[Collins] Two More 51J4 Questions

[Dr. Gerald N. Johnson]

collinsradio.org/archives/manuals/354A-1%2806-55_09-62%29.pdf
appears to be older than the print manual I have. Shows the single value 
fixed resonator capacitors. I print manual I have shows smaller caps 200 
pf on the plate and grid circuits with individual 5 to 20 pf caps on the 
filter inputs and outputs. Most other applications include trimmer caps 
to adjust for each individual filter.

I see the original circuit shows the IF tube plate current passes 
through the filters. DC can change the transducer, it would be a good 
idea to make the mod to isolate that DC voltage and current from the 
filter inputs. Alignment may have changed that DC plate current.

I don't think the filters should be that different.

In the S-line filters the filter tuning affects passband ripple 
considerably and there its a trimmer capacitor.

When I saw the individual caps in the late J-4 manual I expected to find 
"selected in final test" on them but that statement is not there. I 
suspect it could or should be.

73, Jerry, K0CQ, technical adviser to the Collins Radio Association

On 8/23/2013 10:33 AM, Grant Youngman wrote:
> 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
>
> Sent from my iPad
>