Pin 9 on IC601 measures 5.22 V while in play mode. I wonder if the unit is somehow permanently stuck in auto-stop mode?
If the motor is running in stop mode, that is not a good sign. It is one of the symptoms of a failed CX20084. However, before writing off the CX20084, it would be worth checking the Q601 transistor. If you have 5.24V on the emitter and 4.68V on the base, I would expect it to be switching on and passing some current through to the collector. What voltage are you getting on the collector? One more thing to check is C608. If this goes bad, it can cause the motor to run slowly. I had this happen on a SMD version of a D6C recently. I wouldn't expect that to cause the motor to run in "stop" however. The most worrying sign is the low voltage on Pin 5 of the CX20084. I believe that the 1.65V is generated by an internal voltage regulator within the chip and should be present as long as the B+ is present. If the 1.65V is not present and is not being pulled down by a bad C606 then that is a bad sign.
Thanks for the follow up @Deb64. The motor running in stop mode and the random slow movement was a result of the contact cleaner. No idea why. I've had the same situation when I cleaned it previously, where the motor hummed in stop mode, but this only lasted a few minutes (I took the batteries out immediately after finding it was running while stopped and waited a few mins). After that it worked fine. Same thing happened last night as in the video. Within a minute or so after I recorded it, it also stopped making any sound in stop mode, but continued to stay off in play mode. My question was relating to the auto-stop circuitry as it seems to connect to pin 9 of IC601 at least from what I gather on the schematic. It's currently always 5.22 V while in play mode so just wondering if that suggests the auto-stop circuit is engaged and therefore the motor doesn't run. Just a random thought. I will check C608 but not sure if it's worth as the slow motor thing was a temporary side effect of the contact cleaner I guess. As for the Q601, the collector is around 0.05 V but sometimes randomly spiking briefly to around 0.13-0.16 V (play mode). Emitter and base remain at 5.2/4.6 ish.
Some more observations that might be useful... the voltages measured on the IC601 pins vary if I use the star point on the board vs using the battery negative. E.g., pin 8, batt -ve: 5.22 V, star point: 5.07 V pin 7, batt -ve: 5.25 V, star point: 5.1 V pin 5, batt -ve: now it's fluctuating between 0.2 V and 0.15 V, star point: solid 0.025 V
@Deb64 I came across some info you posted in this thread https://www.tapeheads.net/threads/wm-d6c-motor-problem.86161/ noting that pin 7 should drop to 0 V in rew/ff mode when no rotation is detected, so I did some additional tests on pin 7: Stop mode: 0 V Play: 5.25 V Rew/FF: starts at 5.54 V and slowly climbs to 5.78 V and stays there, instead of dropping to 0 V <--- this behaviour seems wrong based on aforementioned info.
It sounds as if you may have a cracked PCB track or an earthing problem. Cracked tracks are often very difficult to see on a D6C, even with a good magnifier. You need to find this before going any further. If the walkman has been dropped, the areas around the PCB mounting screws are high stress points where cracks are most likely to form. These are good places to start looking. Also check the tracks near the DC input socket. I have seen cracked tracks around this area when a D6C has been dropped and landed on a DC plug and the force from the landing has been transferred to the D6C in this area. Is the motor running at normal speed when in REW and FF? The auto-power-off system cuts the power to the motor when the end of the tape is reached in FF and REW. It detects the lack of rotation in the supply side spool by use of a photo sensor and a rotating disc with alternating black and silver reflective panels. When the rotation stops, Q702 is switched on, which pulls IC601 pin 7 low. This sometimes takes a second or two. Finally, a correction to my earlier post. I have checked IC601 Pin 5; it is low (0V) when in "Stop" mode.
Currently the motor doesn't run at all in any mode. Pin 5 is now 0.025 V (sometimes 0.05 V) regardless of whether in play, ff, rew. In stop mode it's 0 V. Pin 7 seems to now be showing normal behaviour: in stop it's 0.05 V. In play it's 5.2 V. In ff/rew it starts at 5.2 V then drops to 0.05 V a second later. I'll have a deeper dive into finding cracked traces. The PCB for sure doesn't appear cracked. Just find it weird, because I've never dropped it and it was working absolutely fine without issue for several months before I found it dead last week, after only about a week of inactivity where it was sat on my desk.
Also just going to leave this pic here, if anyone with a keener eye/knowledge of this board can spot something obvious that I might be missing.
I have noticed a couple of points on your photo. There are a couple of bad solder joints which I can see. The screen of the single screened wire looks rather mashed up. There may be loose whiskers of wire causing problems there. Also there appears to be some contamination or spillage on IC601, around pins 3, 4, 5 and 6. This is a high impedance circuit so any unwanted capacitance or conductivity caused by the spillage will affect its operation. Cleanliness is really important when working on these chips and surrounding circuits. Any kind of spillage or excess flux can lead to bad wow and flutter or stop the circuit from working altogether. The track running from pin 7 or IC601 looks as if it may have suffered from some corrosion damage. I would want to clean off the green solder resist and check the condition of the track and, maybe, fit a by-pass wire. There is light mark between the centre battery terminal connection and the adjacent ground connection (below it on the photo). Is this a loose flake of solder? If so, it could result in a short from B+ to ground. Finally, you mention that you are getting different readings when measuring voltages relative to the battery -ve terminal and the star point. Were is the star point which you are referring to? It would appear that there is some resistance between the battery -ve terminal and the star point so this should be investigated. If I were to attempt to repair this, I would de-solder all of the wires on that part of the board, remove IC601 and the added resistor and capacitor modification, clean everything very thoroughly and then check the tracks for corrosion or other damage. Removing the chip would also give me an opportunity to check the condition of the pads and the tracks which run underneath IC601. I would also check all the solder joints. Once the board appears OK then I would refit the original IC601 as I am not totally convinced that it is the cause of your problems.
