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233D Transmitter Restoration Project

On Wednesday, Jules and I pulled the low voltage power supply to troubleshoot the loss of PA screen and plate voltage on the oscillator and driver tubes. We could find no low resistance problems and the capacitors appeared to be good. The choke that had heated appears to have the correct resistance with no obvious shorts. One of the taps on the biggest power resistor in the bleeder/divider chain had a high resistance, Jules was able to clean the contact and it looks good now. Jules thinks the choke was intended to be a swinging choke because of the tap shown on the schematic. However, it looks as though the tap is no longer being used. Mercury Vapor tubes (866As) were originally used, but our supply had 3B28s, so that is likely why the change was made.

Today, I measured the resistors in the bleeder/voltage divider network to determine the network current and figure out what elements in the exciter used the various taps.

I then inserted some of Mikes pins in the supply socket to see if I could see the excessive load source. Everything looked good except the 807 and 837 plate and feed for the 813 screen resistor. That line measured 468 ohms to ground. With 660 volts no load on that line, more than an amp would be required. So that is likely the source of the fault. Pulled the exciter and the low impedance was still measured. Two terminal strips (AN and N) between the exciter and power supply connectors, so determined that the wire between the N strip and the power supply connector has the problem. The bad wire has an undocumented branch circuit so well need to run that down (and is likely where the fault is), but should be a straightforward fix. Im going to use the single phase supply to check out the low voltage supply before we return it to the rack. Jules plans to bring in a bridge so we can measure the choke impedance, as well.

Encouraging because I did see indications of oscillator activity before the supply shut down. After the fault gets cleared, Im fairly confident well be able to start through the RF stages before the end of the month.