Re: uBitxr4 audio distortion fix

Gordon Gibby <ggibby@...>

Don’t forget those emitter resistors, that Add additional voltage drop.

Someone suggested using a bit of feedback, I didn’t look at the circuits, but wouldn’t that be one solution?

On Jun 29, 2018, at 06:47, Raj vu2zap <rajendrakumargg@...> wrote:


It was too much! Q72 started to get hot but not Q73. Means they are not matched.
Iq went up almost 100mA. So I used a pot where 47 Ohms seems fine.

What may work that instead of these diodes use the same transistor BE junctions!
Then just two may work, need to experiment on this.


At 29-06-18, you wrote:
What if we use three diodes instead?Â

On Fri, 29 Jun 2018, 13:35 Raj vu2zap, < rajendrakumargg@...> wrote:
Got back to town and plugged in the uBitxr4 and found the audio distorted.

Poking around the finals I found that the two diodes D15 and D16 gave only a drop of 1.2V total
but the final transistors needed more like 1.5V. i.e. 0.75v each.

I simply cut the track between D5 and D16 and soldered a 100 ohms multiturn preset as shown
in picture below. (Preset leads used is center and one side)

Start with the two soldered connections of the preset set to ZERO ohms.

Power on and tune to a station at low volume and slowly increase the preset till the audio
becomes clear. In my board the RX power consumption increased by 25mA at the best sound.
Just check and see that the output transistors are not getting hot.

My board sounded great with 47 Ohms between D15 D16 and I run at 13.6V. If you run at the
same supply voltage then you can try a fixed resistor. For 12V supply you could try a higher value
like 56/62 or even 68 Ohms.


Raj, vu2zap
Bengaluru, South India.

Content-Type: image/jpeg; name="121df75.jpg"
Content-Disposition: inline; filename="121df75.jpg"
Content-ID: <>

Join to automatically receive all group messages.