1541 DIAGNOSTICS 2-15-08.
1541 DIAGNOSTICS Some symptoms and solutions for a sick disk drive. latest updates or corrections: 2-15-08 WHEN THE DRIVE IS WORKING PROPERLY To properly diagnose a potential problem, you have to know exactly how the drive should respond when it's working correctly... DRIVE POWER UP: Green power LED comes on and stays on, red activity LED comes on and spindle (which turns the disk) motor runs for about two seconds, then red LED goes out and spindle motor stops. There is no stepper (which moves the head assembly) motor activity at this time. COMPUTER POWER UP (OR RESET): Drive red LED should come on and spindle motor should start, then LED goes off and motor stops within two seconds. READ DIRECTORY: Insert a known good disk and type: LOAD"$",8 and hit the RETURN key. The disk should spin and the stepper should move the head to track 18 and read the directory. The screen will show: SEARCHING FOR $. If it finds it, the screen will display READY. Then you can type LIST to see the contents of the disk. Note: some program disks will not have a directory you can list. If the disk read fails for any reason (drive door open, unformatted disk, bad chips in the drive, etc.), the drive red LED will flash and an error message: FILE NOT FOUND will appear. If you read the disk error channel, it will display: 74, DRIVE NOT READY,00, 00. INITIALIZE: This command from the computer should move the head from wherever it was to track 18 (directory) and the disk should spin. The head will not move (but the spindle motor will turn) if it is already over track 18. If there is no disk in the drive, or you insert an unformatted disk, or if the drive door is open, INITIALIZE should cause the spindle motor to run and the head to seek track 18 (directory) anyway. When it tries and fails, it will pull the head back to track zero and "chatter" as it hits the head stop, then advance to where track 18 should have been. The red LED will flash because of the drive read error. No error message will be shown on the screen, but if you read the disk error channel, it will display: 21,READ ERROR,18,00. More on this later... FORMAT OR DISK "NEW": When you format a disk, the spindle motor will turn and the red light will come on. The drive will pull the head back to track zero and "chatter", then the stepper will advance to each track as it writes from track 1 to track 35. When it finishes the format (about 1 minute 25 seconds on a stock drive), the head will return to track 18 (the directory). If the format fails, the red activity LED will flash, but there will be no error message on the screen. Reading the drive error channel will display: 21,READ ERROR,00,00. Format failures can be caused by write protect (disk tab open), drive door open, bad disk, bad or clogged head, or bad chips in the drive. The format will attempt to write to track 1, then do a read, and if the read fails, the format will terminate, and the head will not move from track 1. If it advances a few tracks and then stops or takes a long time to format, suspect a bad disk or an intermittant connection to the head. If the drive will read OK but fails to format a disk, check the head, UC1, UC2, UA1, and UD2. Swap out drive mechanics to verify the head is bad. It may test good with an ohmmeter and read disks OK, but if defective, may fail to format a disk. Note that all wires of the head should measure continuity (low resistance) to each other. If any line is open, the head is bad. Disconnect the plug from the drive to do the resistance tests and make sure you get the plug back on the connector the same way it came off. For write-protect problems, check UC1, UC2, UA1 and, of course, the sensor. As mentioned above, 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. Note that JiffyDOS provides an easy way to check the error channel... just press the @ key, then hit RETURN. DRIVE DEAD... OR NEARLY Lets take it from the top. Does the drive start up properly when turned on? If the power light (green LED) doesn't come on, or is dim or flickers, you probably have a power supply problem... the 5 volt line is bad. That usually results in a spindle motor that runs continuously with red LED off. Check the bridge rectifier (CR3 in early version drives with PCB# 1540050, and CR1 in later version drives with PCB# 251830) and the 5 volt regulator VR2. Note: if the regulated 12 volt supply fails, the motors will not run at all. That's a rare failure. At power up (without the computer connected) if the red activity LED stays on and the motor runs continuously, it means that the drive failed to complete its startup sequence. The most common causes are a bad DOS ROM UB4 (901229-xx) or failing 5V bridge rectifier. The easiest place to check for correct voltages in and out of the power supply regulators is at the diodes CR2 and CR4 located near the two rectifiers. The anodes of those diodes are connected to the outputs of the +5V and +12V regulators. The cathodes (designated by a line or stripe at one end) show the unregulated source from the rectifiers that feed the regulators. With drive startup problems, some 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 POWERS UP OK, BUT WILL NOT LOAD THE DIRECTORY OR PROGRAMS When the computer is turned on, the reset signal from the computer should cause the drive (and other periferals like the printer) to reset. The red LED and spindle motor should come on and go off within a few seconds. If that doesn't happen, try a substitute serial cable. If that's OK, suspect the interface chips in the computer or VIA chip in the drive. If the computer resets other periferals, it's probably OK. Note that a drive may stay in reset (red LED on and spindle turning) if connected to a computer that is turned off. That's normal. If the computer can't "see" the drive on the serial bus, (serial cable unplugged or drive turned off, for example) you will immediately get an error message: "DEVICE NOT PRESENT" when you try a LOAD command, and the red LED will start flashing. 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 with LOAD"$",8 you will get that error message but LOAD"$",9 will work. If the VIA (UC3) 6522 interface chip in the drive is bad, the drive will likewise be "invisible" to the computer and you'll get "DEVICE NOT PRESENT". If you get "SEARCHING FOR" and nothing else happens, check ICs UB1 (7406) and UA1 (74LS14). These two chips carry data to and from the VIA chip. When one of them fails, if you try to load the directory or a program, the computer will display that error message until you turn it off or reset it. 74,DRIVE NOT READY,00 00 from the drive error channel indicates the computer can "see" the drive on the serial bus, but 1. there is no disk in it, 2. the disk is not formatted, 3. the drive door is not closed, 4. the read/write head is completely clogged or 5. the drive has an electrical problem. With any of these problems, the drive head can't find -any- data. The drive will respond by flashing its red activity LED and may step the head back and forth slightly looking for data. Note that this takes only a second or two before the drive "gives up" and the spindle stops. A partially clogged R/W head may allow the drive to see data but still not read it properly. Other similar false reads would include a corrupted disk or trying to load the directory of the reverse side of a 1571 formatted disk. In any case, if the drive can see data but can't read it properly, it takes some time "hunting" before it gives up trying... more time than if it doesn't see any data. That's an important clue. You may hear the head assembly "chatter" as it bangs against the head stop searching for track zero... a normal process if disk errors are encountered. DRIVE SUDDENLY WILL NOT READ DISKS OR LOAD PROGRAMS 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, gets stuck 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! Some disk errors can do that to a drive and make it look "dead", as can exiting incorrectly from some programs by just turning off the computer. So, if the computer can access the drive, but you can't load even the directory of a known good disk, try the INITIALIZE command (with or without a disk inserted), then try reading A disk again. To INITIALIZE the drive: OPEN15,8,15