The radio was bought by my dad James in around 1965. He used to take it to the footy (Aussie rules) to listen to the commentary. It was playing up and dad asked me to fix it. This was around 1990. Dad left us in 1996, and I never actually got around to fixing it until now with the Covid 19 lock-downs (lockups).
Repairing these things is daunting at first sight because the compactness and the PCB tracks are all unfamiliar.
The circuit diagram has been traced as well as could be done without taking components off the PCB. As a basis for the circuit I based the drawing on a ‘typical’ Japanese 6 transistor radio.
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| similar circuit diagram |
Notes:-
- The mixer transistor is 2SA102, and IF transistors are 2SA101. Typical emitter current is around 0.5 mA for Q1 and 3. Q2 is the AGC transistor and the emitter current varies with signal level.
- The self-oscillating mixer Q1, has the local oscillator signal fed through the emitter of Q1. Accordingly the emitter resistor is NOT bypassed with a capacitor.
- The variable tuning capacitor in these types of radio have usually used different capacitance values for the aerial and the oscillator sections to eliminate the need for a padder. In this design, the two capacitor sections appear to be identical because there is a small capacitor (padder) in series with the oscillator section.
- The bias feed resistors for each RF transistor have been assessed by measuring the ‘Base bias current’ by connecting the mA meter from the transistor base to ground. Q1 and Q3 current: - 0.2 mA; Q2 current 0.085 mA.
- Emitter currents are around 0.5 mA.
- The bias point for Q4 has the emitter voltage at about half the supply voltage; ie -4.5 Volt. The two problems with this radio were (1); the earphone jack had a short which shorted the voice coil and (2); there was an intermittent open circuit between the collector of Q4 and the coupling transformer primary winding.
- The attached images should help to understand the layout. Audio quality is just OK; there is no crossover distortion.
Hope these notes help someone.
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Editors notes:-
I've found with this design of circuit, the 3 electrolytic's tend to fail, especially the 10uf (or similar) between the volume control slider and the base of the first audio amp. Causing low volume, the others cause instability or squealing.
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| Nice publication of Crown transistors |
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