I recently bought a WM-D6c (serial number 518***) with a defective CX20084. The speed was more than twice as high. I replaced this chip with a servo chip from a DD11. Now the D6c plays at the correct speed again, but it takes about a minute to reach this. I hear the speed slowly decrease (with a lot of flutter) until the correct speed is reached. Then it sounds excellent. If I stop the Walkman for a while and then start it again, it still sounds good. If I stop it for fifteen minutes or more, it starts again at high speed. In the DD11 the CX20084 worked perfectly. Does anyone have an idea where I can look for the cause of this phenomenon?
You should check the ESR of C601, C602 and C603. These are the electrolytic capacitors on the FG amplifier circuit. If these have gone high ESR then they will cause the symptoms which you are getting. You should replace all three of these even if only one appears faulty. You will need to remove the volume potentiometer to access these caps so you should also replace C607 while you are doing this. I am finding a lot of problems with the surface mount electrolytic capacitors used on these later D6Cs now. One of my customers who has worked on computer equipment from the 1990s found similar problems with surface mount electrolytic capacitors from that era.
Thank you Deb very much for your quick message! I have now replaced the SMD electrolytic capacitors you mentioned. Unfortunately, there is no improvement (yet). The D6c starts at high speed (with a lot of flutter) and then goes very slowly to the correct speed. Just lasted about 2 minutes. But after that the D6c plays perfectly until I turn it off for a while. Then the cycle starts all over again. The speed tune also works well. The collector of Q601 is about 2,6 Volts. The DC-DC converter gives a decent 11 Volts. So I can assume that the CX20084 is good, it is very similar to charging/discharging a capacitor. Only: which one...
It sounds as if you have a capacitor problem. When you have the speed tune switch set to "On", does it start at the correct speed and stay at the same speed? When you replaced the CX20084, did you clean the old solder flux from the board very carefully? Any left over flux can do strange things on this high impedance circuit. For example if you have two adjacent pads on the board with some flux on them so that it is bridging the pads, they can behave like a capacitor. It may be worth checking C606, which is a tantalum 10uF / 4V capacitor, although I think if it was faulty it would affect operation with the speed tune on and off.
Speed tune to 'on' or 'off' does not make any difference. The D6c starts with double speed. It now also takes longer and longer before the speed returns to normal. I cleaned the pads before soldering the CX20084 with IPA and a Glass fiber pen. I even checked C606. It's 10uF and 1,2 Ohm ESR. Unfortunately I don't know what causes the problem. When the speed is correct, the WM is functioning very well.
The only other capacitor which I can suggest is C608 (3.3uF / 16V tantalum). I had one of these which had failed on a SMD D6C last year. In this case it caused the motor to run very slowly or not at all. C605 is worth checking (3.3nF Film) however it is very unusual for these to give problems. If this doesn't solve the problem, there isn't much more I can suggest. I would need to have the walkman on the bench in front of me. The only thing I can suggest is to check all the other capacitors in the motor drive circuit, and then the resistors. Also check the connections between the components in case you have a cracked track or open circuit via hole.
I'm having the exact same issue on an early version from '85 and I'm halfway troubleshooting. Replaced FG coupling capacitor 602, ESR/Capacity was not that bad, and cleaned grime from the indeed high impedance VCO control voltage circuit - also at the speed switch end. Seemed to improve but not for long. Oh, and the Tune potentiometer was noisy, ofcourse. But ESR isn't everything, ie if C601 goes electrically leaky it will also drag down the FG signal. Next thing I will try is test/replace AC feedback bypass C603 and C606, the latter being a smoothing cap for the output of the internal voltage regulator of the 20084, and confirm output voltage on pin 5 is stable. Which in itself seems the most likely cause especially since Tune switch has no effect. Can't imagine any other caps could be in play here really. Also, does yours sound rattly while it is speeding? I assume the motor drive voltage is fluttering but haven't confirmed yet.
*DTC1000: Yes, when speeding, the sound is annoying, huge flutter. I'm listening piano music. When the speed comes to 'normal' it looks that the servo takes control over the motor again, and piano sounds acceptable. *Deb64: I replaced C608 and the speed slowed a bit. But still too high with much flutter. But after about 2 minutes the WM sounds good. I would like to do the tape speed adjustment (RV 601 & 603) but I cannot find the solder bridge on my SMD version D6c. I made a picture of the print. Do you know where this bridge resides? On my DD30 I discovered that tuning RV601 to 3090 Hz. is very important for stable tape transport. Thanks!
I have labelled the bridge on the photo. Note that the white wire from the two core screen cable, from the speed tune switch and pot, should be attached to the pad on the left of the bridge (it is marked "WH"). You need to tidy up the wires from the FG coil. If they are left taped to the motor then they may pick up interference from the motor, which will affect the wow and flutter.
Thanks!! I know now why I could not find that tiny bridge. I soldered the white core on the pad left of the bridge. I did only tide the wires in order to make a good photo of the board. With the bridge open, the lowest frequency (speed tune off) while turning RV601 = 3150 Hz. Bridge soldered again it is 3060 Hz. With the speed tune pot on the rear of the D6c I can adjust to 3000 Hz. So there still must be anywhere a problem in the servo circuit. I continue searching..
Nice going! I haven't had an opportunity to continue working on mine yet, but am curious to know what the regulated voltage output on pin 5 is. If that fails, everything else is moot. Mine being '85 vintage and yours an modern SMD, also would point at the only component they really have in common and that is the CX...
The voltage on pin 5 should be 1.65V or somewhere close. If it is low then check C606 first; it may be leaky or short.
To complete the 'story' I measured on IC701 in Volts: 1. 2,35 5. 3,5 2. 2,4 6. 0,2 3. 2,4 7. 5,8 4. 5,8 8. earth Does not correspond to the values in the service manual, except for pins 4 and 7, the supply voltage.
Deb, I found the problem: RV601 was very dirty. When I put my finger on it, the motor spinned up and down. I turned it around 100 times. Now I could adjust the WM neatly at 3090 Hz. Replaces the drop of tin on the bridge. The speed problem is definitly over ;-)
Thank you for the update. Some of the speed problems on these machines can be really strange and difficult to fault-find. Well done on finding it @Mathieu.
