Laser Tuning (Multi Mega)
When tuning the Multi Mega there are several potentiometers on the motherboard, as well as one on the laser.
Start by tuning the laser to a general position. The best position I have found is look for the black line on the potentiometer and set it to just slightly right of bottom center.
The usual position that works for the tuning range is between the red marks shown here.
Start with the laser set to just off from the bottom right and then focus on the potentiometers on the motherboard itself firstly. You can come back to the laser afterwards as a final tune.
The most important thing to get right first is the PLL frequency. This is a clock derivative that times the data reading with the clock. This drifts over time and comes out of specification making it much harder for the console to succesfully read games.
Based off several working units the correct frequency appears to be somewhere between 4.32MHz and 4.35MHz.
The adjustment is very small to get the frequency within this range, so you need a good oscilloscope and turn the potentiometer slowly to get it within range.
If your frequency is even 3.45MHz that is too high ideally, or lower than 4.15MHz is too low. The accuracy wants to be as close to 3.3MHz.
Probe the PLL test pad to see the frequency.
And the potentiometer is the top of the 4 potentiometers on the right.
The Multi Mega has the most control over the laser and calibration than most other retro consoles. You have both a lot of power but at the same time a lot of responsibility.
The top potentiometer adjusts the frequency of the PLL. Anti-clockwise reduces frequency and clockwise increases. Tune this to 4.30MHz ideally.
The next potentiometer I am assuming until further research is some kind of feedback control. Generally keep it in the vertical position and only turn it left or right slightly if all else fails.
The third, T. Gain is the gain of the signal received from the raw laser output. Anti-clockwise decreases, and clockwise increases. On a weak laser the gain increasing can help, but too far can saturate and cause slow sinewave oscillations. A good position is as seen above, with a maximum usually being no more than 2 O'Clock.
The final one is the Offset, which I believe is the position vertically the laser moves in order to focus the beam. So basically the distance between the disc and the laser head. This is hard to tune even with an oscilloscope, and without specialise equipment (or a novel way I have not yet invented) it is best done by just turning it in small increments and watching the results as the disc spins.
Take a photo of the potentiometers before you move any so you have a reference point of where they were at stock.
The very first thing I would do is tune the PLL to 4.3MHz. Without that all other steps are much less accurate and just making the problem worse.
After that, tune your laser head potentimeter to just offset to the bottom right. Now test your console and try and get the disc spin and successful load. If it still fails, I would turn the laser head potentiometer slowly in really small increments (as small as you can move it) anti-clockwise so the black line moves up and to the right. Test and every interval.
Find the spot between the red area that the disc spins the most (if at all) and leave it there. If it does not spin at all in any of those positions simply set it back to about 5 O'Clock.
Start by moving the Offset potentiometer slightly left or right (doesn't matter which way you start). The chance is more likely the disc and laser head distance has changes over time with the rubber mounts degrading, so re-focusing the laser with the Offset will give much better results than adding gain.
If moving the Offset left and right (in really tiny increments every test) does not work (you can really move the Offset potentiometer fully to both extremes, although its more than likely only going to need a small movement to be in focus, then you can move on to the Gain potentiometer as a final check.
If you find the disc spins more, or faster in a certain position but a game does not load, leave the potentiometers in those positions, do not reset them as you move to the next potentiometer.
With the PLL set, the laser head potentiometer tuned and the Offset tweaked to try and improve the disc spin and speed, the last attempt is moving the Gain potentiometer. Do exactly as you did with the Offset, and turn it small increments to begin with, likely clockwise to increase gain, but try both ways.
Once all the above is done, if you saw improvements in game load or disc spin speed you can start the process all over again from the top until you fine tune all of the potentiometers together.