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Old 30th Apr 2007, 8:06 pm   #12
Studio263
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Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,574
Default Re: FM tuning drift -problem solved!

Well, I got it in the end!

With the aid of the service manual I quickly noticed that the HT voltage was rather low. I pounced on the selenium rectifier (a common cause) but a replacement made from four 1N4007's and a resistor did not produce the expected improvement in either voltage or stability.

It was then I spotted the real culprit, someone had fitted a cathode resistor of the wrong value in the output stage (should have been 390R, 330R was there, looking hot and bothered). This had lead to the output valve passing excessive bias current, sucking the lifeblood out of the HT supply and making the transformer and rectifier (located right next to the FM tuner unit) get very hot and bothered.

Things are back to normal now and it works properly again, many thanks to everyone that helped.
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