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Old 30th Apr 2017, 10:13 pm   #19
turretslug
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Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: GEC BRT402D Receiver.

Yes, the S130 looks more like something from the 1900s, not the 1940s! I wouldn't be surprised if it was a pre-war GEC (?) item. I'm not sure when gas-filled shunt stabilisers became widespread and commonplace- the AR88 was notably a mass early adopter but there were probably earlier mass production examples. Blumlein seems to have published the classic reference/comparator/series-pass full electronic regulator as early as 1933.

The hum-canceller circuit, as well as adding a dollop of extra heat, also gives us the mother of all "that" capacitors in the shape of a 100nF waxy between raw HT and the grid of the shunt power tetrode. In their keenness to avoid electrolytics, GEC did seemingly get themselves into a bit of a losing spiral- the smoothing valve adds something like 35mA extra HT drain and another amp of heater current, so the transformer got even bigger and the heat just kept building. A smaller transformer and no smoothing valve might even have meant available space for more paper smoothing block in the first place, and surely electrolytics were good enough by war's end to at least get a look in- some, notably US kit, had IO-base electrolytics for quick replacement.

As well as the Achilles' heel capacitor mentioned above, there's an anti-hunting capacitor between the amplified AGC line and the 1st IF amp screen grid- this is also a candidate for early replacement as any leakage could make several W81s rather ill. For some reason (probably physical size!) it's only rated at 250V when even the cathode decouplers are rated at 350V- it's in a particularly tight spot at the front of the chassis, next to the oscillator box and under the selectivity switch....
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