UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - FM tuning drift

Ray Cooper · 26th Apr 2007, 4:24 pm

These earlier valve sets could drift about quite a bit, though 500kHz does sound somewhat excessive.

Just a few ideas to toss about:-

Do you have a selection of ECH81 valves available? If so, it might be worth swapping them about to find the specimen that gives least drift.

I see that you've been changing the 'usual suspects'. The only suspects that'll make much difference, since the set has no AGC, are capacitors round the local oscillator. Some manufacturers specified components having particular temperature coefficients to try and minimise frequency drift: if any of these have been replaced at any time with ones having different temperature coeffs, the drift can only increase.

Many moons ago, there used to be a gadget called a 'Tempatrimmer': looked like a trimmer, but it was effectively a fixed-value capacitor that could be adjusted to have different temperature coefficients. Does anybody remember these gizmos? I've no idea where you'd get one: commercial sets never used them, far too expensive: but if you could insert one in the proper place, and had a vast amount of patience at your disposal, you could tweak it to eliminate frequency drift almost completely.

As a final query, you say that the detector uses germanium diodes, but I note that one of the valves is an EBF89: what are the diode sections used for in your particular receiver?

26th Apr 2007, 4:24 pm	#3
Ray Cooper Retired Dormant Member Join Date: Jan 2007 Location: Porthmadog, Gwynedd, UK. Posts: 199	Re: FM tuning drift - any ideas? These earlier valve sets could drift about quite a bit, though 500kHz does sound somewhat excessive. Just a few ideas to toss about:- Do you have a selection of ECH81 valves available? If so, it might be worth swapping them about to find the specimen that gives least drift. I see that you've been changing the 'usual suspects'. The only suspects that'll make much difference, since the set has no AGC, are capacitors round the local oscillator. Some manufacturers specified components having particular temperature coefficients to try and minimise frequency drift: if any of these have been replaced at any time with ones having different temperature coeffs, the drift can only increase. Many moons ago, there used to be a gadget called a 'Tempatrimmer': looked like a trimmer, but it was effectively a fixed-value capacitor that could be adjusted to have different temperature coefficients. Does anybody remember these gizmos? I've no idea where you'd get one: commercial sets never used them, far too expensive: but if you could insert one in the proper place, and had a vast amount of patience at your disposal, you could tweak it to eliminate frequency drift almost completely. As a final query, you say that the detector uses germanium diodes, but I note that one of the valves is an EBF89: what are the diode sections used for in your particular receiver?