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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 1st Jun 2003, 9:47 pm   #1
Roy
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Join Date: May 2003
Location: Hertfordshire, UK
Posts: 258
Default Philips B3G97U – intermittently dead on VHF

This set is almost identical to the B3G75U (covered by Trader 1342). It’s a small AC/DC set from the late 50s and covers MW, LW and VHF, using permeability tuning on all wavebands. The VHF front-end uses two UF80 pentodes, rather than the more usual UCC85 double triode.

The set worked perfectly on MW and LW, but on VHF it would play for a while and then suddenly go silent. Sometimes, only the stations at the top end of the band would vanish, sometimes the band would be completely dead – there didn’t seem to be any pattern to it, but clearly something was making the oscillator stop. The infuriating aspect to this was that the slightest disturbance (e.g. attaching a probe just about anywhere in the entire set) would clear the fault instantly, and no amount of prodding and poking would make it return. It would only return days or weeks later, long after the set had been reassembled and the details of the latest repair attempt forgotten! Various things were tried, including valve substitution, long searches for dry-joints, valveholder and switch cleaning, etc, but each attempt needed to be followed by a lengthy soak test, so this went on for weeks on end.

To cut a long story short, there are three trimmers in the VHF tuner head, the usual Philips type comprising a ceramic tube with an outer foil and a threaded brass rod inside for adjustment. The brass rod is normally earthed to the chassis via the trimmer’s mounting screw, but one of these trimmers is in a position in the circuit where neither plate is earthed, so it is mounted on an insulating block. It was the connection to this trimmer (a solder tag, screw, nut and shakeproof washer) which was causing the problem. The screw wasn’t loose but the trimmer was tarnished and the connection to it must have had an intermittent high resistance. The connection was cleaned and reassembled, and that seemed to do the trick.

That was over two years ago, and the set has been in regular use ever since, with no reappearance of the fault, so it’s probably safe to say that it’s fixed.

This may not be a "stock" fault, but if it saves anyone else weeks of frustration it's worth a mention.

Last edited by Paul Stenning; 27th Dec 2004 at 1:01 pm. Reason: Fix link or code for vBulletin
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