go to the plates of the other three tubes and to the three input lines. Then the center taps of the other three filament windings are tied together and provide the HV DC output. So in our system it appears that one side of the filament is used instead of the center tap. I suppose that might work, but every other drawing I have seen that used the MV tubes in a full wave voltage doubler circuit used the filament center taps. Iâ€™m fairly sure I have seen other rectifier filaments that didnâ€™t require center taps, but I need to do some looking.
Another mystery with the old beast; schematic shows one thing; as wired is somewhat different. Good news is that so far, no arcs and sparks. But Iâ€™d like to keep it that way, so would appreciate some other eyes looking at the wiring.
Iâ€™ll hold off doing any more until next Wednesday unless someone wants to stop by earlier.
Have some feelers out for the big HV bleeder resistors from Vishay Dale Milwaukee Power resistor division, but havenâ€™t found stock yet. Iowa Radio doesnâ€™t appear to have a relationship with Vishay, but Allied does and they have a Cedar Rapids office. Good news is that Dale has a fairly current data sheet, but likely lead time will be months and the price may be high. Hope to get that nailed down next week.
I am encouraged with how things are working out. Still lots to do, but so far no major problems.
| March 20, 2014