Lovely transceiver arrived today. An iconic set from the late ‘80s. The fault was, the main tuning control did not work correctly. This is quite a common issue with this set and many others alike. The tuning control is an encoder, this one using 2 offset opto-couplers in the same package with a tacho-wheel. The tuning knob module is no longer available, consisting of a small PCD with a dual OP amp providing two digital signals derived from a dual opto-coupler. You can use alternative encoders from other sets with some modifications, but the resolution of the tuning wheel on this FR-736R is quite high.
Here is what you often find and a way of getting around the issue if all else fails. The problem is normally due to one of the IR sender diodes becoming lazy. If you measure the volts drop across each of the two diodes (wired in series with a resistor from the 5 V supply) you will find one will be higher than the other. If you use an oscilloscope to look at the output of the OP Amp. One side of the data chain will be intermittent. This confuses the logic circuit feeding the processor which decides which direction you are turning the tuning control, frequency up or down…
In the past, I have tried to alter the gain of the OP amp in the bad channel. This has never been a good result. If however, you bridge a resistor across the existing one which limits the current through the IR sender diodes, you can breathe new life into it, putting off the inevitable. I shunt the 120R SMD resistor with 220R.