The AM Forum

Hi all,

I am the president of on organization at the University of Delaware called Blue Hen Electro-Magnetics. We are currently restoring a Collins 20V3 Broadcast rig but we've run into a problem where the pre-driver circuitry seems to prefer the 5th harmonic of 630 kHz (stock frequency). Upon using a grid dip meter on the plate tank plug in coils, the resonant frequency of the tanks seems to be around 2.1 to 3.1 MHz. So that explains the doubling, but we are struggling to understand why, as the assemblies look stock as far as I can tell. If any of you have pictures of the original coils for AM band operation, they would be greatly appreciated. Any additional info would be of great help as well

Best,

Daniel DeSantis
KC3FKR
Blue Hen Electro-Magnetics

Daniel,

I have no experience with the big Collins, but one idea pops into my mind. If the plug in coil for the RF pre-driver is close wound enamel covered copper wire, then perhaps there is shorted turn.

Jim
Wd5JKO

pull the tube that load the circuit and measure again, may be a feed back problem.

I don't have any pictures but, I modified a Collins BC transmitter to transmit on 3800kcs...It was originally on 970kcs....I snipped out one of the fixed caps and removed turns from the driver coils until the coils resonated at 3800 kcs (dip meter)  I believe these collins transmitter drivers are fundamental frequency, they don't operate on a harmonic...Good Luck...

Hello all,

Thanks for the input. I have done some asking around and the coils I have were far from stock. They were supposedly unmodified, but they were. This explains why the tuned circuit was spitting out energy on a harmonic.

Thanks for your input

Daniel
BHEM