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Are you sure you have the BFO set and fine tuned??? Check out VK3YE's YouTube video to see how he fine tuned his BFO setting:



He is listening to his transmissions, but if my memory hasn't failed me, the setting affects both TX and RX.

Hope that helps.
--
Mark,
KE6BB

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

Hi I actually have two uBITx radios. One that I have tuned the BFO on and another factory stock. I did swap in the other uBITx into the same cable harness: the "crispy / semidistorted" audio is exactly the same on both of them. Its kind of "cracking up" on the peaks. If i listen to a carrier, I hear the same effect. The tone is not clean.

Here is an example of how it sounds like. SSB signals kind of sounds like someone runs a strong RF clipper on them. If you wait until the last part of the clip you can also hear CW signals. Those signals are not too strong and should not distort.

/g/BITX20/files/180615_1218.MP3

There is distortion, sounds like clipping in the receiver due to overload
by a strong signal.

Also check to see if the Q70 mod is needed, it is listed in a different thread.

The last items is maybe the speaker itself is bad and distorting.

Allison

Hi!

There is also overload on weak signals
Tried Q70 mod: that didnt help, still distortion
Tryed several speakers and several headphones: the same
Tried two different ubitx units: no change, same distortion
I received the units around 15june2018. They were sent in June 2018. Could this be a bad batch?
Anyone else have experienced this? Any more hints?

Receiver overload.? There is no AGC in these.? TO my ear the CW sounded a bit hard.
Try tuning in a really weak signal.

It could be the audio amp.... V3 or V4?


Allison

Are you comparing? the uBITX signal quality with that from another receiver?
I could be wrong but S57DX sounds like a typical highly processed signal.

i was thinking the same thing.? Should try other signals, NOT during a contest.

Disregard S57DX and listen later in the recording. Pls listen to the CW signal later in the recording. This is a weak signal. Even the very weak signals barely detectable sounds distorted.

Its V4

I have used an oscilloscope and a signal generator to more tests regarding the audio distortion.

I have followed the signal thru the audio chain after the last mixer when appying a known clean and weak signal on the input so no front end or mixer overload ocurs. Everything looks clean on the oscilloscope until the output of final two transistors. The signal on the output of Q71 looks fine. On the output of Q72/ Q73 the signal is distorted around the zero crossing. /g/BITX20/photo/59179/3?p=Name,,,20,1,0,0

I wonder, is the biasing actually correctly done?
Should it have been like this instead? /g/BITX20/photo/59179/0?p=Name,,,20,1,0,0


Here is the distorted signal out of the amp (blue trace, note different Y axis scale). The red trace is the signal earlier in the audio chain. /g/BITX20/photo/59179/1?p=Name,,,20,1,0,0

Pls post your comments.

Some more testing results:
I attached an external high quality Hi-Fi type NAD linear audio amplifier via an electrolytic capacitor (for DC blocking). On that NAD amp. the audio is no longer distorted. I have a feeling that the final amp of uBitx V4 has a flaw in it.....

As far as I know, is the most common way of biasing a push pull amplifier to avoid crossover distortion:
To me it looks like there is a problem in the final amp ...

Mike,

Could you do a voltage test around the audio amp and report ? Volume = 0.

Q71/72/73 CBE

Raj

At 14/06/2018, you wrote:

I now replaced the output amp with a MCP6284 that I had laying in the shack. That works.? No distortion. I tapped the signal from the collector of Q71 via an electrolytic cap. This strengthens my suspicion that there is something going on with the output amp.

One thing I also noticed now is that there is a clicking sound whenever I change frequency with the VFO .... Is this known? I wonder if it is due to the clock generator? Or crosstalk ...

I will also do as you suggest : measure some voltages around Q71, Q72, Q73 on BCE.

Raj, VU2ZAP: here are the voltages you asked for.