NEC Super CDROM² Scrambling sound fix

[aje_fr]

Hello there'
My first post to share you one of my fix for my favorite machine.
If you had already repair SCD2, you may have encounter this trouble.
Sound is scrambing within the first minute of running and go fine after few time. Sometime is in the other side, it starts well and after few minutes it goes wrong.
It may be more evident with an audio CD.
This trouble is not fixed after a full capacitor replacement.
(I had it on the 5-10 units I owned in the past)

As I was really borred with this trouble, I traced all the audio signals with an oscilloscope and find from where it comes.

And... It was a conception error, let me explain.

On the SCD2, the sound is mixed by this small pcb that's plugged one the "motherboard" :



And on this board, some components are present to mute the sound, here is the schematics of this parts :



To sum up, a mute signal comes from pin 4, go thru the D601 dual diode to drive two transistor (Q603 et Q604) that switches audio signals to ground (for zeroing the audio signal)

The conception error come from the fact that on the motherboard, nothing is wired to the pin 4.

And with time, either diode and/or the transistor have leaking current. So transistor are more or less activated by this leaking current.

The fix is really easy. You can remove all this circuitry or Q603 and Q604 are enough.
Or, like me, just add a 1k resistor between pin 4 and ground :


It did not prevent you from making a full capkit (it's mandatory on this unit)

That's all folks ! Hope it can help you !

[SignOfZeta]

Great work!

[NightWolve]

Hey aje-fr of France, I see you registered in 2014 and so your first post arrives almost 4 years later! I guess it was worth the wait since it was about doing something productive! I didn't know about muting failures like this, great EE detective work to track down and repair the problem!

[Fidde_se]

The leak voltage comes from the 1000uF cap fluid in the regulation part on the other side of the board, its well documented here before, but I guess this works too.