|[AMRadio] 3.878 & 3.892|
ars.w5omr at gmail.com
Tue May 8 10:27:47 EDT 2007
Todd, KA1KAQ wrote:
> Thanks to Joe, WA2PJP, I have a loaner mod transformer while mine is
> being rewound. Gary, WZ1M took it back to Maine with him after
> NEAR-Fest to test and repair. He sent me a rundown along with pics,
> posted on amfone if anyone would like to see the failure mode:
For those that don't like going to amfone.net, here's the text (don't
know why you couldn't have said it here, Todd)
Heh, well....it is pretty scary when you put all the pieces in place
that caused it.
Thanks to a loaner transformer and some hard work from Joe, WA2PJP, the
KW-1 is back on the air! Joe made contact with it on late Thursday
evening, around 2320 or so. This after removing the zorched transformer
so it could be hauled to NEAR-Fest where Gary, WZ1M picked it up for
The final verdict: shorted windings between the secondaries. Several
people predicted this based on audio quality(or lack thereof), modulator
cathode current, and the fact that the transmitter wasn't blowing fuses
as it would with a short to ground.
Gary emailed me the following description of what he found complete with
photos. Gary is meticulous and prompt, I have no doubt that he's the
right man for the job. Despite the age of the original mod iron, it's
still pretty amazing to see the damage that takes place inside those
"/I got a chance to do some tests on it. I put 120 volts, AC on the
primary side and then measured across the secondaries. Both were the
same, hmmmmmm. Put 120 volts across the secondary, one winding at a time
and came up with the same voltage on the primary side. Looks good so
far. Decided to hypot it.
First I did a hypot to ground from all windings. Hypot was brought up to
5kv and had a good reading. Over a 100 megs. Then I did a hypot from
primary to secondary and still over a 100 megs. This tranny looking like
its ok but had what you told me in the back of my mind about you
changing it out and now the transmitter works. Well, just one more test
and hope for the best, or worse, which ever the case may be.
I connected the leads of the hypot tester to each secondary, started to
bring up the voltage, got to about 100 volts and BAM, my audio alarm and
red light went on. AhhhHAAAA, a high resistance short between
secondarys. I then dug into the task of tear down. By the way, its
potted under those endbells and I will repot after the
I got down to the very last layer on a secondary, which borders the top
layer of the other secondary and low and behold, see the pictures, a
1/4" hole burned thru the insulation which would short out under a high
voltage load. Of all the places for this to happen. If it occurred
somewhere else, it would probably keep on modulating until you smelled
something burning. Glad I found the trouble and will keep you posted as
to whats going on. I should have it ready by the end of the week.
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