[NJARC] AK 80 question - more tsuris

[Al Klase]

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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>
>     -- 
>     Al Klase – N3FRQ
>     Jersey City, NJ
>     http://www.skywaves.ar88.net/
>
>     ------------------------------------------------------------------------
>     ______________________________________________________________
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>
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-- 
Al Klase – N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/

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