[R-390] Newby needs help

[k2cby]

This sounds obvious but the first place I wood check is to be sure that the
shorting straps are in the right places on the rear terminal strips.
Terminals 1 and 2 connect the front panel RF Gain control and should be
strapped together. Terminals 3, 4 and 5 connect the AGC bus for either local
control or diversity reception. 3 should be strapped to 4 for normal
operation.

 

Second, I’d make sure the IF gain pot (one of two on the top of the IF
chassis) isn’t set wide open. The R-390A has a prodigious amount of IF gain.
This pot is usually adjusted to minimize noise but that setting is typically
no more than 1/3 to ½ open.

 

Third. Turn the noise limiter off and keep it there. Particularly at high
settings, it distorts the audio like crazy, and it isn’t much use unless you
have ignition noise.

 

If none of this seems to produce the desired result you should suspect a
problem in the AGC system. To troubleshoot it satisfactorily you need a vtvm
or a digital meter with an input impedance of at least 1 meg (preferably
10). You also need a signal source capable of pushing the AGC voltage high
enough to be in the control region. Your long wire antenna with the receiver
tuned to a loud AM station should do the trick.

 

Tune the receiver to the signal source (BC band station). Back down the RF
Gain control on the front panel. Set the VTVM to 10 volts dc full scale
(preferably 30 volts or whatever the next higher setting is). Connect the
positive lead of the VTVM to chassis ground (Terminal 16). Connect the
negative lead to Terminals 3-4 of the terminal strip. (You are measuring
negative AGC voltage so the meter connections are opposite what they
otherwise might be unless your vtvm has both a +dc and a –dc mode). Now
gradually open the front panel RF Gain control. There should be no action at
first, then the AGC voltage as measured by the vtvm should rise to at least
12 to 15 volts.  

 

If it doesn’t, disconnect the jumper between terminals 3 and 4. Turn off the
receiver and measure the resistance from pin 3 to ground. It should be
pretty close to infinite. If it isn’t, there is a leaky capacitor on the
drop side of the AGC bus (the end that connects to the AGC-controlled
tubes). If this checks out ok, you will have to check the source side of the
AGC bus from the detector through the noise limiter.

 

Good luck.

 

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net 

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