Have an absolutely pristine WM-702, replaced the belt, cleaned head and rollers and it's really working perfectly - but the button functions seem to have gotten mixed up - Stop is now play, Play is FF, etc. Was working fine earlier - bit of an issue with corrosion on the battery pack letting low voltage through - maybe that's responsible somehow? I've seen this before on other units. Works as it should via the headphone remote. Anyone else seen this or know how to resolve it? Thanks in advance for your time.
I have a theory, but cannot confirm it because I don't have the service manual. Some of these walkmans use a resistor ladder to sample the buttons. Each button press gives a different voltage (within a certain interval), that's sampled by the ADC in the microcontroller and each of these voltages is associated to a function. If that's how it is implemented in this walkman, corossion (or just oxidation) on the button contact will increase the resistance and throw off the voltages. You can test that by measuring the button's resistance with a multimeter (battery out of the unit): keep the button pressed and measure, you should get a very small resistance (less than 1 ohm). Of course, the corossion on the battery terminals should be cleaned anyway, but given the unit works well with the remote, it's more likely the problem is related to the buttons.
Similar to indeed how remotes of most the portables work. Do you have the inline remote @Yermawn ? If so, you could measure the resistance when pressing the buttons to verify. Correction: it seems that the entire 700 series uses dedicated lines, shared by the remote and device buttons, into the microcontroller. Below is a screenshot from the WM-701C service manual I found online. The headphone-remote-audio connector is of the wider, more exclusive, green type with another pin-configuration than other connectors used by Sony. The DX-100 amongst others also uses this connector, with a proprietary - to the DX-100 only - inline remote. Link: https://www.manualslib.com/download/1001377/Sony-Walkman-Wm-701c.html
I just came across the WM-EX85 and F85 on Instagram. The resemblance with the (F)702 is striking. Would I have know it before, would I have included these in my eBay searches!
There are Sony remotes that work indeed as described. Button presses result in difference in signals based on the number of resistors in the circuit. The picture below is from the DX100 service manual: Stop - no resistors in the circuit; Fast Forward - all 3.
