Laser Tuning (Multi Mega)
When tuning the Multi Mega there are several potentiometers on the motherboard, as well as one on the laser.
Take a photo of the potentiometers before you move any, so you have a reference point of where they were at stock.
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 successfully 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 top potentiometer adjusts the frequency of the PLL. Anti-clockwise reduces frequency and clockwise increases. Tune this to 4.30MHz ideally.
Next tune 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.
Now test your console and try and get the disc spin and successful load. If it still fails, 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.
This controls the amount of correction the laser does when out of focus. This is often called Focus Gain or Focus Bias.
Generally, keep it in the vertical position and only turn it left or right slightly if all else fails.
Often you can hear if you need more focus error bias as you will get constant whir / constant laser noise.
Probe on the middle pin of the T.OF Offset potentiometer with a multimeter or oscilloscope.
Adjust until the voltage is 2.5V (half of the 5V supply).
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.
The adjusts how much the RF signal is boosted in amplitude, this goes hand in hand with the laser power tuning.
Anti-clockwise decreases, and clockwise increases.
This tune is the most critical after the laser gain. If the E Gain is not exactly right the disc data will be lower amplitude and not load.
Rapid seeking (laser moving up and down disc) is a sign of too weak or too strong gain.
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.