My fat PSX has a dead laser. That’s it.
Or is it?
I came across an interesting post on Reddit about someone adjusting laser on their PSX with an oscilloscope, with unbelievable success. I’ve certainly heard about this method, but never actually tried it nor had a patient that I was desperate enough to try this on.
Let’s see if I can bring my KSM-440AEM back to life.
OK, this is my guinea pig Playstation, an ordinary SCPH-7502, that rocks a KSM-440AEM laser assembly.
The console that this Reddit user did it was not in fact equipped with a PU-22 motherboard, I believe it was PU-18, which would make it a SCPH-5502. It obviously had testpoints in a different place, so I had to find a service manual and check where I should probe.
Remember 2 things: test point is named CL704 and the waveform.
The probing part was extremely tricky – I had to both hold the probe and disk door switch closed with my one hand and adjust the pot with my other hand. All while looking at the scope, manually changing Vmarkers and avoiding exposed components on the PSXs PSU. Oh, and making sure that the console played the last track on the music CD, because let me tell You, it was SO SENSITIVE that even my shaky hands would make it lose tracking.
Anyway, let’s see what I started with.
Hold on, what is that?
WHO STOLE MY EYE PATTERN AND REPLACED IT WITH DOTS?
You know what the worst part is? This scope has a function to connect the dots, or should I say, interpolate.
BUT IT MAKES IT LOOK LIKE A DSO! It makes then a single line, that refreshes so often that I could make myself a breakfast, fix my car, track ISS on the sky, in broad daylight and it still would be on the same screen. All that while making it EVEN LESS readable than these dots.
The scope I’m using is a HP/Agilent 54501A. Its sample rate is only 10MS/s, which makes it unsuitable for being anywhere near high frequencies. And by unsuitable I mean unusable – at its rated 100MHz sync rate and assuming a sine wave it’s going to measure 0.1 sample per period. Its memory depth is also only 500 points, which doesn’t help.
I’m usually using it to diagnose analog audio circuits, so I rarely ever go over ~50KHz, including noise from SMPS, for that purpose this scope works great, but man, oh, man, it really struggles with any signal above 1MHz.
Also, a side note. This is a “digitalizing oscilloscope”, which means that it processes its signal like a DSO, but has a limited analog input. I wonder if it has an auto peak-peak measurement function and I just needlessly troubled myself with markers.
After some calibration this is the highest P-P voltage difference I could get on this laser.
During testing though none of my games would launch. I had to back it up a bit for the PSX to even recognize that it had a CD inserted.
That’s my final waveform. Games lauch more or less reliably, but to be honest, if I didn’t have a scope I wouldn’t be able to tell a difference between where I started and where I ended.
This procedure maybe more needed for initial calibration of new lasers, but to be completely honest, simple pot mod works just as fine and there is way less setup and equipment needed to make your PSX disc drive usable again.
Thanks for reading!