Couple things to look for, the glue that holds the inductors can become resistive with age and shift the oscillator or cause low injection. That sealed tuning capacitor can get internal oxidation and cause erratic tuning. I'll be surprised if it's a semiconductor causing this. Paid for school working at an electronic repair shop with Navy B school instructors who owned the place years ago. Still do some vintage Corvette radios now and then. Fun stuff. Try a hair dryer over selected areas, aged capacitors in the RF, oscillator stages may wake up. Try a tap test with a non-metallic tool and see if anything shows up.
@@mrf246 Your comment is much appreciated 👏 I did try the tap test last night but haven't tried any heat. I have a heat gun and it has a low setting. I'm interested in seeing if any heat will wake something up 😁
The 3904 is ok-ish for AM. At 10.7 MHz IF they are not quite there, and at 100MHz they need to be pushed with at least 5 to 10mA current to obtain some kind of amplification. In short: they are general purpose, but with some reserve. If you happen to decide to substitute the FM RF and Osc transistors, a proper substitute that's easy to buy and I've tested and used many times with success is the MPSH10 or its current reference KSP10. If you may wish to keep the original transistors, you will need to increase the current flow by reducing the base bias resistor in the mixer transistor. I have not seen the schematic but cheap radios use just one with high value. In a radio I repaired with the same symptom as yours, the base resistor was 680k, and reduced to (I think it was) 330k started to work again. Old and early silicon transistors, specially those you have with the epoxy dome style package, tend to go bad with lower gain, and since manufacturers tried to make the radio draw the least current possible to have better battery life, they biased transistors a bit too starved to give optimal performance.
@@joseppuig925 I thank you so much, this comment makes so much sense because when I fed the radio 9 volts, it came to life. So maybe I should reduce the base resistor. I don't mind if it draws more battery life.
I also have an old model AM FM radio. The if still uses a full transistor and it misses the IF settings and makes me dizzy. The radio is just for collection.
Well your oscillator and mixer work else you would not be able to dial in any stations. For the other things. Measure voltages and find out why they differ. Watch out for the AGC which could be the reason that IF stages are under biased which may be the case as you have no sensitivity. Avoid changing transistors unless they are defective which you can easily find out with a multimeter. Changing transistors in the IF might require realignment. If you have an electronic engineering degree this should be easy
@@LutzSchafer yeahhhh it has been years since I done this stuff but I've been wanting to get back into it and now is the perfect time since me and my wife bought our first house just recently. I'll definitely keep the original transistors and replace them ONLY if they're defective. I thank you so much for the info 😁
Couple things to look for, the glue that holds the inductors can become resistive with age and shift the oscillator or cause low injection. That sealed tuning capacitor can get internal oxidation and cause erratic tuning. I'll be surprised if it's a semiconductor causing this. Paid for school working at an electronic repair shop with Navy B school instructors who owned the place years ago. Still do some vintage Corvette radios now and then. Fun stuff. Try a hair dryer over selected areas, aged capacitors in the RF, oscillator stages may wake up. Try a tap test with a non-metallic tool and see if anything shows up.
@@mrf246 Your comment is much appreciated 👏 I did try the tap test last night but haven't tried any heat. I have a heat gun and it has a low setting. I'm interested in seeing if any heat will wake something up 😁
The 3904 is ok-ish for AM. At 10.7 MHz IF they are not quite there, and at 100MHz they need to be pushed with at least 5 to 10mA current to obtain some kind of amplification. In short: they are general purpose, but with some reserve.
If you happen to decide to substitute the FM RF and Osc transistors, a proper substitute that's easy to buy and I've tested and used many times with success is the MPSH10 or its current reference KSP10.
If you may wish to keep the original transistors, you will need to increase the current flow by reducing the base bias resistor in the mixer transistor. I have not seen the schematic but cheap radios use just one with high value. In a radio I repaired with the same symptom as yours, the base resistor was 680k, and reduced to (I think it was) 330k started to work again. Old and early silicon transistors, specially those you have with the epoxy dome style package, tend to go bad with lower gain, and since manufacturers tried to make the radio draw the least current possible to have better battery life, they biased transistors a bit too starved to give optimal performance.
@@joseppuig925 I thank you so much, this comment makes so much sense because when I fed the radio 9 volts, it came to life. So maybe I should reduce the base resistor. I don't mind if it draws more battery life.
What again?? I would scrounge thrift shops and work at fixing.. its better than beer n bars. Keeps me home at 73 years young hihi😊😊
I also have an old model AM FM radio. The if still uses a full transistor and it misses the IF settings and makes me dizzy. The radio is just for collection.
I'm curious how that radio operates 😂
Well your oscillator and mixer work else you would not be able to dial in any stations. For the other things. Measure voltages and find out why they differ. Watch out for the AGC which could be the reason that IF stages are under biased which may be the case as you have no sensitivity. Avoid changing transistors unless they are defective which you can easily find out with a multimeter. Changing transistors in the IF might require realignment. If you have an electronic engineering degree this should be easy
@@LutzSchafer yeahhhh it has been years since I done this stuff but I've been wanting to get back into it and now is the perfect time since me and my wife bought our first house just recently. I'll definitely keep the original transistors and replace them ONLY if they're defective. I thank you so much for the info 😁