I had that pocket radio for a long time, and now I had some time to look at it. It does not work, no surprise there.
Here is some info on it at the radiomuseum web site: http://www.radiomuseum.org/r/philips_90al071_90_al_071.html
On Beka's web site there is a pictorial of its dis-assembly and some documentation on it: http://translate.google.com/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Foldradio.tesla.hu%2Fszetszedtem%2F569philips-zsebradio%2Fal071.htm
The oscillator is not working in my radio.
Here is a pic of the front end schematics of the radio. (I expanded the schematics from Beka's web site.) +2 gets +3V through a 220 Ohm resistor and 47u bypass cap. I also added a 10n cap there.
This is what I have looked at so far:
- DC voltages are in the ballpark.
- Resistors have the proper values.
- Transistor is fine, has a beta of 295.
- Disconnecting L1 and driving it from a signal generator the circuit peaks at the right frequency and with reasonable Q.
- At the osc coil the tap was not connected, dismantled the coil and reconnected it. Just needed a touch up at the solder lug. Now there is signal at the tap point.
- Paralleled C2 with another 10n cap.
- Grounded the base of Q1 with a 10n cap (the osc works in a grounded base configuration).
- Sometimes, at parts of the frequency band, when I grabbed the ferrite bar of the loop stick antenna, as if I have seen the stage oscillating! I have not checked the frequency though. Most curious!
Well, it is still not oscillating!
Only thing I can think of is that the phase of the osc coil is reversed (no positive feedback). It is highly unlikely, as the radio certainly looks used, so at one stage it probably did work and the coil looked original!
I also simulated the circuit and it does not seem to be too sensitive for component/feedback variations.
I would love to hear theories that what could be wrong!