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In Reply to: RE: I saw in your pic of the MPX board... posted by sherod on June 30, 2016 at 19:44:18
Yea, old picture. There's a polystyrene in there now.
If I read the schematic correctly, C3 is 4700uf, 500v. Did you hear an improvement with the styrene cap? By the way, where did you find a good polystyrene with this large of value?
A polypro will be fine, since you've had an alignment, pull and measure the ceramic. Try to match the size of the ceramic with a film. That way you won't alter the separation adjustment. And yes the poly sounds better.
Is the voltage critical? I can't seem to find a decent polyprop that has 500v capacity.
Anything over 25V is fine. Should not be a problem finding a low voltage poly.
I haven't tested the capacitance of C3 cap yet but assuming it's + or - 20%, the only cap I can find with that high of capacitance value from the main distributors, Mouser, Digikey, Newark is a polyester. Do you have a source where you got your poly replacement? Thanks.
Since your's has already been aligned, I would pull and measure what the cap is now since the cap is in the adjustment circuit. Then match that value with the replacement cap or caps. Just get as close as you can to the current value. I looked at my notes when I did mine and that 4700pf ceramic actually measured 3960 and that's what I put in. If you're going to have it aligned in the future, put in the 4700 before its sent out to get back to the OEM nominal.
You may want to look here:
Thanks for the info. I have been confused. The schemo says that all capacitors are listed in uf, but you say this ceramic cap is 4700pf, not uf, correct? I plan on measuring it this weekend, but just wanted to make sure.
OK, in the metric system going from micro to pico means you move the decimal point 6 places to the right filling in the missing places with 0s.
The ceramic is clearly marked .0047, moving the decimal point yields 4700, which is indicated on the schematic.
I read on one of the threads that a ceramic capacitor can maintain its capacitance better at RF frequencies. Could this be why H/K put it at C3 on the MPX board? Just wondering...
Yes that's absolutely true, that's why you typically don't muck with the cap type in the front end, the key being RF frequencies, figure 100MHz for FM. Now once you hit the IF stage the frequency drops to 10.7MHz and then into the composite stage at about 55KHz which is where C3 lives, so no there's no good reason to use a ceramic there. More a cost and availability reason than a technical reason. IMHO
I see you put a poly cap at C15 on the MPX board. Besides that location and C3 are those the only locations you feel benefit from a film cap upgrade? I was curious where C15 was in the signal path?
C15 is part of the stereo switch filter, and on mine it was already a film cap. I did check for value, should be the same on yours.
...just as what is in the deemphasis. I haven't checked for value but was wondering if that location was critical for the sound.
No, not at all.
I have some polystyrene caps on order to put in that location.
I understand now and will measure and order accordingly. Sorry for bugging you so much. Even though this thread seems to be a private conversation between you and me, maybe someone else will see it and the info can help them out.
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