[R-390] R-390A IF deck input impedance, sig gen reading

[Larry H]

I was curious about the IF deck input impedance, so I checked the loading
it has on my URM-25.  It had very little affect on the input voltage, so
the impedance must be high.  IE: I set my sig gen for 150 uv and terminated
it with a 50 ohm load and it measured 152 uv volts RMS with my HP 400FL AC
voltmeter.  I removed the 50 ohms and connected it directly to the IF deck
input and centered the URM-25 frequency to the IF deck.  I then measured
the voltage and it was 304 uv.

For making the IF gain adjust -7 vdc on diode load, the tm-11-5820-358-35
book says to terminate the sig gen with a 50 ohm load and then connect that
to the IF deck input.  Now we know why.  They want a real 150 uv to go into
the IF deck.

Regards, Larry