Audio Distortion in AM Mode with SB-310

[Mark V. Johnson]

Heath team;

Had an unusual problem with an SB-310 that I thought I'd share with the 
team. I welcome feedback and other thoughts as well. As the AM detector 
circuits are the same in the SB-300 and SB-301, this would apply to those 
receivers as well.

The audio in AM Mode was distorted enough to warrant examination. SSB and CW 
modes were OK which mean't it was in the AM detector circuit. I fed 
modulated RF from my HP signal generator into the antenna circuit and scoped 
the detector output right at the input to the AF gain control. When I went 
to greater than 16% modulation, you could see the audio signal start to 
distorted.  Scoping the RF output from the last IF showed a cleanly 
modulated signal. It was distorted at the detector stage. I checked the RF 
bypass cap, the diode load resistor. No problem. I changed out the 1N191 
diode and that fixed the problem. AM sounds nice and clean now. I can take 
Modulation level up past 40% on my signal generator before any sign of 
distoration. Checking another SB-310 I have, this seems to be "working as 
designed" now.

To me a very unusual situation...having a diode behave like that.

Thanks

Mark V. Johnson VE3DJU/VE3DDI

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