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NC-300 Crystal filter alignment
I bought a National NC-300 with a damaged phasing cap in the crystal
filter a while back, so I repaired it and realigned the whole thing. I thought I had it aligned correctly, but after checking the filter operation lately, I'm not sure if it's aligned correctly. On the wider crystal filter positions, it seems to work satisfactorily, but in the narrowest position, the peak is not extremely narrow as described in the manual, and the signals are attenuated quite a bit. I'm not positive if the phasing cap is actually in the mid-capacitance position when it's in the middle, where it's supposed to be to align the filter, or not. Also, when it's in the middle, pretty much all the signals are in the notch, so I have to offset it some to hear any signals at an acceptable level. According to the manual, the signals that are peaked are supposed to be relatively unaffected by the notch, but I don't get a definite peak anywhere. Just a definite notch. I followed the alignment instructions in the manual exactly, and re-aligned the filter several times. It still doesn't seem to work as described in the manual. I would appreciate any tips on getting this to work as it should. I do work at a company that has spectrum analyzers with tracking generators and other similar instruments, so I could probably take the filter out of the radio and sweep it if this would enable me to align it correctly. Thanks, Al, WA2AS |
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