[Elecraft] Linear PS Recommendations

[Alan Bloom]

The 723 regulator has some known reliability issues, but with proper 
design they can be mitigated.  In particular the differential voltage on 
the error amplifier inputs is only rated for 5v.  If one input is 
connected to the 7.15V reference, then if the power supply output is 
shorted the voltage rating is exceeded.  The solution is a 
strategically-placed diode.

Alan N1AL


On 07/22/2016 12:14 AM, Richard Fjeld wrote:
> I was glad that I caught Jim's remark about blowing the 723 by being
> connected to a battery with the PS off.  And also Myron's answer.
>
> Back in the tube TV days, I purchased a bare circuit board at Radio
> Shack, (I think). The board was similar to an Astron circuit board, but
> simpler.  I rewound a heavy transformer from a color TV and made my
> first 13.8 Volt power supply to use for a Heathkit HW-104.  Years later,
> the 723 went bad.  It may have been due to the stated reason, I can't
> remember now.  I did some repair work on it and still have it.
>
> Someone on the reflector provided the following link one time, which I
> think is a good idea for an extra measure of over-voltage protection or
> accidental reversal in polarity. I remember discussion about a reversal
> that happened to someone on the reflector not long ago.
>
> I hope the link still works on here.  If it doesn't, it is on ad5x dot
> com website. Look for a Vprotect pdf file.
> http://ad5x.com/images/Articles/Vprotect.pdf
>
> Dick, n0ce
>
>
>> On 7/21/2016 7:41 AM, Myron Schaffer wrote:
>>> Jim and Clay,
>>>
>>> It used to be that way....the RS linear models could not tolerate
>>> external voltage present when unpowered, not anymore. Astron has
>>> incorporated back-voltage protection through incorporation of some
>>> diodes installed in key locations to prevent this.
>>>
>>> You can verify if your supply has this modification by looking at the
>>> circuit board and see a 1N4001 type diode soldered across 2-pins
>>> (can't remember which ones off the top of my head) of the IC voltage
>>> regulator LM-723 chip and others hung in various places to protect
>>> the pass transistor as well. Usually soldered to the solder side of
>>> the circuit board so they are easy to spot. I can't remember is there
>>> were some small capacitors added as well.
>>>
>>> Any newer (like newer than 1990) linear Astron will have these
>>> installed.
>>>
>>> Myron WVØH
>>> Printed on Recycled Data
>
> -
>
> ______________________________________________________________
> 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
> Message delivered to n1al at sonic.net
>