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Apr-16 ···
233D Transmitter Restoration Project

Mike finished installing the MBOF Tone control so we could look at various I/O points on the assembly. Because of his removal of some of the undocumented changes and reconstruction we now have a fair understanding of how the system works.

The documentation says 50 to 80 volts AC for the dialing pulses (we’re assuming RMS) so we set the controls to 70 volts and that appears to make the Telco selectors work well.

We were also able to verify that the carbon mike circuit appears to be working and that the Turner hand mike is good. The audio level meter on the modulator panel shows some deflection with voice peaks, so ever that looks good. We do need to verify the wiring on the meter plug because the schematic appears to be in error. Next step will be to track audio through the speech amplifier/mod driver.

I think we are having problems with the channel select switch. Previously, we had been repeatable when selecting channels between 0 (10) and 6, but channels 7-9 sometimes made the Autotune motor run continuously. Jules determined that the contacts on the channel select wafer of the exciter assembly that sends the signal to the Telco assembly that the proper channel has been selected, were not making good connection. There appears to be coating on the contacts that de-Oxit will not clear. Rod had to replace some wires with charred insulation near that wafer, so I expect carbon released during the “charring” might have resulted in high resistance conditions. The wafer needs to be replaced, but it looks to be fairly unique, so I expect it is unlikely that we will find a replacement. So we will have to attempt to find a more aggressive method to refurbish the contacts. We can work around the problem, but when we want to fully use the Autotune system, we will have to make it work.

Next step is to get the exciter oscillator and multiplier stages going. That’ll be the focus next week.