[R-390] re: Call for measurements - 100dB carrier reading

[Roy Morgan]

At 09:55 PM 8/30/2007, Jim M. wrote:
>...there is also something going on with the BFO.  When the BFO is close 
>to zero beat with the strong carrier, I think it is also being pulled by 
>some kind of phase locking that happens between a strong IF and an 
>injected BFO - the IF signal feeds back into the BFO and makes it try to 
>phase or frequency lock.

Jim,

There has not been a TMC GPR-90 I've worked on that does not do this.  The 
suspected cause (not proven) is just what you describe: feedback path that 
locks the incoming signal with the BFO.  The cure (in the R-390 which does 
not normally do this to the same degree that the GPR-90 does) may well be 
to correct poor grounds or replace now-ineffective bypass caps.  If you go 
LOOKING for IF signals throughout the radio, you may be surprised where you 
find them: filaments, screens, audio output from detectors, and other 
unlikely spots.

You may learn something from the expected phenomenon that the IF signal 
pulls the BFO *toward* the IF signal.  Up if the IF signal is approaching 
from above in frequency and vice versa.


>... This is a true Collins deck with a Motorola IF, which is very sensitive.

Do you mean that it has way more gain than it needs?  If the IF gain is set 
too high, the signals will be higher than they need to be, especially 
toward the detector where the BFO is operating.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
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