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 Post subject: Homebrew transistor pocket radio
PostPosted: Aug Mon 31, 2020 4:25 am 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
This radio came to be as a result of all suddenly having a lot of free time while working from home as a consequence of the pandemic.

I've had a bag of ГТ309 (GT309) Russian transistors from eBay for a long time. These transistors are pretty universal (despite the low power): hfe = 50, ft = 80MHz and P = 50mW. Also I've had an unassembled Chinese HX108-2 transistor radio kit, with which I was not happy due to the weird solutions in its design (weird voltage stabilization and biasing technics utilized).

I've also had a plastic box, AAA battery holder and a perforated board that fit together perfectly. So I figured I could build my own superhet design, using all the major parts of the Chinese radio (tuning capacitor, loopstick antenna, IF and oscillator coils, audio transformers), but with the Russian germanium transistors. To my help was also a couple of small-size IF transformers that take significantly less space than the ones that came with the kit radio (I still had to use the black IF for the diode and the full-size oscillator coil).
The only missing part was the 10k volume control + power switch, which I was eventually able to find on eBay, 1-2 months into the project.

Unfortunately I lost the circuit drawings with the exact resistor/capacitor values, but the design is totally average: Autodyne frontend, two IF stages (AGC controlling the first one), germanium diode detector, volume control, audio driver and push-pull output. The detector diode is biased to roughly 40mV from the emitter resistor of the 2nd IF stage.
All the transistors are of ГТ309 type, the germanium diodes are OA1160 (but can be of any point contact Ge type).

Probably the most interesting part was the audio amp - it turned out that since the radio is driving a pair of headphones, the audio amplifier doesn't need to provide a whole lot of voltage amplification; for 5mW into 32 ohms (that's loud) the audio amp needs to provide approximately 400mV RMS. The 200-300mVpp coming out of the detector is pretty close to that, so the audio amplifier is set to an amplification factor of 2 with a strong negative feedback. As a result, the miller capacitances on the output transistors are pretty thick and there's no complaint on audio quality.
Also, I noticed that when bringing up the volume, the radio went into oscillation, due to some RF leaking through the detector, being amplified by the audio driver stage and radiated. The solution was a miller capacitor on the audio driver stage as well.

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Last edited by szoftveres on Sep Tue 01, 2020 12:41 am, edited 4 times in total.

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 Post subject: Re: Homebrew transistor pocket radio
PostPosted: Aug Mon 31, 2020 5:09 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36212
Location: SoCal, 91387
Very good. Is there more to the schematic?

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 Post subject: Re: Homebrew transistor pocket radio
PostPosted: Aug Mon 31, 2020 3:15 pm 
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Joined: Feb Sun 03, 2019 4:47 pm
Posts: 104
very impressive sir!


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 Post subject: Re: Homebrew transistor pocket radio
PostPosted: Aug Mon 31, 2020 7:25 pm 
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Joined: Oct Fri 11, 2019 3:03 am
Posts: 40
Location: Seattle, WA 98122 (from Hungary)
fifties wrote:
Is there more to the schematic?


Sketched up and added it to the original post. It's just like any other RF front ends. There's a resistor on the top of the first IF transformer that regulates the current of the converter stage and thus the oscillator amplitude (it also decouples RF together with the attached bypass capacitor). I lost the drawings with the exact values but it's somewhere between 2k2 and 4k7. The RC time constant in the emitter is high enough (> 50kHz) to prevent superregenerative squegging.
The first IF stage biasing also provides biasing to the detector diode, however the 100mV on the diode (corresponding to the current of the biasing circuit) is a bit too high, so a counter-bias voltage is taken from the 2nd IF stage emitter resistor to keep the diode voltage at around 40mV (discussion here: viewtopic.php?f=4&t=374810&p=3174925 )


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