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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Modification to Yaesu FRG7 SW receiver

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The Yaesu FRG7 shortwave receiver remains a popular choice with many listeners and this modification improves its ergonomics. As originally presented, the red LED “lock” indicator is designed to light up to signify that the Wadley loop is not tuned correctly and remains dark when all is well. This can be confusing for many operators not used to this tuning “quirk”.

A common circuit to fix this (as published on the internet) requires a small circuit board, two transistors and several resistors to do the job. By contrast, this circuit requires just three components, including a bi-colour replacement LED. It can be simply constructed on the existing LED-mounting panel.

One extra hole needs to be drilled in this PC board, for the third lead of the LED – see diagram. This also shows how the 3.9V zener diode (ZD1) and the additional 1kΩ resistor are wired to the LED board.

Now, when the loop is unlocked, the red LED section of the bi-colour led is switched on, indicating the mistuned condition (as designed). In this condition, the voltage appearing at the common cathode junction of the LEDs rises above the 3.9V at the anode of the green LED, thus turning it off.

When the lock condition is achiev-
ed, the green LED lights, indicating all is well. The green LED only is supplied from the dial lights, to conserve power when batteries are used.

The circuit is also useful anywhere a 2-LED indication is required and only one switching voltage, without its inverted counterpart, is available. There is no reason why separate LEDs cannot be used, if that is preferred.

Dayle Edwards,
Taylorville, NZ. ($40)

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