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?
Buck, k4ia
Honor Roll
8BDXCC
EasyWayHamBooks.com
On 4/15/2020 3:31 PM, Ray Albers wrote:
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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