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ubitx v4 - distortion in received audio

 

Hello,

I have been using my uBitx (v4) mostly for receiving until now as I was lacking a good antenna. I observe a couple of problems.

1. Calibration is off by ~1200 Hz (down). I believe that is corrected by getting into the calibration menu and tuning at 10MHz with another receiver.

2. More seriously, the received audio is severely distorted. It also feels like the audio is saturated at both low and high volume levels..

I will search the mailing list archive for any past instances of such reports and remedies..

73
--
Ramakrishnan VU3RDD
 

On Mon, Mar 11, 2019, at 6:40 PM, Ramakrishnan Muthukrishnan wrote:

I have been using my uBitx (v4) mostly for receiving until now as I was
lacking a good antenna. I observe a couple of problems.

1. Calibration is off by ~1200 Hz (down). I believe that is corrected
by getting into the calibration menu and tuning at 10MHz with another
receiver.

2. More seriously, the received audio is severely distorted. It also
feels like the audio is saturated at both low and high volume levels..
I also have a 50uV test signal generator at 7040 which I connected to the antenna input. That signal *didn't* sound too distorted when I listened to it. I don't have any other test equipment to poke at the signals..

--
Ramakrishnan VU3RDD
Mark - N7EKU
 

Hi Ramakrishnan,

Not sure if your audio problem is related to the audio amp section, but if it is this link shows a fix that shouldn't be too hard to do:

/g/BITX20/topic/22892553#53357

Make sure you read Allison's post number 53356 in that link for adjusting the operating point of Q71.

73,


Mark.
 

On Mon, Mar 11, 2019, at 6:57 PM, Mark - N7EKU wrote:
Hi Ramakrishnan,

Not sure if your audio problem is related to the audio amp section, but if it is this link shows a fix that shouldn't be too hard to do:


Make sure you read Allison's post number 53356 in that link for adjusting the operating point of Q71.

Thanks so much Mark. I also spoke to Raj. I will try the modification tomorrow and report back.

73
Ramakrishnan VU3RDD
 

I thought the uBITX audio volume was low, until my kids told me to change the battery in my hearing aid.? 8-)
_._


On Mon, Mar 11, 2019 at 9:43 AM Ramakrishnan Muthukrishnan <ram@...> wrote:
On Mon, Mar 11, 2019, at 6:57 PM, Mark - N7EKU wrote:
Hi Ramakrishnan,

Not sure if your audio problem is related to the audio amp section, but if it is this link shows a fix that shouldn't be too hard to do:


Make sure you read Allison's post number 53356 in that link for adjusting the operating point of Q71.

Thanks so much Mark. I also spoke to Raj. I will try the modification tomorrow and report back.

73
Ramakrishnan VU3RDD
 

My v4 came with the bfo out of alignment,? which matters a lot more than frequency accuracy. This might cure it. Do listen with headphones before concluding an issue with the receiver. I hope to incorporate an adjustable audio filter and a final audio amplifier before the speaker and headphone jack.

Curt
 

The V4 use of discreet transistors suffers from crossover distortion in the audio amp final stage. Raj did some great work identifying this and providing a fix:
/g/BITX20/message/52181?p=,,,20,0,0,0::relevance,,crossover,20,2,0,22188783
I used a 10-turn 100 ohm mini preset pot and it ended up between 30-40 ohms. That completely fixes the distortion.

=Vic=
 

On 11/03/19 6:57 pm, Mark - N7EKU wrote:
Hi Ramakrishnan,

Not sure if your audio problem is related to the audio amp section, but
if it is this link shows a fix that shouldn't be too hard to do:

/g/BITX20/topic/22892553#53357

Make sure you read Allison's post number 53356 in that link for
adjusting the operating point of Q71.
Thanks Mark. I made the modification suggested by Raj (cutting the trace
across D15/D15 and added a fixed 47 ohm resistor since I didn't have a
preset). Audio is much better now.

73
Ramakrishnan
 

On 12/03/19 4:11 pm, Vic WA4THR via Groups.Io wrote:
The V4 use of discreet transistors suffers from crossover distortion in
the audio amp final stage. Raj did some great work identifying this and
providing a fix:
/g/BITX20/message/52181?p=,,,20,0,0,0::relevance,,crossover,20,2,0,22188783
I used a 10-turn 100 ohm mini preset pot and it ended up between 30-40
ohms. That completely fixes the distortion.
Thanks Vic. I did more or less the same (I didn't have a preset, so I
soldered a 47 Ohm resistor) and it fixed the distortion as well.

Ramakrishnan

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