[R-390] Image problem cause... sort of

[Jon Schlegel]

Tony,

You may want to include like a 100k in your probe lineup.  Make it 
the last in the lineup (Pictorially:  PROBE---CAP---100k--->  ) to 
minimize shunt C from the probe.  Looks like circuit impedances are 
on the high side around there.  The 100k ohm may not even be high 
enough.  A trick might be to temporarily repeak the associated 
trimmer adjustment after connecting the probe so at least the probe's 
C will be absorbed into the circuit.

Jon


At 09:04 PM 10/23/2007 -0600, you wrote:
>I believe I have a handle (part of a handle) on my image problem. I 
>compared the preselector selectivity of my R-392 to the R-390a and 
>the R-392 is way way better.
>
>I put in a signal from the generator on the balanced antenna input 
>and put the spectrum analyzer on the grid of the first RF stage 
>through a 10x probe and a small capacitor. With the generator at 1.5 
>Mhz I tune the receiver to 1.5Mhz and peak the antenna control. I 
>note the amplitude of the 1.5MHz signal on the analyzer and then 
>tune the reciver to 3 MHz. On the 390a the 1.5MHz signal drops less 
>than 20db. On the 392 it drops over 50db.
>
>I also note that the peaking with the antenna trimmer is much 
>sharper in the 392 than with the 390a.
>
>I want to look at some other bands and see if it is like this everywhere.
>
>Tony
>
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