IC-735 Main Encoder pcb Replacement





Having recently had another IC-735 on the bench for repair, this time with a faulty main encoder, I decided to make a modern replacement for the small pcb that sits on top of the encoder metal enclosure.

The encoder has a rotating disc with graticules that pass between a couple of IR LEDs and corresponding IR detectors. There is a small pcb that sits on top of the encoder that then generates two streams of pulses (one set inverted), as shown in the schematic below.



On my unit, both of the receivers had failed. A good search failed to come up with anything suitable as a replacement, mainly because the originals have a very small form factor. I did find some photo transistors of a suitable size, but the output from these had rather long rise and fall times and did not work well when tried out with the radio.

So I came up with this design:

(Note that the 4457P's are actually photo transistors, not LEDs)

 

This uses two pairs of IR LEDs and photo transistors feeding a 74HC14 Schmitt trigger inverter, that then generates the two pulse streams needed by the radio.  The AM4457 series of IR devices have the right sizes to fit into the encoder and are available from Mouser.

It was laid out on to a small pcb that is a drop in replacement for the original.











One minor irritation is that the IR devices are slightly too tall for the pcb to sit flat against the metal housing, but it all still fits into the radio.

So I now have a working, and reliable, new encoder for this neat old transceiver.

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