03-31-2016, 10:20 PM
What I don't quite understand is why Philco used a 3000pF mica cap that gets switched in parallel with the oscillator low frequency trimmer when from tests I did by putting a 2700pF cap in series with the 3000pF cap which brought the capacitance down to 1421pF got WWV at 5MHz much closer to 5MHz and checking the frequencies at 1MHz intervals between 5 and 10MHz shows they are almost spot on as well when compared to my RF generator's frequency setting.
Pretty sure that Philco would not have used a cap that was so far off the proper value and have that much tracking error on the dial on the low end of the SW band.
That said I still think maybe I have something wrong that lowering the value of that cap fixes unless all Philco 118s are off by that much at 5MHz. If all are off then it makes me wonder why the 3000pF cap was even used unless they used that cap in other models and figured they could save having to stock another part and that the resultant tracking error wouldn't matter.
EDIT:
It appears that some value of capacitor between 1421pF and maybe 2200pF will get the tracking spot on.
If that works I need to figure out why a lower value of capacitor was required.
An 1800pF cap gets it real close so I'm thinking that maybe 2000pF will be just about right.
If it works maybe someone else who is restoring a Philco 118 can try the same value just to confirm if I have an actual problem or not.
Pretty sure that Philco would not have used a cap that was so far off the proper value and have that much tracking error on the dial on the low end of the SW band.
That said I still think maybe I have something wrong that lowering the value of that cap fixes unless all Philco 118s are off by that much at 5MHz. If all are off then it makes me wonder why the 3000pF cap was even used unless they used that cap in other models and figured they could save having to stock another part and that the resultant tracking error wouldn't matter.
EDIT:
It appears that some value of capacitor between 1421pF and maybe 2200pF will get the tracking spot on.
If that works I need to figure out why a lower value of capacitor was required.
An 1800pF cap gets it real close so I'm thinking that maybe 2000pF will be just about right.
If it works maybe someone else who is restoring a Philco 118 can try the same value just to confirm if I have an actual problem or not.
