1541 Drive Repair - Part 2

In my last post I began the process of repairing a Commodore 1541 disk drive. In that post I got the power on self test to pass but the drive would have some other problems that needed to be resolved. 

To continue the troubleshooting process I decided to write a test program using my IF65 in-circuit emulator. There are other ways I could have tested the drive, but I choose this options for two reason. First, I used the IF65 to do this sort of testing at my first electronics job, so there was some nostalgia for doing it this way. Second, this would give me a tool I could use to quickly troubleshoot other drives. 

The IF65 allows the 6502 code to communicate with a terminal so this made it easy to build a interactive test program. Here is a look at the menu for the test program. 

The first test was pretty simple, by pressing 'O' of 'F' on the terminal keyboard it would turn the activity LED on and off with a simple write to one of the 6522 VIAs. The motor test worked the same way to turn the spindle motor on and off. The write protect test continuously displayed the state of the write protect sensor. The last of the first batch of tests I wrote was the head test which allowed me to move the read/write head in and out. 

On the unit under test all of these tests passed except the head test, it was unable to move the head in either direction. I carefully checked the signals in the stepper drive circuit and everything looked ok, so I tried turning the motor manually and it would not move. I finally used a pair of pliers to grab the motor shaft and I able to break it loose. 

Next thing I had to deal with was a minor mechanical problem. The spring for the drive door latch was missing. I didn't have anything that was an exact fit, but I found a spring that was close and cut it shorter to fit. Initially the spring kept popping out so I had the bend the ends in a little to make a tighter fit on the pegs on either end. 

With the basic functions of the drive tested and repaired, next thing was to check the speed. The drive is designed with a very clever way to do this without needing any test software. If you look at the bottom of the drive mechanism you will see this checkboard pattern on the bottom of the spindle motor. When viewed under fluorescent lights it can be used to adjust the drive speed. Fluorescent lights strobe at whatever the AC line frequency is, so there are two tracks on that pattern, one for 60hz as found in the US and the other for 50hz found in other countries. When the speed is correct the marks on the appropriate track will appear to remain stationary. If it's running to fast or slow the pattern will slowly drift in one direction of the other. On the drive PCB there is a potentiometer that is used to adjust the speed. You turn the potentiometer until the pattern stops moving. 

With the speed adjusted I was able to boot a diagnostic disk. I used that disk to double check the speed, and finally do the head alignment.