RF Circuits

Great progress to report, and more dead bits!

The frequency counter unit is now working properly.

The two DC-coupled "front end" transistors in the 40MHz amplifier section were suspected as being faulty and were replaced. No exact replacements or direct equivalents were to hand, but looking at the specifications of the original units (2SC460) I figured that more or less any old small signal RF transistor would do, and as I had a bag full of MPSH-10s in my spares box I used those. The pinouts are incompatible and I ended up mounting them under the board.

Anyhow to my great delight the frequency counter then appeared to work, albeit not quite properly. The frequency count was correct at the bottom band edge (e.g. 7.000MHz) but when I tuned a signal at 7.100MHz the counter read 7.060MHz, near as makes no difference. This was consistent across all bands. The counter was proportionally out of kilter receiving at 7.200MHz, the counter reading 7.120MHz

Owing to the frequency scheme in use in this transceiver, and the fact that basically the counter is fed with frequency of 40.0 to 40.5MHz as the VFO is tuned across each band I figured there was a counter timebase problem.   The microprocessor logic in the counter unit adds the base frequency for each band to the difference between the 40.0 to 40.5MHz and 40MHz producing the correct frequency on the display.
Investigation of the divider part of the circuit revealed a problem with Q5, a CMOS BCD counter (4018) which was producing an output of 83Hz instead of the required 50Hz. Ha! That explained everything!  The frequency being too high would mean a shorter gate time for the counter than is required, and hence the frequency count would be too low.  This signal goes through a further division-by-10 process (a 7490 TTL chip) before the 5Hz, or in the case of this faulty radio 8.3Hz gating signal is sent to the counter signal chain.


Below:- the replaced Q5 - the replacement has obviously been in my spares box for quite a while!