Hello! Two months ago I bought from somebody this radio cassette Record-0-Matic Radio Craftsman; Chicago, Illinois., build 19 . Theoretically works pretty good on AM, Tape and a little bit on FM. The AFC also works.
(http://i67.tinypic.com/dmybh1.jpg)
(http://i66.tinypic.com/kew9ki.jpg)
Here is the diagram from the interior ticket:
(http://i68.tinypic.com/b6bygk.jpg)
The radio cassette has a hybrid design with both Si and Ge transistors.
The minor problem is a screw that is locked and when I tried to rotate broke down. The big problem is that even if is on tape or radio, the device makes from time to time an unusual noise; even if it is on low or higher volume. I tried to change most of the capacitors but in vain. Even the power socket I change, also in vain. I tried to power it from 9V battery but the same noise appear. I cleaned the rec-playback switch and the potentiometers but in vain.
https://vocaroo.com/i/s0K8qYqNnINd (https://vocaroo.com/i/s0K8qYqNnINd)
Note:
-at the end of volume potentiometer if I collect the signal to an external amplifier there is no such noise;
-also at the colector of TR104 if I collect the signal to an external amplifier there is no such noise.
Here is presented a general problem usually with the driver transistor which works on A class: https://www.youtube.com/watch?v=0ebspoaetMI (https://www.youtube.com/watch?v=0ebspoaetMI) The guy has a similar problem like mine. After replacement in this video, the hiss disappears.
I removed 2SB56 driver transistor and after that all the noise disappeared. When trying to connect C120 from the 1st audio transistor to the input of the audio transformer, there was no hiss. I checked with an multimeter 2SB56 Ge transistor and it seems to be ok. Instead of 2SB56 Ge transistor I tried a configuration with a P-channel FET transistor: 2N5640 and it worked almost perfectly, without (more or less) any adjustment. This is the modified diagram compared to the original. The quality is perfect, no harmonics, no distorsion. The only difference should be a little bit less amplification, and when moving the volume, the physiological point via C116 (300pF) is moved a little bit away. I think that instead of Ge transistor (for the moment) could be used this type of FET which is deplation (always on).
(http://i68.tinypic.com/2nvbh1l.jpg)