Collins Aerospace Museum

233D Transmitter Restoration Project

The filament transformer (gray on the left side of the chassis) is upside down because it obviously was a replacement. It was “floating” on top of the chassis when we got it and was wired with spliced leads. I used a mounting plate mounted on hex standoffs to adapt it to the original mounting holes. I’ll paint the plate at some point, but for now it is “functional”.

N as As wired, the supply didn’t agree with the two versions of the schematic. And doesn’t agree at all with the parts list. The schematics show 4 wire wounds in the bleeder network with two of them tapped apparently to function as a divider, but the unit has 4ea 500 ohm resistors with the one attached to ground having 3 taps and the next one next up having one tap. I’ll need to trace to determine where the other two taps go. The bias interlock relay appears to be wired as expected except that the poles are paralleled which is no problem, but not shown that way on the schematic.

The other relay is somewhat of a mystery for now; a note on the schematics says it was not installed on the unit at “EJ”. Since our transmitter has the relay, it was apparently not the one at “EJ” at least when the drawing was done in 1950. I was at McClellan AFB when the Sacramento Airport moved to its new location in the rice paddies west of town. Don’t remember what it was called, but pretty sure it was not “EJ”. So I think it is likely there were at least two similar transmitters. Hoping that some our Western Airlines fishing expedition will result in a big catch.

Larry brought in the high voltage cable to finish the connection between the plate transformer and the high voltage rectifier. He and Mike are working on getting all the cable routing done between the power cart and the power bay, so guess I’d better get to work on the high voltage bleeder network and the high voltage filter. No luck on finding the values we need from Vishay-Dale, so I’m going to rebuild the board to 8ea 10 watt ceramics for each of the 80 watt resistors we need. I am planning to increase the total bleeder resistance to 160K from the original 120K to reduce the heat. That’s still a pretty heavy load and I don’t expect it will impact the regulation.

Jules completed clean up on the 103C-4 relay and power control chassis. (AKA Telco relay chassis). This is somewhat of a mystery because the 103C-4 is commonly referred to in the documentation as the unit used in the “CW” bay. The 103C-2 has four rectifier tubes for 48 volt relay supply and our chassis has no tubes. When installed, the chassis does provide filament control to the RF bay, so that part is working. Next step will be to check dial pulsing levels from the Tone control chassis in the modulator bay to make sure we are getting dial pulse amplification. The rotary dial is fed from a rheostat across the 6 volt AC (60 cps) and then sent through filters and some gain stages to get it to about 50 volts RMS. The tone control chassis gets the filament (and dial source voltage) from the Audio power supply in the modulator bay, but has its own transformer/rectifier for plate voltage.