N9FOY Mods

Steve N9FOY has designed two excellent modifications to the T-368. I currently use the first mod, the N9FOY speech in my T-368 #1. I am quite pleased how well the unit works, the audio is very near broadcast quality. Implementation of this modification require removal of the stock speech amp chassis, and replacement with a suitably sized blank chassis. The mod is then built upon this chassis.

The second mod allows individual bias adjustment of the 4-125A modulator tubes. It is quite simple, and can be installed in the stock speech amplifier with no problem.

Speech Amp Modification

One of the more popular audio mods, and one which I use in my T-3 #1, is the N9FOY speech amplifier mod. It is based loosely upon the AF amplifier used in older RCA Broadcast Transmitters. A information sheet and schematic are presented below, just click on the images to enlarge.

Info Sheet

Speech Amp Schematic

Modulator Bias Adjustment

Steve also has designed a mod for the adjusting the individual bias of the 4-125 modulator tubes. Quite straightforward in it's design, this mod can be implemented with or without the N9FOY speech amp.

Individual Bias Adjustment

The second mod allows individual bias adjustment of the 4-125A modulator tubes. It is quite simple, and can be installed in the stock speech amplifier with no problem.

Speech Amp Modification

Speech Amp Schematic

Modulator Bias Adjustment

Individual Bias Adjustment

1. - This upgrade results in a decrease in harmonic distortion and an increase in frequency response, nearing the quality of a broadcast transmitter.

2. - The feedback ladder assemblies, shown at the right side of Schematic 1, obviously operate at high voltage on the plate connection end. These are best kept separate from the rest of the circuit. It is recommended that these be mounted on an open circuit board located near the modulator tubes, either above or below the chassis deck.

3. - Feedback lines between the two ladders and the audio chassis itself must be shielded using two-conductor cable, and example being Belden 8451. The balanced feedback circuit must be reversed in phase - observe the F.B. 1 and F.B. 2 notations on Schematic 1.

4. - The audio chassis, containing the 2E26 tubes, input transformer and associated components, can be any chassis box which simulates the shape and size of the original speech amplifier assembly. It should have a good ground connection to the main deck.

5. - It is preferrable to install a connector on the audio chassis of the same kind used in the original speech amplifier assembly, thus allowing the existing wiring harness to be plugged in without modification.

6. - A source of 6.3 VAC must be provided for the 2E26 tubes (approximately 2 Amp capacity). This filament transformer can be located on the new audio chassis or elsewhere in the transmitter.

7. - The B+ feed (J3, pin 13) is originally supplied through a large series dropping resistor located under the main deck. For best operation of the new circuit, bypass this resistor to get about 500 VDC.

8. - The feedback ladder capacitors, with the exception of the 0.01 uF shunts, must be rated for 1kV, 300 pF mica types. The 0.01 uF shunts can be just about anything, such as discs, etc.

9. - All circuit resistors, both on the ladders and in the chassis, must be of 2% or better tolerance, either carbon or better yet metal film types.

- TNX to W5KP for supplying these mods.