electronicsRepairing a Superscope R-1220 receiver
The owner of a local sawmill always listens to the radio while he works. But the mill produces lots of sawdust so his beautiful 70s vintage radio slowly clogs up. The left channel is already dead but that doesn’t bother him. One day the radio stops working altogether. But the miller really likes his music. So he asks me to repair the radio as quick as I can, by only repairing the most important bits. I tell him I can fix the radio properly, but he doesn’t want to hear that; he just wants to hear the music. So I only repair the basics: the tuner.
2025-05-02
The problem
The radio has several issues:
- Tuning knob stuck
- Left channel doesn’t work
- After several minutes the radio stops working
The theory
The automated gain control (AGC) of the tuner does not work properly. When tuned to a station I measure -60mV across C121. But when I detuned the signal the voltages rises to -2.5V and the tuner becomes deaf. Retuning does not work and the voltage accross C121 stays at -2.5V.
The voltage across C121 sets the bias of the input transistor H101. When at 0V, the gain of this transistor is at its maximum. The more negative this voltage is, the lower the gain. A voltage of -2.5V almost completely shuts of the input transistor, making the tuner deaf.
After some digging around, I noticed an unwanted oscillation of about 170kHz in amplifier stages H104 and H105. The amplitude of this oscillation is sometimes low, but often it is high. This unwanted signal is rectified by diode H113 and smoothed by capacitor C121. The resulting DC voltage lowers the gain of transistor H101 dramatically.
The cause of the oscillation is the degraded capacitor C129. The power supply is no longer decoupled and a feedback loop via R168 and R153 (upstream in the power supply rail, see schematic in the service manual) is created.
The repair
The radio needed a proper clean. After that the tuning knob already worked much better. But I had to tighten the string by twisting it a few times at the tuning capacitors’ gearwheel to make it work even beter.
At the request of the owner, I didn’t repair the left channel of the amplifier.
Replacing the capacitor C129 solved the tuner problem.
Video (Dutch only)
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- Dutch video Torrent file