[AMRadio] 3.878 & 3.892


Geoff/W5OMR 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:
>
> http://amfone.net/Amforum/index.php?topic=11077.0

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 
rewinding.

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 
windings:

"/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
rewind.

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.
Regards,
Gary.../"


 Yeowsa! Shocked



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