Ray, I reported to this forum (ca. February 2019) a similar, though not identical, problem involving my K3/10 while running from an Astron RS-20A supply. My problem occurred in receive mode rather than transmit mode, which made it a bit more difficult to understand because the current demand for my K3 in receive mode is less than 1 A.
After finding no problem using an alternate supply (Astron SS-30 or 12 V battery), I ripped my RS-20A apart to test the various parts on either side of the regulator board. To keep a long story short, the problem turned out to be the large electrolytic filter capacitor at the output of the rectifier block (ahead of the regulator board). It had no external signs of leakage or bulging, but its ESR (equivalent series resistance) had risen to perhaps 2-3 ohms. Finding it hard to believe this ESR could present the DC voltage and current oscillations I observed, I did a SPICE simulation of the transformer/rectifier/filter capacitor circuit with a load designed to draw about 1 A. A good capacitor (ESR = 0.05 ohms) showed hardly any DC voltage ripple, whereas the capacitor with 3 ohms ESR showed huge ripple. I didn't get a chance to put a scope on the actual RS-20A regulator input to confirm the simulation result, but it appears that the regulator circuit developed a low-frequency limit cycle oscillation when loaded by the K3/10 at 1 A current, likely due to the high amplitude voltage ripple at the regulator input caused by the elevated ESR of the filter capacitor. The DC bus voltage reported by the K3 toggled between two values, 13.6 V and 12.7 V. Motto of this story: even if an electrolytic doesn't appear bad externally, suspect increased ESR and replace it. 73, Mike, K8CN -- Sent from: http://elecraft.365791.n2.nabble.com/ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
