Sony WM-DC2 Rebuild - Stops During Rewind

Hello,

Many thanks for the amazing resources people have posted around here for resurrecting old Sony Walkmans - so useful and appreciated!

I've been building a working WM-DC2 out of two donor units (one had a corroded PCB, one had a cracked central gear) - and I've got it mostly working - but I have 1.5 issues outstanding which I'd be grateful for a bit of advice on:

The main issue is that, in rewinding a decent 90 minute (TDK) cassette, the transport just comes to a halt shortly before it reaches the top of the tape. If I press Stop, then Rewind again - it continues for a few seconds, then comes to a halt again. The gears all look like they're making good contact, and it's not making any unwelcome noises. The tape reels move freely in the cassette housing, so it's not like it's binding.

I'm happy to take the mechanism apart and re-clean and lubricate it if that's the most likely option - just keen not to do so if that's unlikely to be the answer...

The other 0.5 of an issue was happening last night - but not today: I had thought there was insufficient back tension during playback, causing the tape to lift slightly clear of the head. Manually slightly slowing the supply spool would restore correct playback. However I can't see what might cause that to happen - so I don't know if it's just a bad cassette or what!


Any advice would be much appreciated!

Thanks!

 

Getting the FF and REW functions to work 100% on these DD units can be a challenge. The fact that you're not hearing any gears grinding is a good sign for sure. I was once able to fix this on a DD33 by simply tightening the thrust screw on the idler wheel just a bit. I had adjusted it simply going for the lowest W&F, but this meant the spindle started slipping on more demanding tapes. While you're there, it might help to give the idler rubber another clean, just in case a trace amount of oil got on there (I've also had this happen). Another possibility is the supply spindle itself starting to slip. A bit of glue should fix that.

 

@Moisie2000 The first problem seems auto-off related. Does the power LED also turn off when it halts ?

Second problem isn't clear. Backtension isn't needed because cassettes have a pressure pad.
Don't understand how the tape lifts off the head because too little backtension.
Are you sure problem is not related to tape path ? Like tape being pulled up/down at the pinch roller or the tape guide not in line with head guide ?

Backtension is important only on units that have a permanent pad lifter, like some early Panasonic units.

 

Hello,

Many thanks for the suggestions. Sorry for the delay in replying - I've only just got to looking at this again.

I've stripped down the mechanism, cleaned and lubricated it as direct in Marian's excellent guide - and I'm still having the same issue with Rewind stopping prematurely. And yes - the power LED turns off as it halts.

I'm not convinced it's anything slipping as, one it has halted, if I slightly disconnect one of the batteries for a moment, the mechanism (briefly) starts up again when I reconnect it - presumably as though the mechanism has reached a high resistance and gives up rather than forcing it.

I've tried swapping the motor out from the donor unit I have here - but the same issue stands.

Do you think there's anything further I can do?


> Second problem isn't clear. Backtension isn't needed because cassettes have a pressure pad.

I've not had this problem on other cassettes - so I think this was just limited to the one I was using at the time.


Thanks for your time.

 

Your rewind problem is not mechanical, it's the auto-off circuit that shuts the motor off.

Things to check are voltages around Q308, 309, 310. See what voltage you have at base of Q308 when running and when it stops.

 

Thanks so much for the advice - I'll check on that!

 

Ok - interesting...

I didn't think I was being an idiot, but it looks like I might have had some finger trouble before, and the same problem is now affecting both FF and REW:

In either mode, whilst running for the first half-ish of the tape, the base of Q308 hovers around 0.2V. However, as the tape approaches the end of its travel, this voltage gradually rises up to about 0.52V - at which point the motor shuts off and the voltage rises to 0.56V and stays there. At this point, the voltage at the collector of Q308 drops from 0.68V to 0.09V.

 

Hello again...

Still fighting the same issue, but I've now done some poking around the CX20084; whilst in FF or REW, the voltage on pins 7 and 8 gradually drop as it gets through the tape. Pin 8 starts off around 2.36V, with pin 7 around 2.30V; when pin 8 reaches about 2.31V, pin 7 goes low and the motor shuts off (and pin 8 rises up to about 2.5V again).
In play, I think the same effect happens - only more slowly - so the auto shutoff also happens before the tape has reached the end.

It looks to me that the pin 8 voltage is definitely rather lower than the 3V quoted in the service manual - do you think I need to be looking around the power regulation area?

Thanks!

 

First thing to note is the 3V specified in the manual is with alkaline batteries, with NiMH that voltage will be a bit lower (2 x 1.2V = 2.4V).
However using re-charghable NiMH cells should work just fine.

Also as a sidenote, in PLAY the auto-off should never trigger as the motor (and hence also FG signal) never stops as it does in FF/REW.
In PLAY only mecahnical auto-stop should kick in at the end of tape, when takeup reel stops rotating (but capstan still does).

Pin 7 goes low means Q310 turns off when emitter reaches 2.3V.
Question though is what happens at pin 9, as that's what triggers the turn-off of Q310.
Also, removing the PLL bridge changes anything in the behavior ?

 

Hi - thanks so much for your continued input.

For the record, I'm using Duracell alkaline batteries, which are currently producing an unloaded voltage of 2.6V.

Looking at pin 9 of the CX20084: it starts off around 0.2V (wobbling around quite a bit) and gradually rises as the tape progresses in FF/REW mode. When it reaches around 0.7-0.8V, the mechanism stops, and pin 9 quickly rises to 2.3V.

