KW-1 Plate Transformer

[WA5VGO]

Recently lost the plate iron in the KW-1. It's a real beast and shipping it for rewinding would be expensive and a real pain. Has anyone used a Johnson KW plate transformer in a KW-1? I see that is rated 100mA less than the Collins, but I suspect that's not a big deal. Hammond lists a replacement for the Johnson that looks tempting. Being a modern transformer with a tape wound core it's much smaller and lighter.

Darrell

[KY4SP]

Looks like someone has-

I realize that not many people who read this board have a KW-1... but I can pass along some suggestions that may also be applicable to other transmitters

My KW-1 does not have the original Collins plate transformer. It has a plate transformer of the type that is also used in a Johnson Desk Kilowatt transmitter. Although I would have preferred to have the original Collins transformer... a nice thing about the transformer I do have is that it has both: 2900 VAC outputs (either side of CT) and 2350 VAC outputs. I moved two wires to allow me to use the 2350 VAC outputs. This resulted in a reduction of the HV DC from well over 2500 Volts to around 2100 Volts (with a 240VAC line voltage). I adjusted the modulator (a pair of 810's) bias accodingly. I load the transmitter to 300mA of plate current (instead of the normal 400mA). This produces 630W of input power and 375W of output at carrier. All in all... I think the reduced stress on the transmitter is a good idea.

With respect to the fuses:

I had a fuse-blowing problem with the fuses in the primary of the plate tranformer. I measured the primary current that flows when the transmitter is switched from standby to transmit. With the reduced secondary voltage (2350VAC), the primary current peaks at 10A for one half cycle of 60Hz, and then settles down to around 4A (even with a choke input plate supply). I added a step start circuit, with a time constant of around 0.25 seconds. Since then, I have been using 10A fuses, with no fuse-blowing problems.

Stu

Also, my experience has been that you can often have a new transformer made for about the same cost as a rewind. The shipping and labor to disassemble your core compared to the cost of new laminations is just about a wash.   

[WD8KDG]

Well:

Don't own one, wished at times I did.....

#1: Art would roll over in his grave....putting Junkston parts in a KW-1?

#2: On a serious note, resale value. At least keep a close eye on the olde transformer and make sure it follows the KW-1.

#3: Get it rewound by WZ1M, Gary! Little by little keeping the boatanchors alive/intact is getting tough. Fewer people have the knowledge and equipment. Now is the time to build a wood crate of plywood and 2X4's and ship it to Gary. Cost $$$ to play with the bigboys.

Craig,

[AB2EZ]

Darrell

My KW-1 #54 came to me with a Johnson Desk KW plate transformer. I operated it using the lower voltage secondary taps: 2100 VDC (instead of 2500VDC) and 320mA of plate current (instead of 400mA). The carrier level RF output power was 375W (instead of 600W).

I added a step-start relay that limited the peak of the first cycle of primary current to 4A ( instead of 10A). That allowed me to use a 10A fuse on the 240V primary. I disconnected the primary center tap, since it did not appear to serve any useful purpose. That allowed me to use a single 10A fuse on the primary, and a single pole step start relay.

I had to readjust the bias on the modulator because of the lower modulator plate voltage. The existing bias control circuit didn't have enough range... so I built a separate modulator bias supply employing Zener diodes. That was a good thing to do for other reasons besides being able to properly adjust the modulator resting current. I.e. the stock modulator bias supply has too high a value of equivalent series resistance... and the bias voltage will become more negative if the modulator tubes draw grid current on modulation peaks. This is not a big problem with typical voice modulation... because of the capacitor across the output of the modulator bias supply... but it will produce a noticable shift in the modulator bias voltage if you apply an audio sine wave for testing. That, in turn, can mislead you with respect to what the achievable peak modulation index is, and can cause crossover distortion in the modulation envelope when applying a sine wave audio test signal.

My KW-1 worked great for the 5 years that I owned it. I found a new home for it in 2012, when I relocated to a smaller home on a smaller lot.

Stu

[W3GMS]

Recently lost the plate iron in the KW-1. It's a real beast and shipping it for rewinding would be expensive and a real pain. Has anyone used a Johnson KW plate transformer in a KW-1? I see that is rated 100mA less than the Collins, but I suspect that's not a big deal. Hammond lists a replacement for the Johnson that looks tempting. Being a modern transformer with a tape wound core it's much smaller and lighter.

Darrell

Hi Darrel,

You may get a rewind price from Gary -WZ1M since he is in business to do such rewind jobs.  He has a very good reputation in what he does.  I would think that would be cheaper than buying a new one from Hammond but I have never done the price comparison. 

I do have a spare plate transformer for my Johnson Desk KW but want to hold onto it just in case something happens to the one in it now.

73,
Joe - W3GMS   

[WZ1M]

I just finished up a rewind on one of these this past week. 150 lbs boxed is what your looking at and its $120.00 each way to North Carolina vie FedEx ground.
Regards,
Gary

[Todd, KA1KAQ]

It should be fine, Darrell. Both rigs are 4-250/4-400 transmitters modulated by a pair of 810s. Stuff something in there until you can get the original rewound. I can vouch for Gary's work, he re-wound the modulation transformer for my KW-1 and it hasn't zorched yet. Best bet would be to take a vacation to Maine sometime and at least save shipping one way, or find someone to AM Pony Express it up for you.

Stu, I remember when Bob got that transmitter in. We discussed it via email as he was under the impression that Collins also used the Johnson version of the plate iron, which they did not. They did share the same modulation transformer, though the later Johnson versions went to a larger insulator arrangement. Electrically they're identical. Bob got another transmitter in later with RCA broadcast mod iron in it that we discussed, too. He ended up swapping it out for a Dahl transformer. Sad thing is, the RCA was a better transformer, even if it wasn't 'accurate'. The stock mod iron seems to get close to the hairy edge of its abilities.

Johnson wisely did not include a splatter filter like the KW-1 used.