Building the DIY Commodore C64 Diagnostic Harness

1541 CHIPS VS SYMPTOMS - PCB# 251830 or 251777 with Newtronics Twist-door drive mechanism (1984)

The twist-door Newtronics mechanisms seem to suffer a higher failure rate from an open R/W head than the push-down ALPS ones. I don't know why but I speculate that Newtronics didn't seal the heads well enough and moisture 
seeps in and corrodes the wiring. Storing a drive in a humid environment is therefore to be avoided. An open head is not repairable. I usually just swap out the entire mechanism. I did once substitute an ALPS head element into a 

These pertain to the PCB# 1540050 (early version) with ALPS drive mechanism

This reviews most of the main electronic components in the 1541 disk drive having the  PCB# 1540050 (early version) with ALPS drive mechanism which has the push down door.

Make sure the latch clamps the disk properly. Without a disk, move the lever down and see if the spring presses the collar against the spindle to clamp it securely. You can bend the tab down -slightly- (Newtronics drives only) so it makes more firm contact if necessary. A slipping or stalling disk will produce random read and write errors, a problem that's very hard to track down. Some disks may work better than others. The DD ones without a hub ring seem to slip more easily. HD disks should never be used in a 1541. 

Drive misalignment is something that doesn't usually happen all at once. It's normally a gradual process that begins with occasional disk errors (bad disk or intentional errors from copy protection) while loading (red LED flashing), failure to work with some programs, or excessive head chatter (the drive getting "lost" and having to go back to track zero to "find" it's place again.) Drives are forced out of alignment mostly while hot from extended use. Heavily copy protected programs or disk errors can cause the head to chatter against the track zero stop repeatedly.

If you smell or see smoke from your drive, of course turn it off quickly. OK, where did it come from? Look at all the tiny electrolytic capacitors (known in the trade as Tantalum) to see if any are burned. When new, they are orange or dark blue in color and shaped like a teardrop. Those capacitors have a history of catastrophic failure and can pop like a firecracker or just make a 

If your computer setup or components have been moved recently, take note: 
drives or cables too close to a TV or monitor can sometimes pick up "noise"  interference from the high voltage circuits inside the monitor which can garble the data. Move the drive and cables at least a foot away from the monitor and try it again. If that helps, move the drive to the other side of the monitor and keep the cables as far away as possible.

One quirk of the 1541 is the "drive lost" symptom. Normally, the drive will "park" the head over the directory track (18). If the head, for some reason, stops past the directory track, an INITIALIZE command from the computer will return it to track zero and it should then work normally. 

 Note: turning the drive off and back on again will -not- reset it if that's the problem! 

Reading Disk Drive Errors

It is sometimes helpful to read the disk drive error channel when the drive red light is flashing. Here is a small BASIC program 
to do that. It reads the channel, displays the error message, and turns the red activity LED off. 

 10 OPEN 15,8,15 
 20 INPUT#15,EN,EM$,ET,ES 
 30 PRINT EN,EM$,ET,ES 
 40 CLOSE 15

This program and all of the possible drive error messages are listed in the back of the disk drive operators manual. (Appendix B)

-- Ray Carlsen --