[R-390] ACG problem

[Drew Papanek]

On the topic of R-390A AGC problems Bill Abate wrote:

>SSB reception is still distorted but not as bad as when I was running
>255 VDC B+.  My friends 390A receives SSB at full RF gain and sounds
>good.  Maybe I can swap radios when he isn't looking! :)

Perhaps you and your friend could swap IF (or other) modules for test 
purposes- that would certainly help pinpoint the location of any circuit 
fault.  You could take voltage measurements at various points  on the 
substitute module and compare with those taken from your module.   Maybe 
your IF module would provide the desired SSB reception in his radio, telling 
you that the problem lies elsewhere in your radio.

On the same topic Bob Camp wrote:

(excerpted)
>Some 390's have been modified to increase the BFO injection and some have 
>some detector tubes in them that have gotten a bit soft. In this case a 
>soft tube *might* work better than one that's up to spec.

>I would also agree that most R-390's do not work very well on SSB with the 
>RF gain turned all the way up. If I was going to look for something odd I 
>would take a look at the radio that picks up SSB fine rather than the one 
>that has problems.

Bob brings up some very good points.  Again, module swapping or even tube 
swapping (from the other radio) could provide clues.

About B+ voltage:  Bill's 255v seems a bit high even for a radio with 120VAC 
power and no B+ reducing devices.  Where was the B+ measured?  At the 
rectifier cathodes is not a good place because the large ripple voltage 
there would confuse many DC meters, giving inconsistent/inaccurate readings.
A better place would be at the filter caps (remember, the filter is choke 
input).   The screen of the audio output  stage (either Line or Local) is 
convenient-yank the 6AK6, wrap a wire around the appropriate pin and plug 
back in.   The wire can be left in place to provide a convenient B+ shock 
hazard :)

I measure 180 volts B+ at the 6AK6 screen with 120VAC power supply input and 
200 ohms in series with the rectifier common cathode lead (solid state 
diodes).

Drew