From: "Al Parker" 
Subject: [Drakelist] Re:  AC-4 Power Supply  & ESR


HI folks,
    I thought the following would be of interest to the group re: p.s.
problems:
The Problem-- a T-4X, a local club member friend brought to me yesterday.
He said it would stick on xmt after he switched off "tune".  He finally
noticed that his finals were glowing red, also.  He was very worried about
the finals' health.
    The VOX system is the obvious area to look, but why is it idling at
around 300ma plate current?
    I quickly adjusted the bias, which he said (correctly) that he hadn't
touched.  Got it to idle at abt 70ma.  BUT the pot was all the way to the
end of travel, and bias voltage was about -50v.
    The bias voltage had been very low, and it does affect the vox, so the
rig stayed in xmt after he switched to stdby.  With very low bias the finals
idled at abt 250ma or more.
But why?  -  a bad elelctrolytic in the bias section.  And here's the
caveat - it checked OK with an "std" ohmeter, it looked low in res.
momentarily, then went high as it charged.  BUT - thanks to Garey Barrell, I
have an ESR meter.  The 20mf {at} 100v cap tested "infinite" on ESR.  Infinite
ESR isn't really wide open, but is a very high, over 200 ohms as I remember,
showing the filtering at 60 or 120cy isn't the low ~1 ohm that it should be.
    So, this is just a reminder to watch for such problems, particularly as
these power supplies get older.  A simple failure can cause unexpected
results.
73,
Al, W8UT
New Bern, NC
BoatAnchors appreciated here
http://www.thecompendium.net/radio/
http://www.hammarlund.info





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