Neo Geo CD Laser lens - no power? (lens works perfectly in another machine)

The title kinda says it all but here's some more background.

Problem:
I have a bunch of NeoGeo CDs in for repair and all but one of them has been repaired. One of them is causing me some issues (I'll call it Unit A).

I thought UnitA just needed a new laser assembly as it wouldn't read CDs, but when the new one arrived the disc still wouldn't spin.

Note that all the motors appear to work. When the lid button is pressed, the laser moves up and down to focus as it should. If the laser is set to the middle of the CD, when the lid button is pressed, the mechanism moves to the inside of the disc as it should.

Fault finding:

- Visual inspection: On inspection I noticed that the lens wasn't omitting the usual red light beam when attempting to focus.

- swapping parts: I swapped this laser from unit A with a working laser from unit B.

The laser from unit A DOES work in unit B (CD spins, game loads flawlessly).
The laser from unit B DOESNT work in unit A.

So I know the laser is not at fault here.

My next step was to swap the laser controller PCB - the one that sits next to the laser and drive mechanism in the top half of the NGCD shell. When I did so, suddenly the fault swapped from Unit A to Unit B.

It seems there is a fault with the laser control board or the associated wiring harnesses.



- Testing:
A quick continuity check tells me the harnesses are fine, so it is indeed the board.

Knowing that FocusGain adjustment can increase laser power I measured the resistance across the FG pot in UnitA and compared it to the same pot in Unit B although I forgot the exact readings they seemed reasonably similar. I'll check later and post them here.



Could this just be a case of a lack of supply to the laser diode? My knowledge of CD player electronics is minimal.




So this is where I'm stuck. I can't find anybody online with a similar problem, and so I can't find a solution either.

Can anybody here help point me in the right direction?

If no one can chime in with actual knowledge of this specific problem I would just trace the power delivery from the mainboard to the laser controller pcb and compare between the working unit and the faulty one. That should be easy enough to diagnose with a multimeter.

Other than that I'd get ready to salvage the laser assembly for your one working unit.

lachlan said:

If no one can chime in with actual knowledge of this specific problem I would just trace the power delivery from the mainboard to the laser controller pcb and compare between the working unit and the faulty one. That should be easy enough to diagnose with a multimeter.

Other than that I'd get ready to salvage the laser assembly for your one working unit.

Click to expand...

I managed to fix the issue!!
I traced back from the laser assembly onto the controller pcb and found a faulty Transistor. swapped it out for a similar one I had laying around and it works. ive now ordered the exact replacement just because I want it to be as close to original as possible.

Brilliant news!

Very nice, I think I had one with a similar problem and simply swapped it from a parts console without giving it much thought. How did you diagnose the transistor, oscilloscope?