I have this nice walkman from early 2000's and it was working fine until some days ago: it now starts playing for a few seconds, then switch to the reverse direction and than it stops. FFW and REW do the same, just start for a few seconds and they both stop. The root cause is clear, the micro-controller is not detecting rotation from the reels. I have disassembled the walkman to check if the photo sensor is dirty but it looks fine; the disc with reflective and non-reflective slices is also perfect so I guess I have an electronic problem. There is a test point on the PCB (TP36) which seems to be the output of the rotation sensor and voltage is not changing. For comparison I tested another walkman using the same transport (but different electronics) and in the sensor test point I can clear see the voltage bouncing between 0.7V and 1.3V. So it is either the photo sensor (led + photo transistor) PH701 or the transistor Q702 buffering the signal for the micro controller. The signal PHOTO CTL (extreme right of the picture) measure correctly so the photo sensor is enabled correctly during operations. All components are SMD and well beyond my soldering skills; I guess I have to find another working pcb and replace it ... but my question is: can the photo sensor really breaks out of the blue like that ? It's a first for me ... ciao
I would first check that the LED is powered: you can either check the voltage or the current by measuring voltage across R701. Then check the base of Q702. If a signal is present at the base of Q702, but not at the collector, then that's your problem. Of course, that doesn't necessarily mean that the transistor is bad, it could also be R704. I would also check very thouroughly for cold/broken solder joints or tracks on the PCB, as that is something that is much more likely to happen than the rotation sensor going bad. I can confirm that I never seen a rotation sensor going bad, but people who were in this business back in the day (12voltvids youtube channel) and have worked on many more units than I had say they do go bad sometimes.
Thanks, that's more or less the troubleshooting I was hoping to do, but all components are underneath the PCB and inaccessible when the walkman is assembled. Accessing the components requires to desolder battery, motor and plunger contacts so that the pcb can be removed from the tape transport. I would then need to build a rig with many wires to have the pcb partially working, but I gave up. I tried inspecting the board and it all seemed fine, I tried heating with the soldering iron the pin of Q702 but it did not fix anything. Anyway, the outcome of any investigation would be "replace the PCB with a known working one". I had actually bought a suitable replacement, bit it got lost in the mail. Will have to hunt for another one. My guess is that actually I killed it with ESD: this unit worked well for years, but lately I was trying new belts in and out so I might have accidentally hit it with a micro discharge. It's winter and heavy clothing builds up a lot of static charge, I was not wearing an ESD bracelet.
To be honest with you I have never used an ESD bracelet (although I have one) when working on these devices and never had a problem (they use almost exclusively bipolar transistors, which are not as sensible to ESD and FETs), though of course it is possible. You may check the sensor by powering up the LED through the reistor that is already in circuit and at that point measure the resistance between collector and emitter of the phototransistor. This way you can do the measurement with the board out. I forgot that WM-EX models have the circuits on the back side of the board. Hope you can either repair it or find another board, which I guess will be pretty difficult, but not impossible.
No, it's not that difficult, I can take the board from any of these models: EX500 EX501 EX505 RX506. They are mostly the same walkman sold with different accessories and colours and indeed the board on my walkman reads "WM-EX500". I had actually found a nice EX501 for a good price, but it got lost in delivery. There is a suitable board on ebay right now, but with shipping fee it is too expensive. I will wait for the right one, not in a hurry to fix it.
