Monday, March 2, 2015

Apple IIe: Repairing the Disc II Drive

The Apple IIe system that I purchased was not fully functional. One of the Disk II drives was not working. This entry will cover in detail the repair of the disc drive system.

Inside the Apple IIe

Connecting the disk drive to the computer is not as easy as plugging a USB plug into a port. The top of the computer has to be removed for a connection to be made to the disk interface card. 

The cover of the Apple IIe is meant to be taken off with regularity; there are no screws attaching it. By applying pressure to the back tabs it will lift up. 

Inside, there is a series of seven Apple II bus slots at the rear of the motherboard that can take interface cards with up to a 50-pin card-edge. Numbered from left to right, slot 6 typically holds the disk interface card.

The disk drives are attached to two connectors on the interface card by way of a 20-pin ribbon cable. 
The connector is very easily misaligned on the controller card, which will short out a certain IC in the drive; if later connected correctly, a drive damaged this way will delete any disk inserted into it as soon as it starts spinning, even write-protected disks such as those used to distribute commercial software. 
This problem was apparently commonplace and it was the reason why Apple soon started printing several fat-print warnings about checking connector alignment in its manuals and used different connectors that could not be misaligned in its later drives. DB-19 adapters for the original Disk II were eventually available for use with Apple's later connector standard.

Inside the Disk II drive

Removing the disk drive case is simple. Four screws beneath the unit need to be removed and the case will slide back from the chassis. Inside a board on top holds the main electronic components. A quick survey in this case reveals a definite problem with a burned IC chip.

This chip is the Read amp, MC3470P. This IC is apparently the most common component to fail if the disk drive's ribbon cable is incorrectly connected to the interface card. Not only did the chip burn here, but it also damaged the IC socket that holds it in place. Both components will have to be replaced.

Further troubleshooting

Once the MC3470P was replaced, the drive was tested. It still didn't work. Using a copy of SAMS Computerfacts (a copy is here) for the Disk II, I made a check of the relevant components working with the new IC and then started a methodical swap of all the IC's by using the good components from the other working disk drive. 

It was discovered that the IC, SN74lS125N was't functioning. By replacing this component the disk drive now was fully operable and could read and write a floppy successfully.

Both of the replaced ICs, MC3470P and SN74lS125N, I was able to purchase from eBay for a nominal cost.

Disk II interface card

After making the repairs on the disk drive, it became apparent that their was also a problem with the Disk II interface card. The Drive 2 interface was malfunctioning. After some casual inspection of IC replacements, I decided the best solution was to buy a new interface card on eBay.

With the replacement card, I had both drives operating.

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