EA 1978 Accu-keyer improvements

Electronics Australia published an article describing an updated version of the WB4VVF Accu-keyer around 1978.

The update was mainly adaptation to CMOS TTL chips.

The EA design suffered from a number of problems, some inherited to some extent from the Accu-keyer design, the were:

the clock oscillator (which stops after each character) has a shorter period in its first cycle which results in clipping of the first element;
the sidetone oscillator had a raw sound and used excessive current;
the output circuit lacked the flexibility to operate a grid block keyed transmitter, popular at the time.

This article describes modifications to fix / improved these defects.

Fig 1: Modified EA78 Accu-keyer

Fig 1 shows the modified circuit. Click on the image for a full sized image and use the zoom facilities in your browser to view the detail. Changed areas are highlighted in yellow.

Clock oscillator

The oscillator was modified to improve the start up, and the output inverted using a spare gate package to edge trigger the flip flops on the more appropriate oscillator edge.

Sidetone oscillator

The raw sound from the side tone oscillator is cause by the NE555 current limiter being triggered by the low impedance load. A small speaker output transformer transforms the speaker impedance so that the NE555 doesn't go into current limit, improving the sound and dramatically reducing the current consumption, extending the life running on a 216 battery considerably. A revised coupling capacitor also improves the sound.

Output circuit

The output circuit was modified to drive a reed relay for polarity insensitive output, and capability to switch up to 200V. The reed relay does draw more current than the original circuit, but the increased current consumption is more than offset by the savings of the modified sidetone circuit (above).

EA 1978 Accu-keyer improvements

Clock oscillator

Sidetone oscillator

Output circuit

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