Attempted AC power-up was not so successful, but was a good learning experience.
Fuse board silkscreen says F1 and F2, both 10 A 250 V. My PS arrived with F2 blown. The fuse was ceramic, so not easy to judge the category of overcurrent that had made it operate. I used glass replacements.Special tool 1: a nine-inch extension cord with male and female tab terminals
. Allows power-up with full visibility of main board and fuse board. A continuity check, all the way from wallplug end of the IEC cord, showed line and neutral reversed. Spellman would never do that, would they? Problem went away when I substituted a different IEC cord. First cord had been miswired at the factory, before final molding of the connectors.
With new 5 amp fuse at F2, and main board connected, DC resistance measured between wallplug blades was more than 10 k ohms each way. So no DC short circuit. Plug it in. F2 blows instantly, with small noise and light, leaving a couple of fused metal balls visible. I think that indicates a fault current at least a few times greater than nominal fuse value, but probably not 100 A, and certainly nowhere near a metallic short circuit current.
With new 10 amp fuse at F2, plugging in made F1 (as received) blow instantly. It's a BUSS GLH 10, with zigzag-shaped link. Two adjacent narrow points had opened, freeing one whole zig to rattle around in the tube. F2 appears to be undamaged. Did this just demonstrate the difference between fast-blow and very-fast-blow models?Special tool 2, made decades ago: my "ballasting outlet"
. A regular duplex grounded receptacle in a regular metal box, with a 3-wire cord and plug for use as an extension cord. Except the two outlet sections are no longer wired in parallel, they are in series. Front plate is marked to show which wide slot is Neutral, which narrow slot is Hot, and which left and right slots are connected to each other. I tested this by plugging in two 300-W halogen worklights & getting dim light from both.
When I powered up the DXM70 with two new 10A fuses, and one 300-W lamp in series, the lamp came on & stayed on at almost full brightness. I forgot to measure the voltage
drop across the DXM. Am suspecting a semiconductor problem, though as mentioned before, nothing looks or smells burned.If the circuit board assembly isn't repairable, I would keep the unit just for the bottom half
. Probably no active components down there, just transformers & capacitors and diodes. Connections from the upper half are a bundle of 4 similar black wires, a bundle of 2 similar black wires, and a cable of 5 colored wires to J86 "FIL FDBK". Would building a driver from scratch be much harder than building a turbopump motor driver from scratch, as reported here many times?