Hi Ray,
This is probably unrelated to your capacitor failure, but your "big hulking linear supply" is underrated for the K3 unless you run your K3 at less than full output power. The RS-20A might be okay for SSB, but certainly not for RTTY or digital modes at K3 full rated output power. The Astron RS-20A specification: "amperage (continuous) 16 A, intermittent 20A" www.astrondistributors.com/astron-rs-20a-1667.html The K3 manual recommends " 13.8VDC @ 25A continuous duty" and specifies: "13.8 V nominal (11 V min, 15 V max). 17-22 A typical" ftp.elecraft.com/K3S/Manuals%20Downloads/K3S%20Owner's%20man%20A1.pdf 73 Frank W3LPL ----- Original Message ----- From: "Ray Albers" <rayalb...@gmail.com> To: elecraft@mailman.qth.net Sent: Wednesday, April 15, 2020 7:31:23 PM Subject: [Elecraft] Electrolytic Capacitor failure I could use some knowledge and/or opinions on this subject (Radio/Elecraft related only because it deals with a power supply that powers a K3!) The other day I was about to initiate a call, and as soon as I touched the "dah" paddle the K3 instantly shut off. The cause was that the power supply voltage dropped too low when the radio demanded more current. The power supply is an Astron RS-20A, a big hulking linear supply that I really like. Huge heavy transformer and two series-pass transistors mounted on big heat sinks - thus, quiet acoustically (no cooling fan) and electrically (no RF hash from switching). I have two of these - one that I bought for my new radio when I got back into ham radio after a long absence and the other because I was at a hamfest where someone had a pile of them that he was selling for only $20 each - who could resist? So I trotted out the spare supply and got back on the air. Now to troubleshoot the bad supply. I had trouble with this one about five years ago. The output transistors are plugged into sockets, and on one socket the contacts for the emitter pin had gotten loose, and there was a vicious spiral of heat causing more resistance causing more heat until eventually it actually melted the transistor pin and left a black char on the socket. When I replaced the socket that time, I decided to solder the emitter pins on both transistors. It took me a long time to find the problem this time. Various tests told me the transformer, the full-wave rectifier diodes and the pass transistors were fine. Finally - with some help from hints in a great article about Astron supplies on repeaterbuilder dot com - I figured out that the supply voltage to the regulator board was too low, so there was not enough "oomph" available to drive the output transistors when high current was demanded. This supply voltage comes from a center tapped transformer secondary feeding two small diodes (both of which checked OK) to a 1000uF 35 V electrolytic. Turns out the capacitor was bad. It's a typical aluminum case with blue plastic covering (which I assume to be heat-shrink plastic because of the way it is completely molded around the capacitor.) Replacing it brought the supply voltage up to spec, and now everything works fine. But I was surprised about the capacitor failure. Absolutely no visible signs of anything wrong - no bulging, leaking, etc. And the soldering to the PC board is perfect. So here's my question for the group: What do we know about electrolytic capacitor failures? I know that anyone restoring an old ham transmitter or receiver or BC receiver almost always has to re-cap it, because the ancient electrolytics, which are usually wet electrolytics in chassis-mounted cans, will have gone bad. But this is the first time I've ever had a failure of the more modern type electrolytics. A quick search of capacitor failure on Google shocked me when I read claims in several places that these capacitors are only expected to have a two to five year life!! Can that be? I know I've got lots of radio gear that's way older than that and still working fine. To do this repair, I replaced a capacitor that might only be 5 years old with one that is probably twenty years old! Words of wisdom and enlightenment would be most welcome! 73 Ray K2HYD (K3 #8240, KX-3 #6827) ______________________________________________________________ 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 donov...@starpower.net ______________________________________________________________ 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
