Connex Saturn EPT360015B audio is choppy

[QuickdrawMcgraw]

Connex Saturn EPT360015B audio is choppy... I'm working on this thig, the best way to describe this audio is it sounds like you are talking through a fan. I have done a full alignment and eliminated all the mods to this radio. The spectrum analyzer shows that the lower side and the upper side look completely different. every radio out of the 50 or 60 that I have worked on so far have had a mirror image on the upper side and lower side of the display. If I knew what I was doing, I would probably know what to do about this. I have ruled out leakage across almost every diode on the board. I have substituted another IC9 mixer. I would really appreciate input from the excellent users of this site.

 

[nomadradio]

A lack of symmetry from one sideband to the other has to be caused by the 10.695 MHz crystal filter. They do fail in this way from time to time.

I would try eavesdropping on the PLL's local-oscillator output, listening for any interruption to the steady carrier it should be. Key the mike, talk and see if that distortion is audible on either the VCO or the carrier oscillator.

Does changing the mike gain have any effect on this distortion? Does turning it down to a Watt or two PEP clear it up at all?

Not a symptom I see in new radios. Older radios will do this when electrolytic caps go bad in the ALC circuit.

73

 

[QuickdrawMcgraw]

Thanks nomadradio… I will look at your recommendations. The mic gain makes no change in the distortion. The power level does affect the amplitude but the symmetry is still off. That’s what I was trying to show in the pics. I suppose I could have provided details with each pic. Thanks again!

 

[QuickdrawMcgraw]

Update... The 10.695 crystal and circuits are good. Double checked the PLL Alignment... all is perfect. I'm hoping someone can help me understand the principles that effect the symetry of what is shown on the spectrum analyzer. I've hit the books on this one, but nothing I can find addresses this. Thanks in advance.

 

[nomadradio]

I would tune in the PLL's VCO output and listen. If the spec-an has a demod audio output, listen to see if it remains a steady carrier while transmitting sideband. Demod the VCO carrier as if it were a sideband signal. Any "warble" in the heterodyne tone would be a hint. If RF from the power stages was somehow leaking into the PLL that could explain it.

Maybe?

Shot in the dark.

73

 

[midnight special]

i had a galaxy 2527 that had choppy mod,,, the fix was to disassemble the front of radio and resolder the ground behind the mike plug,, the mike plug on mine was also kinda loose but tightened while there but fixed the choppy audio,,,, just an thought,,,,,

 

[QuickdrawMcgraw]

Final update... after much study and learning more about the spectrum analyzer. I learned that modulation is what skews the amplitude measurement. NOTE (AM modulation is by default asymmetrical) but not as drastic as illustrated in my original post. The nonsymmetrical reading that I was getting was due to modifications to the audio amplification chain. I investigated the resistor and capacitor values for every component after the IC4 mic amp. What I found was the R240 and R241 resistors had been changed to different values than specified in the schematics. One was 5 ohms and supposed to be 15 ohms. The other one had been clipped and resoldered but matched the specified resistance. Thanks for your help nomad radio. I also suspected leakage from a diode initially.

One day maybe I'll know something!

 

[QuickdrawMcgraw]

Thank you midnight special!