Three diodes won't fix the problem that the bias on the base of Q73 is
highly dependent on Q71 collector voltage.
If you ever redesign this circuit I would use capacitor coupling from
the collector of Q71 to the bases of Q72 and Q73. The base bias
of Q73 would then be set with a resistor to ground. Then everything is
symmetric and distortion should be minimized.
On Fri, 29 Jun 2018 15:45:51 +0530
"Ashhar Farhan" <farhanbox@...> wrote:
What if we use three diodes instead?
On Fri, 29 Jun 2018, 13:35 Raj vu2zap, <@Raj>
Got back to town and plugged in the uBitxr4 and found the audio
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
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.
Bengaluru, South India.