AGC and AM Audio Mods
The AGC performance of the original DX-394 and the 'A' or 'B' model is at best poor, with bursts of distortion, gain pumping and low bass cross-modulation. The 'A' version had an additional factory fitted AGC modification PCB that attempted to correct the original design defects. It failed miserably but not content with this the designers made it a permanent part of the 'B' version. The circuit diagram in the service manual was published in 1995 for the original DX-394. No supplements or revised manuals for the 'A' or 'B' versions seem to be available. The circuit diagram shown below is the result of reverse engineering a DX-394 B model. The modifications have a very positive effect on the DX-394 'B' with the bursts of distortion, gain pumping and low bass cross-modulation significantly reduced. Its new AGC parameters raise its performance to be equivalent to that found in several much costlier receivers.
The AM audio performance of the DX-394 has been described as distorted, clipped, lacking punch, no bass, or just poor. This is mainly due to the ANL (Automatic Noise Limiter) circuit which is permanently activated. It eliminates impulse and static noise peaks, but reduces the dynamic range and clarity of the audio signal. The modification inhibits the ANL function in favour of audio quality. You can make improvements to the frequency response of the audio by increasing the value of certain capacitors in the audio signal path. You may have noticed when switching between the AM and SSB modes a significant volume difference. The modifications will balance out this discrepancy reducing the need to keep adjusting the volume control.
and AM Audio Modifications
Remove C188, C191 and C68.
Solder a 0.1uF (104) ceramic capacitor across the surface mount capacitor (C ?) at the output of D35, see photo.
Using a double layer of 'Quick Stick' pads mount a 220uF 16 Volt capacitor at the position shown in the photo, with its leads across the surface mount ceramic capacitor C163.
Solder a 0.047uF (473) ceramic capacitor across the surface mount capacitor C73.
Solder a 0.01uF (103) ceramic capacitor across the surface mount capacitor C72.
Solder a 3.3k 0.6W standard resistor across the 10k surface mount resistor R80.
Mount vertically a 0.01uF (103) ceramic capacitor and a 10k resistor at C68. The capacitor goes to the PCB pad closest to the surface mount resistor R80, see photo. The resistor goes to the other pad of C68. Join together the free ends of the capacitor and resistor then solder them as shown.
On the side facing C72 fit a wire link across one half of the surface mount diode D17.