07-11-2018, 01:55 PM
Oh dear. I do get obsessive compulsive with these things.
Looking again at the schematics, I realise another detail overlooked, and blind-sided by the previous fixer(s). The RF bypass caps after the 2nd IF should be 120 pF on the "earthy end of the IF secondary, and 100 pF on the 6SQ7 anode circuit. These had been replaced with film caps, larger in value. The one picture found on-line, and a quick shot in a repair video clearly shows that originally these were the lozenge shaped micas used elsewhere in the set. Fortunately I have suitable values among the parts just received.
Further up-stream, I am working out why the spread band is hard to align (air trimmer all the way at max). Since this is 2-18 pF, it won't take much change elsewhere to confuse matters. I've done a few calculations already to help understand which component might have the most significant effect.
A shiny new capacitance meter (Actually capable of Capacitance, Inductance and resistance) is already helping out, and with care, I can make sense of the first few pF value caps I have been able to test out of circuit. I might put up a review of this instrument elsewhere on the forum. For around $30 from a well known online source, I think it is a handy bit of kit.
Looking again at the schematics, I realise another detail overlooked, and blind-sided by the previous fixer(s). The RF bypass caps after the 2nd IF should be 120 pF on the "earthy end of the IF secondary, and 100 pF on the 6SQ7 anode circuit. These had been replaced with film caps, larger in value. The one picture found on-line, and a quick shot in a repair video clearly shows that originally these were the lozenge shaped micas used elsewhere in the set. Fortunately I have suitable values among the parts just received.
Further up-stream, I am working out why the spread band is hard to align (air trimmer all the way at max). Since this is 2-18 pF, it won't take much change elsewhere to confuse matters. I've done a few calculations already to help understand which component might have the most significant effect.
A shiny new capacitance meter (Actually capable of Capacitance, Inductance and resistance) is already helping out, and with care, I can make sense of the first few pF value caps I have been able to test out of circuit. I might put up a review of this instrument elsewhere on the forum. For around $30 from a well known online source, I think it is a handy bit of kit.
I don't hold with furniture that talks.
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