> Also, removing the PLL bridge changes anything in the behavior ?

To be clear: you're suggesting I entirely remove IC602? Presumably just removing the solder bridge next to R610 wouldn't achieve the same effect (without so much delicate soldering)?

 

No, I suggest removing the solder bridge not the IC (hence why stating "PLL bridge").

Pin 9 should only reach 0.7-0.8V when the motor stops (no FG signal), otherwise it should remain below that threshold.

 

Hello,

> No, I suggest removing the solder bridge not the IC (hence why stating "PLL bridge").

Sorry - I completely misinterpreted what you said! Understood now.

With the PLL bridge removed, the voltage at pin 9 starts off a little lower - around 0.16V - and as the tape progresses and the voltage rises, the voltage seems to become increasingly wobbly (it did this a little with the PLL connected, but it seems to be worse with it disconnected).
The mechanism stops again before it reaches the end of the tape - but the voltage has spent a few seconds with peaks above 0.7-0.8V before this happens.

 

You can isolate the fault by removing R616. See if the voltage at pin 9 keeps raising even with that part of the circuit completely disconnected.
If the problem goes away in this scenario, check or replace (if you don't have the means to check) C313 and C618.

 

Thanks for the continued guidance - and apologies for the continued messages... still trying to understand how this circuit is supposed to function...

I've removed R616, and found the voltage at pin 9 still keeps rising throughout the length of the tape, and now the auto shutoff does not kick in: the tape reaches the end, and still the motor tries to keep going. This is true whether the PLL bridge is in place or not.

I'm guessing this means I don't need to check/replace C313 and C618 - but please let me know if I've misunderstood that.

 

Yes your guess is correct the auto-off circuit seems fine no need to check those 2 caps.

The voltage at pin 9 rises the same as before, right ?
Also, does this voltage rise if there is no tape inserted ? Just because the walkman has been running for some time ?

And if you let it run, does the mechanical auto-stop un-latch the mechanism or not ?

 

Thanks!

It rises - yes - but the characteristics are a little different: we now see the signal rise above 0.7-0.8V as the mechanism starts to struggle* with the reduced torque as it approaches the end of the tape. The voltage reaches about 2.0V at the end of the tape.

* With fresh batteries, the voltages are a little higher and it struggles less. However the original problem still occurs when R616 is reinstated and fresh batteries are used - the auto shutoff kicks in well before the tape end.
* Also, to note, the cassette I'm testing with does not feel mechanically resistant when turning it by hand.

Ahhh - good question!
No - with no tape inserted, the voltage at pin 9 remains around 0.16V (with or without R616 in place) and does not rise. With a tape inserted, the voltage starts around 0.16V and quickly rises as the tape progresses.

No - the mechanical auto-stop does not kick in - so the tape remains at the end (obviously not moving), and the power LED remains on.

 

Interesting. Now I fully understand something that have missed before, the motor is actually struggling not having enough torque at the end of tape.
This is why the auto-stop doesn't kick in and possibly also why the auto-off triggers.
Which makes me thing: is this problem actually electronic or mechanical (motor) ?

Can you power the motor with 1.5-1.8V DC and measure the current consumption with no load (motor removed from mechanism) ? A good motor should have a current draw of 10-20mA.
Also does the motor turn freely by hand in both directions ? How about the takeup torque feel by hand ?
EDIT: Another thing that can be checked before first thing is how does the motor voltage varies.
At the end of tape, when it struggles, does the voltage increase or decrease ?

 

Free running, this motor draws about 10mA with a supply of 1.6V; for comparison, the spare motor I have draws about 13mA under the same conditions.

Yes - both motors turn freely by hand, and feel the same to me. The takeup torque also feels fine to me - not too onerous at all; though I'm not the most experienced of telling what this model of Walkman might object to, I've felt plenty of resistant cassette mechanisms (and cassettes) over the years - and this feels fine to me.

Obviously with R616 removed again: the motor voltage starts at about 1.9V, then rises to 2.1/2.2V on the latter half of the tape. It drops about 10-20mV as it approaches the end of the tape and starts to struggle. The voltage does jump around a bit as it goes, but there's a definite slight decrease as it nears the end.

 

Motors are both fine from the perspective of current consumption, even more so if they turn freely in both directions.

At first glance, the servo is doing what it shoud: the speed drops so it increases the motor voltage to compensate for that drop.
That's up to a point, when Q601 goes into saturation, point at which CX20084 driving the transistor harder won't render any change in motor voltage.
The 2SB798 has a Vce(sat) of -0.4V. If we take into account fact that +B is around 2.5V with NiMH, at a motor voltage of 2.1V we are already in saturation.

One last thing to check is the total current consumption of the device. Spec is 98mA in PLAY. This shouldn't increase a lot from beginning to end of tape.
Like if you get 150-200mA at the end, there's a mechanical problem.

 

Hi @Valentin

Sorry for the delay in updating here - life overtook me for a while.

I think you might be onto something here... I'm measuring the current whilst in PLAY, as you suggest - and I'm already seeing ~150mA at the start of the tape.
I've tried loosening the capstan wheel top screw - and even with it almost falling out, the current consumption hasn't dropped at all.

On this basis, do you think I need to dismantle the mechanism and clean it up again?