Sunday, August 24, 2008

BBC FM radio transmitters and broken switches

This weekend brings a new FM aerial and the second Leak Trough Line is brought back to life.

As hoped the second Trough Line arrived, albeit with no valves, in fact no glass at all, the fuse and 'fuse lamp' were both missing. Before replacing the valves I checked continuity of the mains transformer and on/off switch. The transformer was OK but one half of the switch was faulty. So I removed the switch from the circuit. Connected to the mains, with chassis earthed, gave ac on the heater and HT windings. So I put an EZ80 rectifier in the appropriate socket and checked the HT dc. All fine. With the rest of the valves in everything was still good. With aerial and amp connected I now had a working tuner. Unbelievably easy.

Checking the on-line Maplin catalogue for fuse lamps was a disappointment, it seems they've gone the way of variable capacitors and 3mm (1/8 inch) wander plugs. Such things are still available from some sources, e.g. Farnell have Belling Lee wander plugs, not cheap though. Then I noticed that my "donor" tuner, bought for £4.99 on Ebay, had 6 of them. They're 8V, rather than 6.3V, but perfectly usable. The main purpose of the donor was to provide a stereo decoder. I'll describe how I extracted it in a later post.
A few minutes use revealed that not only had the mains switch failed, but so had the two slide switches. Fortunately my Trough Line surfing had already take me to Keith Snook's page on repairing and tuning the Trough Line. I pretty much did exactly as Keith described, though I noticed that there were some fine cracks in the rubber cushion behind the brass strips. Which made me think that the rubber was probably quite a bit harder than when new. As a, probably temporary, remedy I cut a couple of small pieces of self amalgamating rubber tape and super-glued one to each of the cushions. Oh, and to clean the contacts I rubbed the ball from an old PC mouse over them. Now both slide switches are working well, and I've replaced the volume control, and its faulty switch, with a rotary switch.

As for the aerial, I've replaced my cheapo omni-directional loft aerial with a cheapo 3 element directional one. Which way to point it though? Perhaps this is another job for Google Earth - see BBC FM.

More Trough Line links -

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