233D Transmitter Restoration Project
With Rodâ€™s encouragement, I connected the big 3 phase Variac to the output from the contactor assembly and it appears to work well. All three outputs adjust from near zero to 210 volts and the old iron doesnâ€™t groan or smoke so it must be wired correctly. Mike checked the 460 number and the Variac was purchased in 1964 for $470; it likely has been used on lots of great stuff for the last 50 years.
Kelly managed to get the big choke from the high voltage network extracted from the power bay with comparative ease; I had struggled with it for some time, but his nimble fingers are obviously better than my short fat old digits. Weâ€™ll use it along with the balance of the HV network parts to mock up the circuit to support testing the high voltage system outside the cabinet.
Next week, we plan to connect the Variac output to the plate transformer and check the AC output with the Jennings voltmeter before we attempt to connect the HV rectifier. The main purpose for using the Variac is to slowly bring up the voltage on the 872A Mercury Vapor rectifier tubes to condition them to prevent flash-over that could be destructive to the tubes and other components. After that, we will reinstall everything back in the power bay and complete HV checkout. If things go well, we expect that to be complete by the end of the month.
Mike and Kelly used a function generator to pass an audio signal through the big modulation transformer. The transformer appears to pass the audio signal without distortion and the level appears to track what we expect given the I/O spec. Mike observed that the response was fairly broad, so it is likely that this transformer could have been used for broadcast work. Mike will bring in a 4K load to see how that works.
Jules took the end cap off the Autotune motor to inspect the bearing surfaces and it really looks better than expected. He is planning to lube the bearings and replace the sleeving on the power lead.
| May 7, 2014