[Les Dickson]

If I’m reading the schematic correctly the high voltage winding on my power transformer is not center-tapped. If that’s correct how do I check the resistance in the high voltage windings to check that they’re not shorted or open. I’m used to checking pins 2 and 3 of the rectifier and then between the center tap and pin 2 then pin 3 where the last two readings total up to the first.

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[Alistair Thomson]

You are correct – the power transformer secondary (high voltage) winding is not centre-tapped. Note that the low end of the winding is connected directly to ground, so all you need to do is to check the resistance between the plate of the rectifier (which is where the hot end of the winding is connected) and ground.

This will check for an open or high-resistance circuit, but not for a short circuit, unless you know what the secondary winding DC resistance is supposed to be.

If the rectified tube is faulty, it might have leakage to ground from the plate, so it’s probably a good idea to remove the tube before testing, unless you’ve already tested all the pins.

[Les Dickson]

Thanks Alister. I guess I’ve never come across a radio with a power transformer that used a half-wave rectifier. It all makes sense now.

[Alistair Thomson]

Yes, particularly when the PSU smoothing caps have such low values. The speaker uses a field coil, L7, and maybe they managed to hum-buck the set with a specific design of speaker. Or maybe L7 is HUGE!

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