Re: uBitx has a bit too crispy / semi-distorted audio even on low signal levels #ubitx #ubitx-help

Jerry Gaffke
 

If the audio is loud enough and has sufficient bass and treble and an SSB transmission is intelligble,
then this is not likely a BFO issue.  Could be a fault most anywhere between the antenna port
and headphone jack.   Though I would expect this problem to be somewhere from the demodulator
on through the audio amps.  

I would probably first try injecting a low level audio signal into the bottom of C50.
Could use audio from an mp3 player or broadcast radio headphone jack, but reduce the 
power level by a factor of a million or so (60 dB) first to avoid over driving Q70.
That means a voltage ratio of sqrt(1000000) = 1000  so from headphone jack to a 1 meg resistor
to a 1k resistor to ground, add a wire from the top of the 1k resistor to the bottom
of C50.  You also need a ground connection.  If that sounds good, then trouble is further back in the radio.
I'd look very hard at the demodulator if it isn't in the audio stages.
Trouble in the RF sections of the radio might mean birdies or weak signals, not so likely to 
create distorted audio.

Not sure anybody here knows what objectionably "crispy" audio sounds like,
you might give a link to an audio recording.

If there's another uBitx or Bitx40 in the neighborhood, you could patch in a signal from that other
radio into the your rig.  For example, inject a signal into c30 on your failing radio from the corresponding
point of that other radio, if the receiver now works well that means the fault is closer to your antenna jack.
Ideally remove c30 from both rigs, then have a 0.1uF cap in series with the
signal wire from one rig to the other to keep impedances about where they should be.  That wire is 
ideally a piece of 50 ohm coax, ground both ends of the coax braid. 

Good luck!

Jerry


On Thu, Jun 14, 2018 at 10:18 am, Mike wrote:
I have completed the uBitx and calibrated both frequency and BFO. The sensitivity appears OK. However audio has a strange crispy / semi-distorted sound to it.

I read the post from W3JDR https://w3jdr.wordpress.com/2018/04/11/ubitx-dynamic-range-improvement/#comment-1507  about biasing Q70 more strongly, however that only reduced the gain somewhat but didnt seem to help the crispy / semi distorted sound.

I have tried several headsets and a speaker. Same issue.

Anyone have any hints to what may be done to achieve better audio and get rid of that distortion effect?

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