Neutralizing the TR-3
By Dan - KJ7FX - kj7fx@tdn.com

     Well here goes. I'm sure this procedure is old hat to many on the list, and for that matter, probably has been offered on the list a number of times already. The important thing I suppose is that at least two of us have recently muffed the job and caused some damage to our TR-3s. It needs to be stated again.

     Any idiot can do it; I can!

     My 1991 ARRL Handbook describes a method whereby a source of RF (a hefty one at that) is fed into the output of the PA tank circuit and an RF sensing device (scope is ideal) is connected to the input to the PA. The plate tuning and loading caps are then tuned to maximize the RF out at the input end. The neutralizing cap is then adjusted for minimum reading. Simple, but alas my signal generator didn't put out anywhere near enough signal to be seen by my scope at the input end of the PA.

     My 1952 ARRL Handbook on the other hand described a method using the rigs own grid drive circuits to power the tank from the input end. To accomplish this method of neutralization it is necessary to first disconnect the PA plate and screen B+ leads. These are easy to find under the chassis of the TR-3 (and probably the others).

     In the TR-3 the PA cage extends to below the chassis where the plate and screen connections are made through pass throughs to the bottom end of the plate choke and via 68ohm resistors to pin 7 of the 12jb6's. See the schematic. Drake recommends disconnecting these for the first half of the alignment procedure anyway.

     Next we connect the oscilloscope probe to the output of the tank. Because the T/R relay will be energized during the procedure, just connect it to the antenna connector of the transceiver. Turn everything on and tune the transceiver up on 10 meters. I chose 28750kcs. Obtain a nice sine wave on the scope, of about mid height and tune for maximum output. It only remains to adjust the neutralizing cap for minimum output to finish. I repeated the procedure several times to be sure and I suppose it might have taken about 10 minutes total.

     Reconnect the plate and screen connections and wham bam your done! If you don't have a scope I think a sensitive VTVM and RF probe will probably do fine. If you don't have one of those; GET ONE!

     Thanks for the bandwidth and GL

     Dan - KJ7FX kj7fx@tdn.com

Additional Note:

     Also an easy way, especially if you don't have any test equipment, is to follow all the steps listed, and then listen for the signal in another receiver, tuning the neutralizing cap for minimum S-meter reading on that receiver.

     Jon, K1VVC JONWEINER@aol.com