CDROM DRIVE & LASER REPAIR:

If your machine tends to play better when you turn it upside or on it's side then it is a sure candidate for laser wear and adjustment.

The first thing you try is to adjust the bias and gain controls, on earlier machines this will only provide a temporary cure as the plastic sled may be in need of repair. Adjustment usually provides good results on the later machines that have the alloy sled.

These controls have the following effect....

BIAS : This control sets the default DC voltage flowing through the servo coils in the laser that move the laser lens up and down, basically it sets the default distance between the surface of the disk and the lens. This adjustment is the one that has most effect when adjusting to cure problems.
GAIN : This control sets the voltage that is added or subtracted from the bias voltage by the servo circuits for a given fluctuation. So when you have a slightly warped disk the lens can bob up and down and follow the surface accurately, if this setting is not correct the laser will not accurately track warped disks.

Adjusting the bias control slightly clockwise will increase the bias voltage and therefore move the laser slightly closer to the disk, this is usually sufficient to cure the problem for most machines. With an earlier machine that has a worn laser this may not cure the problem, the laser will either have to be replaced or repaired/modified.

If you have messed around and twiddled the bias and gain pots and your machine no longer operates correctly you can try to set the controls up to there normal position by using a digital multimeter. Connect the ground on your meter to the metal shielding and the positive probe to the following test point near the laser connector.



With the machine switched on (beware of live parts in the power supply) and no disk I.E. door switch up, set the voltage at this point to 1.70 volts, on a good laser this should the correct voltage however if your laser is worn you might need to increase this slightly. Once you have set the bias put a disk onto the spindle press the door switch (you may need an assistant to do this) the voltage should rise to something like 0.15 volts higher than with no disk (E.G. 1.85) if its to high or too low adjust the gain control till it is somewhere between those values. Your machine should now behave itself again.

Modifying the laser is quite tricky so I suggest only real electronics engineers attempt this. Basically the modification has to lift the side of the laser that has dropped because of sled wear. The best way to achieve this is to disassemble the laser and add a small piece of thin sheet metal between the sled parts... see the diagrams later. Only one principle is behind to making your CD-ROM Drive work again, the sled that carries the optics (lens) should be strictly level or parallel with the CD, and all you need to do is devise something to make it level again.

Firstly disassemble your machine and remove the laser as per the instructions for installing the chip. Once you have removed the laser turn the worm gear on the underside to move the lens into the mid position, otherwise the lens will get in the way of the cover if it is left in its rest position when disassembling the laser.

To fix the problem we need to make the above sled perfectly horizontal again, the simplest way to do this is to add a small piece (approx. 3mm by 5mm ) of sheet metal around 0.3mm thick to the sled unit at the position shown in the diagram below. You can use the metal cover from an old floppy disk and then cut it with a sharp pair of scissors. The piece can be glued in place with a tiny amount of super glue.



While the glue sets re-lubricate the sled rails with some decent quality grease, white lithium if possible silicon grease is also good. You can then reassemble your laser assembly and set up the BIAS control again. Your machine should now work fine.