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Radio Modifications

Realistic DX-394

Realistic DX-394

455kHz IF Mods


Introduction

There are four main design shortcomings in the 455kHz IF system.

1. The electrical energy is supplied most efficiently from the signal source (Q6) to the load (CF1 and CF2) if the signal source impedance and the load impedance are the same. If these impedances are mismatched, this energy escapes in form of a reflection. To match the signal source impedance and the load impedance is called impedance matching. This is very important for ceramic filters as improper impedance matching may cause centre frequency shifts and increased ripples in its frequency characteristics. To correct this a 1.5k resistor is placed in series with C51, see circuit diagram.

2. The 455kHz IF signal is influenced by the input circuit of the noise blanker. To reduce this effect a 4.7k resistor is placed in series with C43, see circuit diagram.

3. Strong AM stations within 10 to 30kHz of the station you are receiving can be heard "bleeding" over in the background. The problem is caused by the fact that the designers forgot one of the AM filter (CF1) bias decoupling capacitors. At the junction of R58, R61, R62 and R64 should have a bypass capacitor to ground from this point, see circuit diagram. Without it there is a resistive signal path through R57, R58 and R61 that bypasses the AM filter (CF1) and causes broadband bleed through. A 0.047uF ceramic capacitor soldered directly across R64 will significantly reduce the AM crosstalk.

4. The AGC feed to the IF amplifier transistor Q7 is not decoupled to ground, see circuit diagram. Any stray RF or IF signals can influence this stage so must be decoupled by a 0.01uF capacitor. A small amount of extra IF gain is provided by placing a 1.5k resistor across R66.


Circuit Diagram
DX-394 455kHz IF Modifications

Parts List
Component Type Value Quantity
Resistor 1.5k 0.6W 2
Resistor 4.7k 0.6W 1
Ceramic Capacitor 100pF (101) 50V 1
Ceramic Capacitor 0.01uF (103) 50V 1
Ceramic Capacitor 0.047uF (473) 50V 2

The resistors are Metal Film 1% 0.6W. Dimensions: 6.5mm x 2.5mm dia.
However, you can use 2% or 5% resistors at 0.5W or 0.25W.

The small blue capacitors are resin-dipped high quality multilayer plate ceramic.

Procedure

Remove the 100pF surface mount ceramic capacitor C43.
Remove the 0.047uF surface mount ceramic capacitor C51.

Mount vertically a 0.047uF (473) ceramic capacitor and a 1.5k resistor at C51. The capacitor goes to the PCB pad closest to the surface mount resistor marked 332, see photo. The resistor goes to the other pad of C51. Join together the free ends of the capacitor and resistor then solder them as shown. At this junction solder one end of a 4.7k resistor and take it horizontally to a 100pF (101) ceramic capacitor mounted vertically back down to the PCB at C43, see photo. It goes to the pad furthest from C51.



Solder a 0.047uF (473) ceramic capacitor across R64 and a 1.5k resistor across R66, see photo.

Locate TP2 and fit a 0.01uF (103) ceramic capacitor from TP2 to the PCB ground track. You must scrape away some of the green solder-resist coating from the PCB to expose the copper track.

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