Zorched Mod Iron - Caution: Graphic!

[Todd, KA1KAQ]

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!

[w3jn]

Glad you got the old gal back on the air, Todd!

The problem is ALWAYS in the worst place possible

[W9GT]

Heh, well....it is pretty scary when you put all the pieces in place that caused it.


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.

Glad that you got the rig going again Todd!  Also appreciate the graphic evidence of the failure mode!  Same thing happened to my 30K-4 mod iron...except it was the primary side between winding layers...at least that is what it appears to be...have not dissassembled to confirm.  I also have a 300 watt Thordarson CHT mod iron that went into the zorched mode with similar symptoms. 

Again,  thanks for the report!

73,  Jack, W9GT

[WA1GFZ]

So one winding was modulator B+ and the other was a modulator tube plate.
This means there was tube saturation voltage to 2X plate voltage across a couple layers of paper. Sounds like a good place for some kapton.

[AF9J]

Glad to see you got it going Todd!  You were starting to go through radio withdrawl.

73,
Ellen - AF9J

[Rick K5IZ]

Fantastic pix!  It's not often we get to see the problem up close and personal, thanks!  Gary, you da man!

Rick

[WA1GFZ]

It only takes a little more effort to make a great transformer compared to a stock one....having designed a few
Imagine if Todd's transformer had two more layers of insulation.

[N3DRB The Derb]

thats a first class zorch. 

peoples on this board are scary sometime with their knowledge of ray-dios. make me wanna scream and holler we gots so much talent. I forget, somebody called it right on the money first, but I dont remember who?

that third picture is like transformer scat porn or somethin. It's so gross and disgusting, you cant look away from it. 

[WZ1M]

No amount of extra insulation would of saved this tranny, in the long run. While stripping it down, the old varnish that covers the wires was falling of like a bad case of dandruff. It wouldnt of been long before the whole thing went up in smokonium. When this puppy goes back together, I will take pictures and send them to Todd. Every layer gets a coat of varnish before the next layer is put on. 10 mil nomex or 15 mil nomex paper will be used to insulate the seperate coils from each other. I use only triple coated 200C wire. Glass sleeving, blah..blah..blah.
TNX, Gary..WhiZ1M

[Tom WA3KLR]

If you had a Class E transmitter you wouldn't have that problem.

[W1GFH]

thats a first class zorch. 

peoples on this board are scary sometime with their knowledge of ray-dios. make me wanna scream and holler we gots so much talent. I forget, somebody called it right on the money first, but I dont remember who?

that third picture is like transformer scat porn or somethin. It's so gross and disgusting, you cant look away from it. 

Know whatcha mean. Are there any more disgustingly graphic zorch photos about?

[Todd, KA1KAQ]

I'm pleased that Gary is so meticulous and pays attention to detail. I had an image in my noggin of what it might look like, nothing like it does. The old insulation gave up the ghost, some 54 years after being made.

You are right, Derb - the collective knowledge on this board is amazing, it sometimes leaves me speechless (hard to believe, I know) when trying to explain to others outside of AM. When the transformer zorched, I immediately got reports from folks who could still hear me - W1IA, WA1QIX, then the next day from VE3AJM, VE6PG, VA3ES and others who had been listening the night before or who were on later when I ran some tests. Not only did most everyone diagnose it as blown mod iron, most also defined it as shorted turns in the secondary - just by the sound of the audio and action of the mod cathode current meter.

I've known Gary and known of his work for many years. He's unintentionally been one of the best kept secrets in the old radio world. I still find folks who haven't heard of his work, thought I know many of his satisfied customers. He puts in the extra effort to make sure the job is done right, meaning far fewer returns than Dahl for 'singing' mod iron. In fact, I had heard that Dahl quit rewinding mod iron because of this problem.

Anyhow, #89 is back on the air with loaner iron for the time being, at least until after Dayton. We went with the conservation plate to plate winding taps just to be extra safe, and because Joe had to remove the spark gap and shorting bars to get the thing to fit in.

[steve_qix]

Hi Todd,

Too bad about the failed mod tranny - but things do seem to be looking up    To this day, I remember very well the first time I lost a modulation transformer.  It was a very nice, high quality transformer.  I was using seperate power supplies for the modulator and final, and for whatever reason the modulator power was on and nothing else was !!!  A good bump of the microphone and it was ALL OVER !!!  I had this aweful sick feeling - and I still remember it.  This was back in 1972 !!  Of course, I was only 17 at the time but still...

Good chatting with you at Near-Fest !

Talk later and Regards,

Steve

[WA1GFZ]

I've rewound a number of transformers in my lap. The design is the hard part. The whole thing about repairing a transformer is the potting. Some people dip the transformer and hope for the best.
High quality stuff you pull a vacuum and suck out the air replaced by potting material.

[wa1knx]

glad your back on the air todd. mod xfmrs are magic, the
rca one I have on the 833 x 304tl rig works beautifully,
and I thought the movers might have killed it. it slid into
another xmfrs terminal pin and pierced it. but it wound up
being ok.  Steve, somehow I remember you wiping out
a mod xfmr back then. was that was when you lived in the
unfinished attic at your mom's in Holden? Dean

[N3DRB The Derb]

the Thordarson CHT I had in my hb rig was a work of art. It was the m77 300 watter, black crackle and brass.  Those 805's sang for their supper, as Tron would say. Loved those plugs and jacks.

[WA1GFZ]

Gary,
Great on the wire. I have never seen anyone paint each layer but sounds like a good idea to make sure each wire gets coated. I like vacuum impreganated windings myself. All you need is an old pressure cooker and a fair pump to suck it down to about 25 inches. All you need to do is put the winding in a can of varnish and set it in the cooker and seal it up and suck it down. Let it sit for a while to suck all the air out.
I would use more nomex between the 3 windings myself. if it will fit.. 

[WZ1M]

I do vacuum dip the windings. I use, as you stated, an old pressure cooker which I have mounted a vacuum gage, a releif valve and a place to hook the vacuum pump. I only need to pull 15 inches of vacuum for a good penetration. When winding anything audio, I mean anything, I alwyas coat each layer. Helps to keep the "SINGING" down. Also the core or bobbin is wedged to the iron to help secure it. As with Todds iron, it will be repoted as original for the final touch.
Regards,
GAry...WZ1M

[WA1GFZ]

Gary, That is a great idea and I'm sure a lot of extra work BUT that is how you produce a quality product. I bet the new transformer will outlast Todd.
I wonder if you do anything special for top finish windings. I like to use Teflon wire myself for a device with leads and teflon tubing on a transformer with terminal lugs.

[Todd, KA1KAQ]

Gary, That is a great idea and I'm sure a lot of extra work BUT that is how you produce a quality product. I bet the new transformer will outlast Todd.

Hey now! Is that a challenge? 

Gary, I am back from Dayton and will contact you to tie the ribbons on this.