Collins Aerospace Museum

233D Transmitter Restoration Project

I made sure the crystal switch was on channel 4 by seeing the 51K resistor in the 837 grid circuit. When I keyed the transmitter, I still go no indication of the oscillator starting. Jules had built an adapter/extender for the 837 and we brought wires out for the suppressor and screen grids. I have expected to see -190 VDC blocking on the suppressor grid when unkeyed, but I just see a few millivolts. I have a breakout box in the keying relay circuit, that will let me verify that the relay is being energized. When the rig is unkeyed, the relay contacts ground one side of the bias supply and should feed the both the 837 suppressor and the 807 control grid with the negative voltage, cutting off both tubes. When keyed, the relay opens the ground on the power supply output and the suppressor and grid should get close to zero DC. We have grid and plate current meters for the 807 and neither of them deflect when the radio is keyed. Before the keying bias transformer blew out, I did see some indication on the 807 grid current meter when the radio is keyed. But none since the transformer has been replaced.

Jules noticed that the exciter tuning Autotune is not getting the stop set correctly after the motor stops. I have only been concerned that the pawls were correct for the selected crystal and had verified that by checking for the grid resistor at the crystal socket. We noticed that the exciter tuning Autotune is turning much slower than the other unit on the exciter and that is the one that is not getting the stop set correctly. Each Autotune has a slip clutch behind the panel, so we think there is mechanical binding somewhere in the couplings that is preventing the Autotune from completing the tune cycle.

We have decided to pull the exciter and remove the front casting to determine where the exciter tuning control is binding. While the exciter is out, and the front casting is off, I’m going to power up the 837 and 807 and make sure those two stages are working before we reinstall the exciter.

We are also seeing binding on one of the Autotunes on the PA output network assembly. So what we learn chasing the problem on the exciter should help us solve the PA Output network problem.

Hoping we don’t find any unobtainium parts that are damaged beyond repair.