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THE SIGMA RECONSTRUCTION PROJECT |
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| I bought this amp at a
hamfest. I had never seen one before. I was told all that
was wrong with it was a bad tube and some corrosion on the tuning
cap. The cabinet was repainted and it looked great. We
negotiated a price that I couldn't pass up.
When I opened it up, I found that it was compartmented and well shielded. I also found that it had spillage into the unit that ran onto the chassis, causing some corrosion, but did not affect any components. The problem with the plate tuning cap was not corrosion. It had been arcing. Now it kind of looked like corrosion, but when a high voltage DC snap comes between two aluminum plates, metal gets transferred just like a welder. One plate gets thicker, and the other gets thinner, and they both look rough irregular and flaky. Arcs will take the path of least resistance, and with high voltage, a sharp point flaw will cause a corona breakdown. Once the arc has occurred, the resulting roughness causes the arc to reoccur at the same point. The previous owner obviously made this observation, and he thought he had a solution. He simply took a pair of dykes and clipped off the corners of the plates that were arcing. Now this may have worked momentarily, but then the arcing proceeded to the adjacent plate. By the time I got the amp the cap was pretty chopped up. I suspect the only "problem" may have been insufficient loading and/or excessive drive that caused arcing on voice peaks, that got progressively worse, as the arc path was established. The next step was to "cold check" all the components with an ohmmeter. All major components looked good. The Sigma used a plug in board on the bottom of the chassis that contained the metering, ALC, and HV rectifier string. A couple of the rectifiers were shorted, the shunts were good, but the series HV metering resistors were out of tolerance. I replaced the bad parts. I wanted to give a good load test to the filter caps. The bleeders checked good, so I attached a variable DC supply to each cap one at a time, and brought the voltage up in increments, and let the charge current settle down to the bleeder current, until the rated voltage was reached. My observation was the same for each cap. |
The Sigma with plate tuning cap and tube sockets removed Now I had to test the tubes, since this amp was reported to have a bad one, and I could not see it with an ohmmeter. You just don't take 3-500Z's to the drug store, so I built a jig so I can connect a large tube to metered supplies and check against the published plate curves, using a series of static tests. Check for current stability around design center maximum as well as proper cut-off. Not as difficult as one might think.
THE "BIG TUBE" TESTER |
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