http://elecraft.85.s1.nabble.com/Electrolytic-Capacitor-failure-tp7659760p7659761.html
I don't know that I would run a K3 off a 20A power supply. You're
stressing both. Did you mean to say 30A?
> 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)
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