The AM Forum

Need some guidance. I am in the process of going through a 1939 National NTX-30 transmitter and NSM modulator. The original modulation transformer is long gone possibly due to a 6X5 cathode to filament short in the feedback circuit. I would like to duplicate the original mod iron and not modify the original circuit if possible. The design may be somewhat unusual in that the secondary is tapped for volume compressor voltage above the modulation output to the Class "C" NTX-30 finals. I attempted to contact Hammond's Peter Dahl department rep twice so far but no reply yet. I am willing to pay engineering expense if necessary and I realize the cost will likely be high. A copy of the National NTX-30/ NSM equipment manual is attached which has schematics and further explains the circuit. National calls it Class "B" in the manual which is now Class AB2 I think.

Thanks in advance, Greg

(http://i104.photobucket.com/albums/m191/wa1kbq/NationalNSMModulator.png?t=1346897021)

NSM Photo courtesy of KC8DDH

I guess all I really need is a recommendation for a modulation transformer custom builder. I would like to have the special high tapped secondary to retain the designed negative feedback circuit voltage. I realize I could accomplish that with the usual standard replacement kludge by adding a battery to the feedback circuit but I would like to maintain originality as NSMs are pretty rare. Anybody know who might be willing to do the job? I will probably be interested in having more than one made if the cost is not over the top.

Thanks, Greg

Try Gary
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/

Hi Carl,

Yes, I did contact Gary Brown. He's rewound transformers for me before, and his work is first class, but he said he only rewinds existing transformers. I received another referral via PM which sounds like a very promising lead. I will be sure to post results here in case anyone else has a need for a custom job.

Thanks, Greg