Have you checked that the tuning pot is supplied with the correct voltage? I suppose it is hooked up as a voltage divider.

Preset potentiometers were indeed a mess - actually the 'preset' one (10kOhms) was salvageable, the FM one (50kOhms) was open circuit. An equivalent was fitted (not a cosmetic or size match, but all that was available). I can't understand from the circuit why the potentiometers are different values, or indeed why there is a separate one for the FM circuit as all the presets and the FM circuit are effectively similar, parallel circuits.

Remaining problems: the preset circuit will adjust to the 4,5v tuning voltage (with the preset tuner set to just below 88MHz) but the FM voltage will not go above about 3,5v. Thus the tuning is inaccurate and doesn't cover the full range... and there is a loud 'buzz' between stations, too.

Time to take it all apart & start again.

If your Beomaster still has the black square rectifier in the corner of the tuning board, I suggest you replace it.
They will almost always be faulty and sometimes even falling apart physically.
Note the pinout and twist the pins of a modern replacement to fit.
Replace the 470uF cap next to it as well, it's almost guaranteed to have a bulge outwards.

Will the presets (fingerwheels) cover the full range ?

Martin

The rectifier & adjacent capacitor both look (physically) fine but I will check them & try to find replacements.

The frustrating part is that the presets cover the full range - the main FM tuning is the problem. And I didn't have the 'buzz' between station before I started - I think I have mis-wired or disconnected a 'ground' somewhere. I'll take it all apart & start again.

There are small wire "leads" from the tuning board to the board with the switches.
Depending on the version, these will be either short flexible leads (good) or one-core stiff leads (bad).
The solder joints will often crack at the tuning board with the latter.
Same issue at the connections from the tuning board to the FM frontend casing.

Martin

kimberley wrote:

I can't understand from the circuit why the potentiometers are different values

If they're connected as voltage dividers, the resistance value isn't critical. Maybe they just used what they had a good supply of?