Last night, as I was listening to my WM-D6C, after about 30 minutes of playback, the sound began to distort. I checked both line out and headphone out and the distortion is there.  The output also attenuated and is worse on the left channel.

After resting the unit, I played a tape and it sounds okay for about 5 minutes then the problem comes back. 

Anyone have experience with this? It seems like a bad electrolytic cap to me but not sure which one it may be.  Perhaps, after some heating up, the bad cap creates the problem?

Are you listening using batteries or mains? If it’s battery change the batteries.  

It is not the batteries.  They are fresh.  Also, it happens with AC mains.

When you first play it after some rest, it sounds fine.  It takes about 3 minutes for the distortion to appear.  Mostly in the left channel.  Seems like perhaps an electrolytic cap problem?  Perhaps, once it is charged up.

E 'likely that the problem may lie in the dc-dc converter or its two capacitors.
If the serial number of the D6C is 270000 or up, the capacitors are 2 of 225 microfarads and 25 volts are on the mother board. If the serial number is less than 270,000 the capacitors are 3. One of 220 microfarads and 10 volts and a 100 microfarad 16 volt and are located on the dc-dc converter. The third capacitor from
100 microfarads 16 volts is on the mother board.
To be sure, however, that this is not a fault near the amplifier circuit is necessary to analyze the D6C.

Thank you for your reply.  The serial # is below 270000.  It is early in the 18000s.  Why would it sound okay for the first 3 minutes or so then the sound distorts?  Would a bad dc-dc converter do this?  I had a similar problem in another D6C that was a bad muting transistor.  These transistor's are no longer available.  But with that problem, the problem was constant. 

The flaw exists only on the output headphone or also on the output line?

Both line out and headphone out...

If the fault is on both channels L + R, it is likely that the capacitors on the dc-dc converter.

But we can not exclude a defect on an amplifier IC or a passive component in the vicinity of it.

It is mostly the left channel that has the problem.  The problem just happened the other night.  It was working fine and then one day, it just showed this problem.  The unit has been cared for and never dropped or any trauma...

I'll replace the caps on the dc-dc converter and go from there.  Thanks for your help.

Very good.
The capacitors in question are: C315, C316 and C317.
If, after replacing them the problem still exists, follow the signal from beginning to end with a signal tracer.
Good job.

Well, this might be beyond my patience and tech level.  Replaced the 2 caps on the dc-dc converter board.  The problem is in the left channel as far as I can tell.  And the problem does not happen immediately after the unit has been in rest.  It will play and sound fine for about 30 minutes and then the problem appears.  Suspecting C139 but not really sure on how to be sure.  Also, C110, C112, and C115?  But these are tantalum caps.  I don't think likely to fail.  The fact that it is okay when cold is a hint, I think.  The problem appears after about 30 minutes of play.  Any ideas?

why does this thread require Two monitor's to read properly ???

LOL.  That was what I said!  Just click the compatibility button in your IE.

It's the tantalums more likely, they do actually go bad, have fixed this problem before. Tantalums have intermittent problems while electrolytics just get progressively worse. Sometimes it's the DC-DC converter here. I've had issues with those, there should be 12 volts coming out.

Do you remember which tantalums?  I am suspecting the 4.7uf 10v C110 and C112?  It is very hard to see which is the solder point for these small caps. 

C110, 4.7uf 10v first cap in the playback path  C112, 4.7uf 10v is path for noise filtering and C115 1uf is the second cap after the 3 stage EQ amp. I would also replace Around the Dolby chip there are a number of caps in noise filtering C130, C132, C127, C122, C123, they are known to have issues in this model. Also replace all electrolytics.

If the problem is the distortion, it is more likely that the cause is a transistor or an IC, also a capacitor, but is perhaps even less likely.
If the signal is interrupted can also be a simple cold welding which opens with the heat, even on a simple cable.
We must arm themselves with patience and follow the signal with a signal-tracer or a oscilloscope.
If you do not have this equipment might even going to trial, but you may lose whole days without pecking at the exact point.

Yes.  I am afraid it might take days for my level of tech.  I don't have a signal tracer but do have an oscilloscope. 

I do have a service manual but it is not even clear which caps are C110 on the PCB.  I will give it some more effort though. 

Do not waste time searching for the single component (C110, etc.). Connect a probe to the oscilloscope, and helping with the diagrams on the service manual, follow the signal from the head to the exit line. Always do a comparison with the other channel. At some point you will find the defective component.

Use the oscilloscope trace out P/B head to the pre-amp, 90% sure it going to be those tantalums I've fixed 12 of these that had that same problem, you get severe distortion  after 30 mins or so, usually on one channel or humming. In rare cases it sometime is the IC for the amp section that goes bad, but yet to see that, it's always been those tantalums, Sony made a bad batch of theses and have these known issues.

Honestly I do not ever a bad tantalum on a D6C, however, anything can be.

Okay, wish me luck.  This will be a challeging project.  I will need to study a bit on how to use the oscilloscope to trace signal on the PCB.  The tantalums are the blue caps, right?

exact

They are blue in color, also check the muting transistors to. Q304 or Q303. Q304's no. is 2SC2458. This transistor switches Q303 which is labelled Q803: 2SB810. Q303 drives the 386 headphone and Dolby amps. Like "dottor walkman"  said, it could be your problem, you do need to scope the paths to narrow it down to the circuit. But those tantalums are known issues with this model.

I had another WM-D6C some years ago that out of the blue started having distortion in both channels.  This one did not need any warm up and was a consitent problem.  The tech that restores all my Nakamichi decks (ESL) took a look and told me the problem was the muting transistors.  But these were now long un-obtanium. 

I agree, corresponds with most of my experiences with this type of problem.

So if it is the muting transistor, I guess I am out of luck as ESL told me that I should "retire" the unit as they had no more of these transistors and didn't know where to get them.

Easier just getting another working unit on the market...

If by your investigation should be transistor faulty, you can use an equivalent. You may also choose a sample with characteristics superior to the original.

Okay, with this information, I will try to see if I can isolate where the problem is coming from.  I need some oscilloscope lessons on how to trace audio signal from PB head towards the output.

Act only when you're sure to have acquired some experience and skill. Since the examination is carried out with the D6C in play, be very careful to avoid short circuits. The tracks and pads are very small and close.
Congratulations and good work.

Thanks for your help.  Indeed, the circuit is extremely tight.  I'll see if the magnifying light on my bench is of any use.