[R-390] New Project - Latest Issue

Mark Huss mhuss1 at bellatlantic.net
Fri Mar 2 09:13:20 EST 2007 

Joe, by "Chuck's Instructions', I take it to mean that you are following 
those instructions given here:

http://r-390a.us/R-390A%20Alignment%20v2.htm

to the letter. Now, Connect your signal generator to your R-390A Antenna 
as follows.
1) center pin of RF goes to one pin of the balanced input. Shield goes 
to the other pin. There is an adapter for this, or you can look it up.
2) Set generator to:

    * 750 kHz.
    * 4 uV.
    * 30% mod at 440 Hz (the new standard has been 1000 Hz for about 30
      years, but I find 440 Hz more soothing :) Does anyone out there
      know why the modulation standard was moved to that annoying 1000 Hz?
    * RF On.
    * Modulation On.

3) Connect VTVM to Diode Load (DVM acceptable, VOM is not).
4) Tune R-390A for best signal. Adjust Ant. Trim for strongest signal. 
Set AGC to MGC, and adjust RF Gain to maximum. Volume should be about 3 
(if you have a 600 ohm speaker). Bandwidth should be 16 kc.
5) On the speaker, the volume should be at least comfortable. The meter 
reading should be more negative than -7 volts. Switch bandwidth to 8, 4, 
and 2 kc, noting that the Diode Load voltage stays more negative than -7 
volts DC.
6) Switch back to 8 kc. Switch AGC to FAST. Switch AGC to Medium. No 
radical audio change should be noted on either position. Switch AGC to 
Slow. Audio should mute for about a second and a half. Carrier meter 
should PEG!, then drop down to its previous reading.
7) Increase RF Generator output to 400 uV. No radical audio change 
should be noted. and the signal strength should change about 20 dB.
8) Repeat steps 4 to 7 at least every octave or so (1.5 MHz, 3 MHz, 6 
MHz, 12 MHz, 24 MHz), or at every 1 MHZ (1.5 MHz, 2.5 MHz, 3.5 MHz...) 
if you want to get thorough.

What I am looking for is a basic Stage Gain and AGC test to make sure 
that the Stage Gain is right, and that the AGC is basically working 
properly. 4uV is an absolute maximum for -7 Volts out. Usually less than 
2uV will give you -7 volts out of the Diode load. And I seem to remember 
that you seemed to be having AGC trouble before.

Another note. the IF Gain adjustment is a simplified adjustment. Setting 
it by the manual results in excessive gain, not too little gain, the 
exact opposite of what you seem to be getting. Alternately, the AGC is 
not kicking in at a low enough level, which is why I suggest step 6 and 7.

One thing Chuck does not cover is setting the IF Gain properly. First do 
the IF Gain setting as called for in the manual to insure everything is 
up to snuff. What will result is an R-390A that meets original specs. It 
also results (if your R-390A is working properly) in the receiver noise 
floor being set by the IF amplifier rather than the RF Amplifier! Now, 
if you want to optimize it, you can do an S+N/N measurement. Then reduce 
the IF Gain a bit. Repeat until you get max S+N/N. If you happen to have 
a signal Generator with external modulation input, a Function Generator 
with square wave output large enough that if you feed it into the EXT. 
MOD. input of the Signal Generator it will cut off the RF Out (0% 
Modulation), and an Oscilloscope with a bandwidth of about 10 Hz to 3 
kHz (even a sound card will do that!), then setting the IF Gain pot 
properly becomes a snap using a technique developed by ASA called 
tangential noise measurement.

Set up your equipment as follows.

   1. RF Signal Generator:
          * Frequency of choice, say 1.5 MHz.
          * RF Amplitude, 4 uV.
          * Modulation External.
   2. Function Generator:
          * Connect output to RF Generator EXT Mod Input.
          * Output Frequency, 100 Hz.
          * Output Waveshape, Square Wave.
          * Output Amplitude sufficient to cut off Signal Generator
            Output and drive to greater than 90% modulation. You may
            need to adjust DC offset, depending.
   3. Connect RF Signal Generator to R-390A Antenna Input through Adapter.
   4. Connect Oscilloscope Channel A to Diode Load.
          * Set Channel A Coupling to AC.
          * Set Channel A gain to keep waveform on scope display. It
            should be a square wave of 100 Hz, with noise on the top and
            bottom of the square wave.
          * For better stability of the display, connect the trigger
            from the function generator to the ext trigger of the scope.
            Or use a T- connector on the square wave output.
   5. Set up the R-390A to receive the signal generator output, AGC
      should be set to MGC, and the RF Gain to max. Bandwidth 16 kC or 8 kc.
   6. Now comes the fun part.
          * Reduce the amplitude of the signal generator until the
            bottom of the noise signal riding on the top of the square
            wave is level with the tops of the noise signal riding on
            the bottom of the square wave.
          * Reduce IF Gain Pot. What you should see is the noise on the
            top and bottom of the square wave move apart again as the
            amplitude of the square wave is reduced. What you are seeing
            is the S+N/N ratio increasing in real time. No muss, no
            fuss, no calculator, Nein Flippen ze switches ud twisten zee
            knobs. If you do not, then you have a noisy front end, and
            you might want to substitute some of the V200 tubes to find
            a good one.
          * Repeat the above two steps until you get no improvement.
   7. Doing this manually has always been a pain for me. Since a 10 MHz
      scope is more than sufficient, and even E-Bay sells those for
      under a $100, it is probably worth it to get a scope just for
      this. And if not, I am sure a local Ham will let you borrow one.

Another Reference is 
ftp://bama.sbc.edu/downloads/miltest/st32152/ST-32-152x.pdf It does not 
contain the above procedure in paragraph 6. But if you have a signal 
generator capable of FM Modulation, it makes alignment of the 
transformers in the IF Deck a treat.


Carole White-Connor wrote:
> With your help, I'm getting there. The set is performing nicely.
>
> My new issue is this: if I adjust the IF Gain per Chuck's instructions, I lose a
> lot. With weak stations, I have to adjust the AF Gain to maximum to be able to hear
> the station so that I can understand it. If I just the IF Gain high (about 1/8 turn
> from maximum), I get more background noise but I can hear the station with the AF
> Gain at about 6. The background noise does not drown out the station. This occurs
> both below and above 8 mhz.  I don't think I'm losing sensitivity because I can
> still hear the weak stations. On strong stations, they come in loud and clear pretty
> much wherever I set the IF Gain. I've been listening a lot on 49m and 41 m. Those
> bands are alive with stations.
>
> All the tubes in the set test good. The weak ones have been replaced. I haven't done
> any work in the IF module except for replacing the caps that Chuck recommends
> replacing. I haven't touched the RF module.
>
> Any ideas? Any particular things to suspect? Any troubleshooting tips?
>
> Again, thanks for your help (and your patience)
>
> Joe Connor
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>
>   


-- 
----------------------------------------------------------------------
Some people are like a Slinky .. not really good for anything,
but you still can’t help but smile when you shove them down the stairs.
----------------------------------------------------------------------

More information about the R-390 mailing list