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

[Graham Baxter]

Hi

I would urge you to test three things. One is the dc resistance of the
AGC amplifier anode coil, Z503, should be no more than 17 ohms and
peaked for max AGC voltage.

Second, with the multipin connector off the RF deck, check the DC
resistance to ground from pin E, positive probe to ground. This should
be about 1.8 Meg at any voltage up to 250.

The third thing is to test the dc leakage resistance of the AGC line
in the IF module. Unplug the IF multipin connector. From pin 6 of the
connector on the IF module to ground I see about 50 Megohms once all
the capacitors have charged. This is with 250v test potential, again
with the positive potential to ground.

Any significant leakage here will mess up the AGC, and sadly a
potential culprit is one or more mechanical filters. Lets hope not.

I have a second EAC here. It is slightly less convenient to check the
levels on that one because it is not near the test equipment. If it
becomes important, let me know and I will check it. However, on
broadcast signals it gives very similar S readings to the one I
measured.

I have to say, I have no complaints about the standard R-390A metering
and AGC circuit. The zero pot is a bit twitchy but once set mine tend
to stay put. Is it possible you have a fault?

One item of interest might be the diode load voltage.
With the measured 100mV unmodulated at 1.45MHz its about 14v

Graham