I have an early 13xxx WM-D6C. One of the better sounding ones in my collection. Anyway, sometime ago, it started to exhibit a strange behavior. It will play fine for about 45 minutes then the audio would start to distort. I planned to fix it but alas, I never got to it.

After accepting the fact that due to my career, I will never have time to fix this unit and decided to call on a tech. I tested it out and to my surprise, I can't get it to distort no matter how long I play the unit. It played through a 90 minute tape with no problems... Weird. What can this be? I was told by a tech back then that it probably was the muting transistors. Can muting transistors fix it self?

Well, found out the answer.  The problem returned after about 2 hours of playback.  Left channel starts to distort and then cuts in and out.  What can this be?  Muting transistors?

Well something is missing behaving when it gets warm. Could be failure in the amplifier IC, bad electrolytic cap or even a dry joint on the circuit board. A tech will be able to trace where it is going wrong with an oscilloscope, they could compare the faulty left hand channel with the good right hand channel. They would be able to see where the signal was cutting out. (And if they played a tape with a simple sine wave on it where it is distorting) Half the battle is actually getting the walkman to go into failure mode in the tech's presence!

I can remember these kind of symptoms when the muting transistors of my low serial d6c was wrong. The longer I waited the longer it played normal. But only for minutes. It never fixed itself. After replacing it, the problem went. The muting component is obsolete. Getting one is hard, but you can use the part of a newer version (not the smd version).

But further investigation of the problem is adviced. Like Seb said there could be something else wrong with it.

Looks like the muting transistor package is a comon fault though, could be a good place to start!

Okay, looks like I will need to get a competent tech for the wm-d6c.  Any recommendations?  I am in NYC.

Want to add that this problem happens on both the headphone out and line out if that gives more clues..

It's in the pre-amp section then, could still be muting transistors; but yes you need a tech, I'm in the UK so I can't recomend a good tech in your area!!! Good luck anyway.