Collins Aerospace Museum

Apr-28 ···
233D Transmitter Restoration Project

Started troubleshooting the 406B-1 audio power supply. Real simple, but doesn’t match with the schematic or the parts list. One of the chokes in the filter was getting extremely hot and the bleeder resistor was open. Schematic shows an 8 mfd oil cap on the high voltage line then a choke another 8 mfd, then another choke and another 8 mfd. It looks as though one of the chokes is open and there is about 352 ohms across one of the caps. I have some small chokes that should work and some tubular caps that should all fit under the chassis and a new bleeder, so I think I’ll sub those in for now. The tube is a 5Z3, but the parts list calls for a 5U4. I think they are pretty much the same, but I think the 5U4 handles more voltage, although I think the 5Z3 handles more current.

I had thought that the 175R-3 Tone control had its own plate supply because it has a 6X5, but turns out there is no plate transformer, so the 406B-1 is necessary to make the dial pulsing work. The 175R-3 also gets filament voltage from the 406B-1. If the CW side of the 175R-3 was populated, there would likely not be room enough for another transformer, so I guess it makes sense to have the separate supply.

Before I figured out I didn’t have any plate voltage, I was getting about 40 volts pp into the grid of the dialing amp stage, so we may be good as soon as we get the gain stages powered.

If I manually actuate the impulse (dialing) relay on the Telco chassis, the Telco relay advances properly and then homes when I actuate the homing relay. The two time delay relays seem to work properly, although I’m not entirely sure how they are used. The transmitter off relay seems to shut things down properly. Only thing I haven’t tested yet is the emission mode relay. And I haven’t tried the Autotune start and stop relays. But it does appear that the Telco chassis seems to be working. Next big step is to check the operation of the impulse relay with the dial signal.

The documents call for Gargoyle Arctic C light lubricating oil from Socony-Vacuum Oil company for lubing the Autotune motor and the Autotunes. Mobil has Gargoyle Arctic C oil available intended for use in refrigeration systems. It is synthetic now and appears to be like mineral oil. Have a feeling sewing machine oil would work fine, unless some has spare quart of Gargoyle Arctic C on the shelf.