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Sep-9 ···
233D Transmitter Restoration Project

 
Failure to turn on any of the “plate” voltages to enable tune or operate has been traced to loss of bias. Bias voltage must be present before most of the operating voltages can be turned on. So that part is working as it should. I had thought that the bias supply came on with filament voltage, but that does not appear to be the case. It appears that when any of the operating modes are selected, the bias supply comes on and then the low and high voltage supplies can be brought on. The rectifiers on the bias supply were originally 866A mercury vapor tubes, but those were replaced with 3B28s. I prefer to stay with the 3B28s because the MV tubes are somewhat more troublesome. Jules looked at the 3B28s in his tube tester and we think they are likely good, but we really need to investigate more. The low voltage supply also uses 3B28s and we had to select tubes in that supply to make it work properly. Interesting part is that the bias supply has been working well since it was rebuilt after the chassis was re-plated. It doesn’t appear that there are any system load issues, but the next step is to remove the supply and power it up from the single phase source and check it more thoroughly.

Also critical at this stage is to determine why the Test mode is not coming up. Expect a control wiring or relay issue. Doesn’t appear to be related to the bias supply problem.

We need better documentation on the power control sequence. We got lucky when things came up properly, but now we need to have a better understanding help troubleshoot. I think the documentation describes the sequences, but there are some lack of clarity issues that need to be resolved.

Jim