[Elecraft] K-Line DC Supply

[Richard Fjeld]

Back in the late 70's, I took a large high voltage color TV transformer 
apart and rewired it to build a power supply for 13.8 VDC.  I used #8 wire 
for the secondary.  At the least,  I doubled up on the duty cycle of all the 
components.

It ran for decades. until I found the crowbar circuit had operated. It is 
still fine, but either the 723 is flakey, or the SCR.  Recently, the Astron 
at our repeater site did the same thing, only I couldn't put the smoke back 
in it.  (It was being used at one forth of it's rating.)

Both supplies were old. I'm wondering if a transient on the AC can still 
produce a spike on the DC which triggers the SCR, or is age affecting the 
components that will allow it to happen? Maybe both.

Linear supplies hum, switchers have fans which are louder.  But switchers 
can be reasonably mailed or shipped. That in itself would be nice.  I think 
Wayne could produce a good unit, but would it be priced competitively?

Rich, n0ce


----- Original Message ----- 
From: "Howard Hoyt" <hhoyt at mebtel.net>
To: <elecraft at mailman.qth.net>
Sent: Wednesday, September 19, 2012 1:17 AM
Subject: [Elecraft] K-Line DC Supply


> >From: "Ron D'Eau Claire"<ron at cobi.biz>  said:
>>I'm not sure what you mean about the regulator
>>not being "quiet". The LM723 is a simple linear
>>I.C.; no switching noise and no acoustical noise.
>
> Hi Ron,
>
> I was referring to the broadband thermal and shot noise present at the 
> Vout pass transistor drive output of the regulator itself.  Yes it is a 
> linear regulator, but at least with the parts I have used, the 723 
> internal reference voltage and error amp are fairly noisy by modern IC 
> standards. I have tried to use these regs in audio equipment and I find a 
> quieter and wider BW regulator can be made inexpensively using modern 
> bandgap references and low-noise opamps.  I am sure that Elecraft designed 
> their circuitry to have as good internal sub-regulators and as high PSRR 
> as possible, but having a really quiet supply can't hurt.
>
> Of course, maybe anything better than a 723-based supply is just 
> unnecessary overkill:  I do not have an Astron or other 723 supply here, 
> so a good test may be to compare a K3 or KX3 powered by a battery, with 
> the same setup powered by a supply.  In order to provide the maximum 
> sensitivity, the antenna should be disconnected and the RF and AF gains at 
> max to allow hearing farther into the rig's own noise floor.  I just 
> performed this test on my KX3 using a Xantrex supply (@13.8V, <400uV noise 
> RMS 10Hz-300KHz as measured on an HP3468) I hear zero noise level 
> difference between the supply or internal batteries, so if this is the 
> case with an Astron, and it's regulation is good enough, then the 723 
> based circuit is adequate for this particular application.
>
> I agree with whoever stated that a power supply should be out of the way 
> and out of sight; having no controls or displays that we need to see every 
> day there is no need for it to take up desk space.  I would like to get a 
> small 13.8V dedicated supply that could handle a KX3 and the pending 
> KXPA100, but others have very different requirements, such as power for 
> multiple hundred watt amps, etc.  My feeling based on my experience with 
> the KX3 is that if Elecraft made such a supply it would be no compromise 
> for those of us who have such a need.  One of the main reasons I would 
> like to see this is the same reason I like the Astron: it is made in the 
> US.
>
> Just my $0.02 worth,
>
> Howie - WA4PSC
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html