Hi everybody. Im working on a R707. It is a nice compact recordable Sony cassette player. I have replaced the rubber belt on the unit and I have it successfully spinning tapes. The issue I currently have is that I am getting no audio out at the speakers - not even static. To me this seems unusual. The unit itself doesnt look worn out and pinch rollers are in great condition. I have some theories in my mind regarding what it may be causing it. - Im not sure if the tantalum capacitors in the unit contain fluid but if they do they may have leaked have a high esr or shorted - Perhaps dry joint or ribbon damage - Perhaps IC damage The unit itself looks as if it has never been opened and stored well do i don't see how it could be damaged. Has anyone had any experience with this unit in the past or similar unit to help me with my diagnosis before I get more in-depth Edit: Ive noticed on a youtube video I found there is actually 4 electrolytic capacitors on the back of the board. I did not see them mentioned in the parts list. These could explain the reason I have no sound. I guess I'll have to take them off and test and replace Thanks in advance!
Tantalum capacitors don't contain fluid and are not a problem in general on cassette walkmans. IC damage I would put last, it's very unlikely unless the unit has been powered with reversed polarity. But then it's usually other things that get damaged like the motor servo. Dry joint would render audio if you press on the PCB in certain areas, while a broken flex cable will be visible. Those 4 SMD electrolytics are likely going to need replacement, although even failed ones render some low volume sound unless the connection to the pads is open circuit. Haven't worked on this particular unit, more famiiliar with 701C/702/EX80 which don't have any electrolytics, so can't tell for sure if those caps tend to go bad but you can measure them.
Thank you for your reply Valentin. With the tape playing and the unit covers all screwed back in, I tried pressing hard on the front main body of the unit and the back main body and suddenly i received some static noise. It is very low volume. I can hear some very out of calibration playback also, but the signal is severely attenuated. If the 4 electrolytes are coupling capacitors for the left and right channels Im guessing they are definitely faulty, possibly leaking and the case of the attenuated faint audio signal. I dont know however if I have permanently fixed the 'No audio' fault however as all i did was press hard. I will have to report back when I do a full analysis and replace capacitors
Can't find a manual for R707, but depending on amplifier topology it's possible 2 of the big 220uF caps are coupling caps. However the location of the caps isn't typical for coupling ones: these are usually close together and close to the headphone connector. Looking at F707 manual (can't find an R707 one), it uses the TA8145 headphone amp which has a topology that does not require coupling caps. Not clear what IC the R707 is using. Looking forward to the full analysis.
I unsoldered one of the 220uF caps and there was some small leakage and fish oil odor. The component test couldnt identify it and the multi meter couldn't record its capacitance. The multi meter also measured a constant 1M ohm where it should reach infinity when fully charged. So its partially shorted and unable to hold charge. I'll have to do a special order as I dont have any of these caps. From some research, I think the big 220uF caps job may not be inline capacitors, but "Feedback/Bypass Capacitors" positioned at the emitter of a transistor with a resistor to shape amplifier frequency response and gain. More specifically to compensate for magnetic tape high-frequency roll-off. Im not 100% certain of this though. Also, my analysis is also based off the F707 manual as I cannot find the R707 manual. There is another issue with this unit. It seems like I need to push with way too much force to get some (or all) of the push buttons to respond such as the play button. I believe they are tactile dome switch technology like most Sony Walkmans. Can we fix them without removing the membrane?
Looking in more detail at F707 seems very similar to R707. Thinking of it the 2 are likely identical, R707 just doesn't have the radio board in the door. C328 (220uF) is a filter for the 2.4V rail, important. C106/206 are feedback capacitors and those 2 can explain the no sound. C308 is a Vref filter, also important. Overall all of these capacitors should be replaced, would also check the THT one (C407) although that is related to bias oscillator so will only affect recording. With the switches ideally the membrane should be removed and cleaned. The force is required even if you press directly on the domes with your fingernail ?
