[Elecraft] Electrolytic Capacitor failure

Thu Apr 16 13:09:50 EDT 2020

If Ray has a K3/10, then the power supply is perfectly fine. The recommendation for that is "13.8VDC @ 6A for K3/10”.

There are a lot of K3/10’s in the field. Nothing in his post indicated which transceiver he has.

However, if indeed he has a K3/100, then Frank is right (as usual). Ray should probably be using a heftier supply. Margins are important.

73!
Jack, W6FB
ps Thanks, Frank!

> On Apr 15, 2020, at 10:22 PM, donovanf at starpower.net wrote:
> 
> 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" <rayalbers at gmail.com> 
> To: elecraft at 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) 
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