Distorted audio v4 Main Board?

An initial report from Mike (callsign not known) of distorted audio on a v4 main circuit board was tracked down to the audio amplifier.  The v4 main board is the new board shipping from HF Signals from around the beginning of June 2018.

Mike solved his problem by bypassing the audio amp and using an output amp as indicated bottom left in the photo above.   You can see that he has also incorporated a mic amp board in his prototype.

We will be watching out (or rather listening out) with interest to see if others are experiencing the same distortion issues from the audio amplifier section.  Hopefully this is just a one-off!

Reference

Follow up

Mike has now fixed the problem with crossover distortion.  It was a biasing issue in the audio final.

Raj VU2ZAP suggested lifting up D15 or 16 and disconnecting one end from the board. Then, solder the center and one end of a 47 or 100 Ohm preset between the two diodes.   Make sure the preset is at minimum. Power up and slowly increase the preset till distortion goes away, while also checking that  Q72  and Q73 don’t get hot when the volume is low.

 It is possible that this problem may reappear on other new v4 rigs.

Redoing the biassing as suggested by Raj VU2ZAP solved the issue.  Mike  took a 500 ohm pot and paralleled that with a 125 ohm resistor.   He  says, “The mod doesn’t look so good (LOL) but it works. One of the transistors became warm if the voltage difference due to the pot became too large. I adjusted this while feeling the temp and listening to a CW carrier”.   The setting point was where he observed low/no distortion AND low temperature.

The photo below shows how Mike fixed the issue.

Reference

7 Replies to “Distorted audio v4 Main Board?”

  1. I’m surprised by the position of the authors and sellers of the transceiver.
    despite the obvious shortcomings main board of version 4, they did not recognize the problem, do not comment on the situation, do not offer compensation for inconvenience, do not do not remove the main board from the sale.
    those who bought main board v4 did not even receive an apology for the inconvenience.
    why is that?

  2. Mike initially fixed the problem by substituting an off-board audio amp. Then, ‘Mike has now fixed the problem with crossover distortion. It was a biasing issue in the audio final.’

    What was his final fix?

  3. My oscilloscope clearly shows crossover distortion as well. Also I was looking into ways to get 5-6 dB more audio gain when I measured both sides of R51. The high side was 11.7 and collector side was 1.94 V. I was thinking of changing the collector resistor from the 4.7 K to 10K but with 0.69 Volts on the base it implies about a beta of 400 on the 2N3904! Anyway that may require two resistor changes. Not sure yet if I need it however but that transistor is NOT biased half way.

  4. Crossover distortion occurs whenever the amplifier load is lower than about 200 ohms. At 8 ohms it sounds like a torn speaker cone, at 60 ohms (headphones in series) it is quite tolerable but noticeable. A 8 ohm-to-200 ohm transformer (5.6:1 turns ratio) solves the problem if you have an efficient speaker. A 100 ohm resistor in series with series-connected ‘phones is perfect. Since I will employ a switchable CW filter I will bypass the existing amp and use one with a lower current drain.

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