233D Transmitter Restoration Project
Bias supply appears to work well although the voltage may be a little low with regard to the documentation about PA tube grid requirements (-600 to â€“800 volts), but the lower voltages appear to be correct.
But the real issue with the bias supply is that the chassis is really in bad shape and the rectifier filament transformer has been replaced and the â€śnewâ€ť transformer is just sitting on the chassis on long leads. So we either need to find a new transformer or fab a new mounting arrangement for the existing transformer which appears to be adequate. Replacement transformer may be difficult to find because of the 220 primary.
The audio power supply appears to be functional, but it appears that the output capacitor may be shorted because the second inductor gets very hot and the output voltage is near zero and the input to the inductor is about 150 volts versus an expected of at least 350. Rectifier tube is a 5Z4, whereas the parts list shows it as 5U4 G. Other issue is that documentation shows an additional 6.3 VAC filament transformer on this supply that isnâ€™t there on our unit. This supply is used for the Speech amplifier filaments and for the tone control chassis. I think the 6.3 is coming from another source.
Rod started work on the big output network in the RF bay and had some good results from the cleanup, but found the two big 2000 pf, 10,000 TV (test voltage) caps were both under 50 pf. He thinks they are likely oil filled and there is some indication that the caps have leaked on the chassis below the network. I think we can get some equivalent ceramic high voltage caps to replace them, but we need more search.
I will check the filaments on the exciter chassis next week. I also expect to test the big plate transformer next week by applying the single phase to each leg and checking the output with a Jennings high voltage (100 KV) voltmeter. If those two tests are successful, every power transformer in the transmitter should be operational.
Chrome plating: it appears that the Bias supply chassis will need major tear-down to remedy the problems as previously describe. So it would be a good time to go ahead and have the chassis re-plated along with the modulator tube and HV rectifier chassis.
I sent an RFQ to Quality Plating in Sterling, IL (Lawrence had some good experience with them sometime back) and they responded with an estimate of $150 per chassis and 6 to 8 weeks shop time. I responded that if this was acceptable to the club, I would tear down the three chassis and get them over to Sterling in the next 2 weeks. My assumption was that they would provide a firm quote when they see the chassis and then we could arrange to make payment by the time the work was done in late July. Please advise how the club will handle this.
By the end of next week, I plan to have the Dayton â€śshopping listâ€ť complete. If the parts are not available at Dayton, Iâ€™ll then ask for local sources and then put a request on the CCA reflector. As a final resort, we may have to do re-engineering to address parts shortages.
Larry has been looking for antenna connections. That will be a major aspect of the project. The other major issue is 3 phase power. I think we may be able to operator at reduced power on single phase, but we will still need 3 phase for the Autotune motor. If we can find a small 3 phase power supply (single-phase to three-phase converter) that might be the short term answer.
| May 2, 2013