[AMRadio] Mod Transformer Available

W5OMR/Geoff w5omr at satx.rr.com
Sun Sep 25 23:21:22 EDT 2005

Paul Sokoloff wrote:

>    I have a 1940s vintage 500 watt modulation transformer made by RCA for
>sale.  It has 5700 (approx.) ohm P-P primary and a 5700 ohm secondary.  I
>would like $250 plus shipping.  Local  pickup near Philadelphia preferred.
>I checked the windings and they have continuity.  There is also another
>winding of a higher impedance maybe for screens.  If interested please
>contact me directly at dogfaces at comcast.net

Sounds like an old Liberty Ship MCW transmitter mod transformer.  
Typically, those that I've seen were 1:1 @ 5,500 Ohms of Impedance.  
These old buzzard transformers have been around for a good long time, 
and were plentiful, and easily had, at one time.  I think there's still 
a lot of  'em around, but they're doing duty more as a door stop.  Lots 
of folks don't use 'em, because of the plate on the secondary, that says 
"MAX 0.200 AMP DC". 

Don/K4KYV wrote a nice article in here a while back about the liberty 
ship transmitters and their MCW modulation transformers, that had screen 
windings on them. 

I never did quite understand if the 200mA on the secondary was the 
Maximum current it could take, or if 200mA was the max, before the core 
was saturated. 

K5SWK ran/runs one of these on a pair of 833's.   I ran a pair of 
250TH's on one, but would never draw more than 200mA in the final... 
more than that, the audio response was severly restricted, until I 
strung some chokes together to shunt the DC off of the modulation 
transformer secondary.  3000v @ 300mA.  Life was good, for a while.

73 = Best RegardS

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