Medium waves small radio

[craciun1965]

Hi,
I've designed back in the 90s a small medium waves-short waves superheterodine radio, powered at 1.5V

Above is the schematic, and below the PCB:

It has a very good sensitivity and selectivity taking in the account how simple is the schematic.


BR/Daniel

 

[Audioguru]

I haven't used an AM radio for about 55 years but the ones I used all had Automatic Gain Control so that local stations and distant stations all had the same loudness. Yours doesn't have AGC and it might even be overloaded by local stations.

 

[craciun1965]

Hi,
I've only designed the PCB of the schematic , the schematic was from a Romanian electronics magazine called "Tehnium".
As I specified it's a simple schematic but with very good sesitivity and selectivity.

BR/Daniel

 

[betwixt]

It does have a crude AGC system - look at the bias on the IF stage, it is reduced by the voltage from the detector.
Not perfect but better than nothing.

Brian

 

[Audioguru]

I see that its R4 applies some bias to the IF transistor for some AGC.
But it is the input transistor that will be overloaded by strong local signals.

 

[craciun1965]

Hi,
One of my friends asked about the value of C7, the correct value it's C7=10microFarads.
Below it's the modified schematic as the original was with a wrong value for C7:

 

[betwixt]

Unfortunately, with such a simplified superhet circuit, it isn't possible to apply AGC to the first stage. Changing T1 bias would also shift the tuning to some degree as would any kind of 'voltage controlled shunt' across the input signal.

10uF seems a reasonable value to use for C7, I'm not sure if the original 10mF actually meant microfarads.
If any part could be easily improved it would be the audio amp which has no negative feedback and seems to rely on the 1.5V supply not being capable of burning out the 8 Ohm load!

Brian.