FT-920 on AM

I'll have to dig up the specifics, but here are the broad strokes:

1. I bypassed the whole audio chain. The normal audio feed to the balanced modulator sounds great and has various eq effects but only in SSB. For some reason on AM only, the DSP sounds like a bad cold and I couldn't change it. Totally restricted. No highs, no lows, no punch. Awful. I installed a transformer isolated diff amp (including a phase switch) to drive the emitter follower that feeds the balanced modulator. In signal generator tests, the modulated waveform before the 6 kHz filter is basically flat from 80 Hz to 50 kHz. I use a relay with an external toggle switch to re-engage the original audio if desired. (Again, it's great in sideband.) External to the rig is a Behringer C-1 mic, a little Behringer mixer, an ART equalizer, and a Behringer stereo compressor limiter. Nothing expensive or exotic. I use one side of the compressor for light compression and for peak limiting. That output goes to the other side which I use for overall control of output level to the rig.

2. There are three loops that drive the ALC: Forward power, Reverse power, and PA current. I used a transistor switch to kill the Forward power loop only when in AM. The other loops are intact. I couldn't drive the positive peaks above 50% or so until I did this. The 920 has a lovely DC signal to switch various things into AM mode. I use the same signal to switch the transistor to short out the Forward power signal to the ALC system.

3. I found that there was a gain distribution problem in the balanced modulator and the stages downstream of it. The DC current injected into the balanced modulator that sets the carrier level is fixed and way too high. Using my external audio chain I could drive enough audio to fully modulate that high level but the resulting peaks got clipped downstream in the IF and mixers. I had to increase the value of the current limiting resistor that feeds that DC current to reduce the carrier signal. Then the final modulated signal would fit into the IF chain. At the end of the chain, just before the PA, is a dual-gate FET that sets the overall power level via the front panel Power Level pot. It has a great range and easily sets the required output to drive the PA.

With those three items dealt with (good audio, no forward ALC, and proper gain distribution) I now have the best fully modulated 25 watts that will go through a 6 kHz filter. My inverted vee on 75 is fed with homebrew OWL and is pretty efficient. I've been getting good reports in MA and the South, generally expressing surprise that it's only 25 watts. Especially when on AM, I always use my scope to keep an eye on things. Even though the power is low, I don't want to be a band hog.

The next move will be bypassing the 6 kHz filter in transmit and to rely on my EQ to keep things civil. But that's for another day.

Best 73,
Chuck, NA3CW