[Collins] Two More 51J4 Questions

[Grant Youngman]

1.  I could swear that this issue did not exist when I started work on the radio 2 weeks or so ago.  

I now find a 17dB difference in gain (well, ok, loss) between the 6 KHz filter and the 3.1. After aligning the radio this week, it seemed "deef" with the 3.1 filter selected.  Measured the overall gain with the 3.1 filter to be -17dB  relative to the 6 (and the F500F94 also).  The difference follows the filter, so it isn't a fault in the filter module.

Would I be correct in assuming that there should not be this much difference?  I have another 3.1 filter on the way -- perhaps it will have better characteristics.

2.  In comparing early and late schematics I found a difference in the way the filters are terminated in the filter module.  The "early" radio (which I have) has a single 240 pf capacitor that switches in parallel with all of the filter inputs.  There is a corresponding 240 pf terminating capacitor on the outputs.  I presume these two caps roughly resonate the input and output windings.  

The "late" schematic shows individual small value caps across each filter input and output.  These switch in parallel with a fixed 200 pf cap to yield input vales of 205, 208, and 212 pf on the 1, 3, and 6 filter positions respectively, and 212, 215, and 218 pf on the corresponding outputs

I'm at least contemplating pulling the filter unit again and making these changes.  Anyone with 51J4 experience have a thought about whether it would be worth the effort?  Are these values critical?  Collins must have decided it mattered somewhere along the line I suppose ...

Grant NQ5T

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