Chinese QRP VHF-FM radio restoration – continued

The replacement transistors BG1, BG2 (3AA9, 3AA7) are obsolete and next to impossible to find as it’s been 40 years since the radio was built. However, given the known voltage/power/frequency parameters and a friendly help from experienced radio-engineer ham, a reasonable replacement was identified: 2SA1930 from digikey.

Later, I found more data online at http://www.114ic.com/

Model: 3AA7 Model: 3AA9
Manufacturer: China semiconductor companies
Production of materials: Ge-PNP
Nature: RF / high-frequency amplifier (HF), power amplifier (L)
Package: in-line package
Limiting Voltage: 35V
Maximum current allowed values​​: 0.5A
Maximum operating frequency:> 120MHZ
Pins: 2
Beta: 30
Maximum power dissipation: 5W
Possible substitution: B772
Manufacturer: China semiconductor companies
Production of materials: Ge-PNP
Nature: RF / high-frequency amplifier (HF), power amplifier (L)
Package: in-line package
Limiting Voltage: 25V
Maximum current allowed values​​: 0.5A
Maximum operating frequency:> 120MHZ
Pins: 2
Beta: 30
Maximum power dissipation: 5W
Possible substitution: B772

The parameters were exactly as predicted. After installing new transistors, the radio came back to life. However, predictable the output power is too low – less than 100mW.

The bias/feedback seems to be wrong as the bias voltage is different for silicon (0.6V to 0.7V for silicon vs 0.2V to 0.3V for germanium). Also, the operating frequency (50MHz) is too close to the 200MHz limit which also limits the gain.

It may require readjusting the circuit / voltage dividers to ensure proper bias and gain.

While googling the radio, I also found some decent translation of the operating manual: 884_manual_translation

As the radio works on 45-50.1 MHz, it only slightly touches the lowest CW portion of the amateur 6m band. Re-tuning it to 50-55 MHz may not really worth the effort so I may put it on eBay. Would be nice to fix the finals though first.

The full schematics are here:

884_p1

884_p2

 

 

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About va3paw
Ham radio enthusiast, software developer

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