233D Transmitter Restoration Project
Mike and I got the cable from contactor to the power bay connected and now have front panel control of the filament system. Green light comes on, big relay hums and then goes off as it should when the off button is pressed. Next step is to connect the power bay to the RF bay and bring up the filaments for the final PA tubes. That will also supply 3-phase power to the Autotune motor. Then weâ€™ll connect to the Modulator bay and power the filaments for the big Mod tubes.
I sprayed all the contacts on the 103-4 Telco relay chassis that is pretty much the control center for the Autotune system. Jules took the chassis home for some additional cleaning. Jules: one of the relays has a broken lead from the contacts to the screw terminal. Should be obvious; we need to replace the wire with something flexible enough to all contact movement.
I stopped by Iowa Radio to pickup a can of deOxit to continue to clean the hundreds of dirty contacts; unfortunately, they have not stock for about a week. While there, I talked Warren about availability of large wirewound resistors for the HV bleeder network. As I had thought, 80W resistors at 50K are extremely hard to find. He is doing a search for us, but thinks it will have to be old stock. It looks as though we will have to series/parallel 10 watt ceramic resistors to get the required resistance. Problem is that 27K is about the largest R value in the 10 watt power rating. To get to the 300K total bleeder resistance in the original circuit, about 80 resistors will be required to handle the power. The 10W resistors are about a dollar each, but Iâ€™m not sure we have enough room to mount a board with that many parts. Warren will check with suppliers to see if they have some recommendations. Wonder what the HF-8020/8030 uses for bleeders? I expect the 233D bleeder may load the plate supply heavily, and that may be necessary for good regulation with the LC filter and relatively small caps. Any thought would be appreciated.
We also need some recommendations of cooling for the RF bay. The power and modulator bays both came to us with fans on the back panel, but no fans were on the RF bay. The drawings show fans on all three bays, so we are missing something. We will need to work something out before we try to do any full power tests.
| February 27, 2014