Walkman X, changed belts, played, nice sound but only right channel. Changed capacitor, played, still only right channel. Resoldered jack port, played, only right channel. Just by incidence I've touched volume pot and heard a crack sound on the left channel. I thought - what if it's volume pot? Worked it in and out, a bit of ipa and after a minute or so ... left channel works too. Why I didn't consider volume pot? There was no problem with sound on right channel so I stupidly taken for granted that it means it's the same for both channels. Lesson: Always start with working the Volume pot first after belt replace!
Volume potentiometers usually do need cleaning on older devices. I have seen many instances where in some positions, one channel would be completely gone. So good tip ! I use Kontakt 60 cleaner for potentiometers, spray a little in and then rotate the pot about 10-20 full turns. Then I leave if for 15-20 minutes and repeat the job one more time. After this, it usually works flawlessly, unless there is battery or capacitor leakeage in the pot, in which case more cleaning will be needed.
Its something that most people don't think or know about. With boomboxes I go straight to the function switch for a good cleaning, I've fixed a ton of boomers that way. Over here we use a product called Caig DeOxit https://caig.com/deoxit-d-series/ There's different formulations for different switches but these will be better than IPA. Different countries will have other brands but over here you can get them at the local guitar/musicians stores, the last thing somebody cranking an amp wants is cracking noise.
I have an Aiwa HS T120 that plays the radio fine on both channels but when I play a tape there's no sound on the left channel, I've tried re-soldering the headphone jack too but no luck. I've cleaned the volume pot but it does work for the radio. Any ideas of what to try next?
The connections to the head itself, especially as they get moved every time you press play. Beyond that there will be circuitry on the circuit board that is only used for tape playback. Even the tape/radio switch could cause your problem. The good news is that if the radio works O.K. the volume control, amplifier, and headphone jack must all be O.K. I'm guessing you don't have an oscilloscope to trace the signal (not that an expensive one is needed). Back when they were out of reach of most hobbyists a "signal tracer" (basically an amplifier connected to a test probe) or crystal earpice were sometimes suggested as alternatives.
Just remembered buying a cheap DDII described as having a faulty channel as in no sound from left channel and also had a scratchy volume control. After using plenty of contact cleaner on the volume pot and several full range movements of that pot the scratching stopped and both channels came through and at the same level. I have also found that if pulling the headphone plug partly out of the jack restores audio to both headphones it can also be a symptom of a cracked carbon insert inside the volume pot and not anything to do with the phone jack. Of course if it's an Aiwa HS-G08 with that ribbon cable it's a whole other story...ugh!!
Indeed DD series almost always need a very good potentiometer cleaning and in many cases one channel will go away in certain positions of the pot. As for the HS-T120: given the radio works fine on both channels, 3 things can be excluded right from the start: jack, power amplifier and potentiometer (as @Longman already stated). Then it's very important if unit has auto-reverse or not. If it has, it's very unlikely that exactly the left wire going to both sides of the head is broken. If it's not auto-reverse, this is way more likely. If it has auto-reverse, it's important to see if the switching between sides is done electronically (in most cases it is) or via a mechanical switch. If it's a mechanical switch, it could be dirty or the wire going from switch to PCB may be broken. In most walkmans the electronic switch inside the head preamp is driven by a mechanical switch, so a switch will always be there on autoreverse units, it's just that it's not always switching the signal itself. Then there is the head preamp, the Dolby amp (if it has Dolby) and of course that TAPE/RADIO switch. Given it's a walkman, the switch is likely to be a very simple single pole double throw switch (given the limited PCB space) and the actual switching is done inside an IC electronically. Of course, last but not least, a broken/corroded trace or via or an open electrolytic capacitor cannot be excluded as potential problems. In my opinion, there are too many variables at this point and we cannot draw a direction on the forum without knowing more. Of course, having an oscilloscope or even a "signal tracer", as @Longman mentioned is going to help a lot, especially given a service manual or schematic is probably only available in printed format.
