[NJARC] AK 80 question - more tsuris

Mon Sep 11 22:28:23 EDT 2017

The "distortion" is constant, at all volume levels, and signal levels.

Fed modulated signal from my RF generator, see attached scope traces.  
Very low signal at the IF plate, additionally my 'scope loaded it down.

The oscillator grid resistor measures 103K, and the voltage is -7 
relative to the cathode.

Looking at the trace, it's not really that distorted.  A tempest in a 
teapot??

On Mon, Sep 11, 2017 at 09:35 PM, Al Klase wrote:

     Bill,

     Does the distortion decrease with decreased "volume" (that's
     actually RF gain) or weaker input signals?  Stick your ear right in
     the speaker.

     Scope the IF plate looking for clipping.  There a balancing act 
here
     between the gain of the 1st detector / IF and the 2nd detector
     /audio out tubes.

     You might check that the latter are working properly.  If you don't
     have enough audio gain, then you're going to run the RF/IF into
     distortion before the 3nd detector.

     Also, is the oscillator running well?  Check that the grid resistor
     is really 100K.  There should be quite a few negative volts on the
     grid when measured with a hi-Z meter.

     Good luck,

     Al

On 9/11/2017 8:13 PM, William S
       Zukowski wrote:

Tried Al's idea (grounding the
         cathode), and do I seem to get more gain (volume), thanks Al. 
         However, there is slight audio distortion (this way and with 
the
         original wiring).  Happens on strong and weak stations, long 
and
         short antenna.

         Cleared the audio stage, the audio is distorted from the IF
         going into the 2d detector.  Confirmed with a demod probe and
         signal tracer.  What would cause audio distortion out of the
         IF?  Tried changing the IF tube, no change.  It's not really
         that severe, just want it as perfect as possible before giving
         it back

Thanks, Bill

         On Sat, Sep 09, 2017 at 08:55 PM, Al Klase wrote:

         Just remember

           Reply = Poster

           Reply All = Everyone

           _________________________________________________________

            Bill,

           Well, this is pretty disturbing.  Clearly there are errors in
           the documentation, and maybe the original wiring.  Can you 
say
           FUBAR?

           I'd disconnect the oscillator cathode from the rest of the
           circuit, and ground it.  The slider on the volume control
           raises the cathodes of the !st Det and IF above ground, while
           their grids are at ground for DC.  this should, indeed, 
effect
           RF/IF gain control:  more cathode voltage = less gain.  BTW,
           35's are remote-cutoff tetrodes, so they're intended for this
           sort of operation.

           The alternative would be to leave the oscillator cathode
           connected to the volume control, and clear any ground on that
           circuit node.

           Good luck,

           Al

On 9/9/2017 6:37 PM, William S
             Zukowski wrote:

             Just remember
Reply = Poster
Reply All = Everyone

_________________________________________________________

             Just restored an AK 80.  After replacing all the caps, 
noisy
             resistors, two bad tubes, most of the rubber wiring that 
had
             most of the insulation gone, it actually works.

             My question is about the "volume control" (see schematic):
             It's in the cathode circuit of the oscillator, so when it's
             adjusted, it sounds almost like a regeneration control,
             oscillating, etc..  It also looks like the schematic is
             wrong.  It shows the cathode grounded, which would negate
             any effect of adjusting the control.  It's not grounded in
             this set.  The radio is basically untouched, except for two
             electrolytic caps already replaced.

             Thanks, Bill

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