Modification of GM805 Disk Systems

by L.C. Waring

Old Gemini GM805 units, which use Pertec FD250 disk drives, can be converted to 40 track, 180K formatted capacity by a few simple modifications.

With reference to the diagram of the drive unit on page 22 of the “Pertec Operation and Service Manual”, these modifications are:

1. Removal of head loading mechanism (22). The head loading solenoid is secured by one screw at the side of the drive. This screw should be removed, the two connections to the solenoid cut, and the solenoid itself discarded.
2. The distance of head travel must now be increased. To do this the clamp support beam (10), secured by screws (1), must be removed. Care must be taken to avoid damage to the index photo transistor assembly (19). The carriage shaft (25) is held in position by a clamp, not visible in the diagram. The two screws securing the clamp should be removed. The lugs of this component are asymmetrical, and it is the broader lug which restricts head travel. This should be filed on the inside until it is slightly thinner than the other lug. The heads should now be able to travel inwards a sufficient distance to allow 40 tracks instead of 35 per side. The carriage shaft (25) and clamp support beam (10) should now be reassembled.
3. Adjustment of head pressure may be necessary if it is found that errors occur when accessing tracks 35 to 40 on either side. This may be achieved by placing some packing between the end of the head pressure spring and the head assembly (20). It was found that about 5mm of packing increased head pressure sufficiently to enable correct reading and writing of tracks 35 to 40.
4. The disk primitives must be altered to correspond to the increased number of tracks available. [Ed. – With Gemini BIOS 3.2 or later this merely means increasing the number of tracks in the relevant .CFG file.]

The writer has modified several disk drives in the above manner, and all have subsequently performed satisfactorily.