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Apr-24 ···
233D Transmitter Restoration Project

We finally got some smoke!
 
Mike finished the power contactor cable and we were making sure all the relays were working. Everything appeared to be working properly, but after a couple of sequences, Mike spotted some smoke coming from the tune level contactor. I had noticed some black substance on the coil, so I assumed that it was probably a shorted turn. But there was no evidence of heating, so we cycled it a few more times and Mike said he thought the “smoke” was rusty appearing. I sprayed a small amount of the radio man’s friend (de-Oxit) and that seemed to stop the “smoke”. Both contactors make quite a lot of noise as did the big filament relay. It looks as though there is some rust/dirt between the metal surfaces that may be preventing complete closure. With the cover on the contactor, likely won’t be a major problem, but we really need to see if we can get some relay experts to provide some assistance.

Mike’s next project will be to inject an audio signal into the modulation transformer and do some general resistance checks to try to make sure nothing major is wrong. The schematic shows the center tap of the primary (modulator tube side) goes to one side of the secondary. Seemed a little odd, but I expect that the high voltage goes to the transformer primary to go the plates of the modulator tubes and then goes to one side of the secondary where it is modulated before it goes to the plates of the finals in the RF bay. My previous experience with plate modulation was in early 1957 when I built a 6L6 modulator for my Lysco 600. I used a separate supply for the two modulator tubes.

I had always thought the two 450TLs in the RF PA were operated push pull, but it appears that the plates are tied together, so they must be in parallel.

Jules and I started going through the Autotune circuit to see why the motor runs constantly. There is apparently a limit switch in the exciter that stops the motor when the bandswitch in the exciter matches the position of the telco relay. There are 11 relays on the Telco chassis, so there is quite a lot to learn about how they all “ function. Jules is comparing how the Telco system works in the 233D with that of the 231D-20 and TDH-2 transmitters. There are some “mysteries” to solve: for example four of the relays are listed as having 48 Volt DC coils, but I have yet to find the source for 48 volts DC. But, since the filament start relay works, it must be getting the voltage from somewhere. Good news is that the schematic matches the chassis that we have; bad news is that the chassis is for “CW”. Don’t think that is a problem.

Really would like to get back on the Tone Control chassis, but think it may be necessary to troubleshoot the problem on the audio system power supply first because I expect the Speech amplifier plays some role in dial pulse formation.

Jim