This rig is a lousy performer on AM: Low modulation level (at 30w carrier, peaks never reach 70w), with muffled sound.
There are three serious design flaws who affects mostly AM mod.
- The ALC circuit loop in SSB mode keeps the RF peak at presetted value. In AM, a capacitor is connected as a low pass filter, to keep average RF value constant. This is done by a saturated bipolar transistor, who behave as a diode, not actually connecting the capacitor to ground.
- Replace C305 (10uF) by a 100uF one.
- Replace Q306 (DTC114EU) by a FDN339AN mosfet
- The first tx mixer (IC400) has gain in excess leading for IMD distortion in SSB and low peak level in AM.
- Replace R407 (220ohm) by 680 ohm. Re-adjust R331 to a higher RF level.
- There is a thick felt muffler in front of microphone electret unit. Remove it.
After modification my rig presents excellent AM, and the worst case IMD distortion in SSB 2-tone test dropped from -24 to -32 dB.