Commodore 64 1541 DIAGNOSTICS: Some symptoms and solutions for a sick diskdrive
DRIVE DEAD... OR NEARLY
Lets take it from the top. Does the drive initialize properly when first turned on? The startup sequence (drive does a self-test) should be: green power light on, red drive light comes on and goes out while spindle motor turns for a few seconds and stops. If the power light (green LED) doesn't come on, or is dim or flickers, you have a power supply problem... the 5 volt line is bad. That usually results in a spindle motor that runs continuously and red LED off. Check the bridge rectifier CR3 and the 5 volt regulator VR2. Note: if the regulated 12 volt supply fails, the motor will not run at all. If the drive activity light (red LED) stays on and the motor runs continuously, it means that it failed to complete the startup sequence. The most common cause is a bad DOS ROM UB4 (901229-xx). Other chips to check are: UC4 (6502 MPU) and UC2 (6522 VIA). The smaller "glue" logic chips are pretty rugged, but do sometimes fail. Check UA1 (74LS14) and UD2 (7407)... they have also been known to cause those symptoms.
DRIVE INITIALIZES OK, BUT WILL NOT LOAD PROGRAMS
When the computer is turned on, the reset signal should cause the drive (and other periferals like the printer) to reset: the drive red LED and spindle motor should come on and go off within a few seconds. If that doesn't happen, suspect the serial cable (try a substitute) or the VIA chips in the computer or drive. If the computer resets other periferals, it's probably OK.
If the computer can't "see" the drive on the serial bus, you will get an error message: "DEVICE NOT PRESENT" when you try to LOAD something. The default (factory setting) for a 1541 is device #8. If your drive is hardware modified as device 9 for example, and you try to read the directory (LOAD"$",8), you will get that error message. If the VIA (UC3) 6522 interface chip in the drive is bad, the drive will likewise be "invisible" to the computer. Check also UB1 (7406).
A "DRIVE NOT READY' message indicates that the computer can "see" the drive, but there is no disk in it (or it's not formatted), or the drive door is not closed. A dirty read/write head can do the same thing. The drive will respond by flashing the red activity LED and banging the heads (looking for track zero). If the computer can access the drive, but you can't load even the directory of a known good (formatted) disk, try the INITIALIZE command (with or without a disk inserted), then try reading the disk again. To INITIALIZE the drive:
OPEN15,8,15 <return>
PRINT#15,"I" <return>
CLOSE15 <return>
One quirk of the 1541 is the "drive lost" symptom. If the head, for some reason, gets stuck past the directory track (18), INITIALIZE 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!
Some disk errors can do that to a drive and make it look "dead", as can exiting incorrectly from a program by just turning off the computer. I've done that many times. As an alternative to Initializing, you could try formatting a disk... it takes longer, but it will also return the head to track zero. Lastly, if you insert the transit card (shipped with the drive) with the drive turned off, it will push the head back to track zero. Inserting a disk is not the same thing! The transit card has a tab on the front of it that does the job. Don't have your transit card? With the top cover off and metal shield removed, you can push it back with your finger. The drive must be turned off, of course, or the head will not move.
PROGRAMS FAIL TO LOAD COMPLETELY OR COMPUTER LOCKS UP
If your computer setup or components have been moved recently, take note... drive cables too close to the monitor can sometimes pick up interference from the flyback tranformer in the monitor and 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.
If you have re-initialized the drive and it still doesn't work (can't read a disk), it may be out of alignment. Try formatting a disk and see if it can read the (empty) directory of that disk. If it can't, clean the head and try it again. If it can, but can't read other disks, misalignment is a good possibility. There is one other thing you should check first: see if the head assembly rails are sticky, especially on a drive that has been unused for a time. With power off, the head assembly should slide back and forth easily. If it seems sticky (experience necessary to know the difference), the rails should be cleaned with strong solvent (acetone, MEK, paint thinner) and either run dry or relubed with a tiny amount of graphite or silicone lube. Oil on the rails will work for a time, but eventually picks up dirt and they will get sticky again. Avoid the use of spray cans on a drive. The spray goes everywhere.
DRIVE ALIGNMENT
Drive misalignment is something that doesn't happen all at once. It is a gradual process that begins with occasional errors while loading (red LED flashing), failure to work with some programs, or excessive head banging (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 use) by copy protected programs or disk errors that cause the head to "bang" against the track zero stop repeatedly. If the alignment is far enough off, you will get "FILE NOT FOUND" and red LED flashing... the drive tries and "gives up".
To properly realign a drive, you need special software. I use "1541/1571 Drive Alignment" by Free Spirit Software. The flipside of the program disk is the alignment disk, and as such, should not be copied (a copy is only as good as the drive that made it). The program provides a menu screen that indicates what track you're on, drive speed, etc. You make adjustments to the drive while watching the screen. The instructions even tell you how to load the program when nothing else will load. You can -check- the alignment of the drive without taking it apart, but of course realignment requires disassembly.
Drive speed can drift over time, but it's rather rare to find it off far enough to cause problems. Spindle speed is reset with a small screwdriver adjustable control on a small PC board near the spindle motor. On older belt-driven spindle motors, the belt may be slipping. On all drives, the spindle or collar can get sticky and a tiny bit of lube helps. (Don't overdo it... excess oil will be thrown off and could get on the disks). 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 tightly. You can bend the tab down -slightly- so it makes more firm contact if necessary. A slipping disk will produce random read and write errors, a problem that's -very- hard to track down.
I ran through some of this rather quickly. If you have any questions, email me at: rrcc@u.washington.edu or rcarlsen@i-link-2.net
Ray Carlsen
CARLSEN ELECTRONICS... a leader in trailing-edge technology.
email: rrcc@u.washington edu or rcarlsen@i-link-2.net
