A block diagram would probably help you if there is one in the service manual. My guess is that Q103 is the radios local oscillator and Q104 is a buffer to get the signal up to a suitable level for the frequency counter. If you know how a Superhet Radio, (which most radios are) works the local oscillator gets tuned to a frequency a fixed amount away from the wanted broadcast. The oscillator and the incoming signal are mixed and then all further amplification is at a fixed frequency(often 455KHz for AM and 10.7MHz for FM). This is far easier to do. If you measure the frequency of the local oscillator and then add or subtract the difference, you get the frequency the radio is tuned to. The next step up is to use a Phase Locked loop circuit giving channelized tuning and allowing memories etc. In 1981 it was Sanyo who managed to make the first single chip PLL IC for CB rigs giving 40 channels at around 27MHz which was an impressive feat at the time. I have commented before that back in the late 1970s /early 1980s Digital Displays were the latest and greatest thing. Back in 1979 a colleague bought a frequency counter module (which cost about £25) and fitted it in a normal HiFi tuner. He brought it into work and impressed everyone in the office in the same way a cool raspberry Pi project would impress people today.
Thank you very much. Looking at your explanation, I probably understand the working principle of this machine. At first I didn't understand how digital monitors get frequency changes. I know it now.
This is the datasheet module (or one very similar) that my colleague used. http://www.milair.co.uk/FC177.pdf The circuit inside the Sanyo will be very similar.
