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233D Transmitter Restoration Project
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Kelly and I finished “conditioning” 6 of the 872As with the Variac today and things appear to be working well. Attached are images from the test. We need to check the “calibration” on the voltmeter, but I think it is pretty close. The red light on the dark image is from the pilot light on the box used to supply the filament transformers, not anything in the tube. The left rear tube does not show any lighting because there is an internal housing that prevents seeing the filament on that particular tube. We expect the MV to light up with more current draw and should provide some impressive flashes with voice peaks. The last image is some of the lash-up we are using to deflect some of the flying debris if things start coming apart. Two sheets of ¼” plexi that might not stop bullets, but should give us an opportunity to dive for coverage if things go bad.
Brought in a laser guided IR thermometer to check for heat on the bleeder and other elements. Highest temps on the HV bleeder is about 150 deg F after full power on for about an hour. Interesting part is that resistors on the edge of the board are about 40 degrees cooler than those toward the center of the board. The board is mounted on 1” conical ceramic insulators sitting on a ¾” chunk of plywood with no air movement. In the cabinet, the board will be mounted in the metal top of the rack and the fan is just about 6” to the rear, so I think heat should not be a problem.
I’m going to keep the “conditioning” kluge together until next week. I have a “NOS” 1943 872A that cost $22 shipped coming. I want to run it through the conditioning process so we’ll have a good spare. Our two spares are both unusable. Both have fairly large black material flakes on the tube envelope and when subjected to HV, some spectacular Arcing is observed. My understanding is that the condition gets worse and could result in destruction of the tube, but worse, could damage filter parts, particularly capacitors, especially oil filled types.
Testing high voltage and intermediate voltage filter networks is next. Both are choke input with fairly small oil filled caps on the output. Parts have been checked and I don’t expect problems, but applying high voltage is always a concern.
I’d like to apply high voltage to the modulator transformer while it is easy to access. High voltage is supplied to the CT of the primary to feed the modulator tubes and to one side of the secondary to feed the two output tubes (in parallel). Would like some suggestions about how to terminate the windings. I’ll make sure the transformer case is grounded because my main concern is winding to case shorts. Might be worthwhile to terminate the loose ends of the windings with a high impedance. Thoughts?
Jules got the Autotune motor remounted and is checking with Grainger to see if they have couplings for the shaft available. He has painted the case and replaced sleeving on the cable so that it looks as new. We should have 3-phase power restored to the racks in two weeks, so we can continue with the testing.
Lots of work left, but looking better every week! No predictions on when RF will be generated, but “ Summer 2014” might be reasonable!
Jim |
| May 28, 2014 |
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