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#18
03-01-2012, 07:31 PM
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It's not a quality issue. Yes, the calibrated temperature coefficient components were used to reduce the temperature dependence of resonant circuits.
Capacitors have several purposes. When they are used in resonant or timing circuits, their capacitance stability is of importance. In filters they are less critical. For bypassing and for power supply ripple control they are not critical at all. 
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#19
03-01-2012, 08:38 PM
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#20
03-01-2012, 10:34 PM
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I have no idea what circuit uses that in your unit, but if they specified a coefficient, it's probably wise to stick with it. I don't know if you still can buy those but I see no reason why not.
Before you order anything, look on the data sheet for the part (yes, capacitors have data sheets too) and see what temperature coefficient of capacitance it has. Do a little survey, perhaps of the bigger manufacturers and see what they offer. On the other hand, if you are restoring this and don't plan to use it much, it probably doesn't make a lot of difference.
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#21
03-01-2012, 10:43 PM
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from a previous post I was told to use temp stable in critical circuits but these calibrated temp coeffecient caps just kind of caught me off guard. I will do what I can to get similiar caps. This set is going to see a lot of use once I am done. Still need a couple mica/ceramic caps and then I will tackle the turntable. Its a webcor needs a little work which I will post later.
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#22
03-02-2012, 05:46 AM
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Why are you replacing the micas? Usually they are OK. I think it's best to replace the papers and electrolytics and then see how the set operates. If you shotgun the micas you may have it way off alignment in a complicated set.
Edit: whoops, I forgot in the beginning you said the set drifts. Pay no attention to my query!
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Reece Perfection is hard to reach with a screwdriver. Last edited by Reece; 03-02-2012 at 07:08 AM.
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#24
03-02-2012, 02:25 PM
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#25
03-02-2012, 05:56 PM
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It's just temperature. With the proper temperature compensation components it shouldn't drift so much that you ever have to retune. So that -750 part may be important after all.
Some engineers would cheat by adding a transformer to keep the oscillator tube warm but that only reduces drift caused by tube temperature variation. Another approach is to make the whole thing a temperature controlled oven, with a thermostat and heater/resistor so that there is essentially no warmup drift. Or there is the compensation approach. None of these is ideal, as they only reduce drift. It's impossible to eliminate it, short of synchronizing with an external or atomic standard. Some modern radios use a synthesizer, with an accurate crystal oscillator as the standard. My ham radio transceivers work that way; they drift less than 0.5 ppm, something not achievable with an LC oscillator.
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