[R-390] Mechanical filter tests?

[Charles A Taylor, WD4INP]

At 12:32 PM 2/17/2007 -0500, you wrote:
>Thanks for the information, Charles and Bob.  Unfortunately, I just did 
>the realignment of the radio, including the trimmers on the filters, so 
>that's not it.  And right now I can't dive back into the radio- I put my 
>wife off for a week while I recapped, and now I've got to dive into her 
>project.  Since I don't use the filter, I'm not doing anything right 
>now.  But I would like to sort out whether the spares I have are good.
>I'm using an HP 400 series voltmeter, which has a 1 Meg input impedance, 
>probably similar to the grid of the tube.  And I was using the 
>unattenuated output from the URM-25, which I believe is high impedance.
>The strange thing is that I tried to trim the filter since the cap was 
>gone, and got no change in throughput.  On the output side the filter had 
>a fixed cap.  When I clipped the cap to connect a variable, the signal 
>jumped about 6 dB, so the insertion loss was only 6 dB.  I certainly 
>didn't expect that!  There must have been a test setup for the filters 
>prior to installation into the radio- anyone know about it?
>Ed  WB2LHI


Ed,

The filters were consistent in their characteristics. Ought not to have 
been a "selected value" sort of situation.

Appears from what you write that the input capacitor that parallels the 
trimmer has been removed, and a wrong value was been selected to parallel 
the output trimmer. You need to hand-select the proper values.

Navy manual calls for 110 pF total capacitance across either side of the 
filter. My Motorola has 8-50 pF trimmer either side. That implies somewhere 
near 47 pf
for the fixed padding capacitor.

Suggest that as a starting point.

I'm putting this on the reflector for informational purposes.

73,

Charles


        -----
Charles A Taylor, WD4INP
Greenville, North Carolina