CBM 1541C latest updates and corrections 3-28-22 The 1541C is a cost reduced version of the earlier 1541 disk drive, but is identical in its basic functions. This drive uses single-sided double density disks. High density disks will NOT work. The short board 251854 is unique to this last "standard" 1541 and has fewer, more integrated chips. Like the 1541-II, it has a single DOS ROM, 251968-01 or -02. It also has a photo-sensor zero stop like the 1571 but that sensor was disabled at the factory due to compatibility issues. The DOS ROM code in this drive makes the stepper go to track zero and chatter as it hits the mechanical stop on power up and with computer resets. This "feature" was installed by Commodore to eliminate a common problem with all 1541 drives... the drive getting "lost" past the directory track with its subsequent no-read symptom. An Initialize or Format would restore normal operation as would opening the drive and pushing the head back, but such a drive might look defective to the user since turning it off and back on again does not restore normal operation. One other feature of the 1541C is the spindle motor enabled when a disk is inserted, much like a 1571, and it runs for several seconds afterwards. The write protect sensor triggers the motor so disk access is faster. If it is desired to re-enable the sensor to eliminate the chatter at startup and reset, the user can cut jumper J3 near the large plug-in connector at the edge of the board. The jumper resembles a fat letter H and the tiny dash between the ends can be cut with a knife. I don't know how this change will affect other drive functions. User beware! A better way to eliminate the start-up chatter is to replace the original DOS ROM 251968-01 or -02 with one from a 1541-II: 251968-03. Note that CMD's JiffyDOS uses the same code for both drives. Note that changing the DOS ROM to the later one will not prevent stepper chatter on Formats, Initialize, disk errors and some copy protected programs. This drive is housed in a white case but not all 1541 drives in a white case are true C drives, only the ones with the short board. Commodore apparently used whatever hardware was available when building their disk drives. Some will have Newtronics (twist door mechanism) and others will have the older ALPS (push-down door) mechanism. Any drive that has the photo zero stop sensor will normally have it disabled. This design is what Commodore called "cost reduced" because it uses fewer, more integrated and some proprietary ICs. A reduced chip count can make troubleshooting more difficult and also makes finding some replacement chips difficult. Fortunately, most of the chips are interchangeable with earlier 1541 models. Exceptions are as follows: The R/W head amplifier chip UD1 is an LSI (large scale integration) with a non-Commodore number of H36A2U57. The one in a 1571 has the same number with a suffix D... I don't know if they would sub for each other. The gate array IC UC4 is the same as in a 1571: 251828-01. Other chips match the earlier 1541 drives and most large chips are socketed. This drive has no device select switches but just jumpers on the controller board, namely J1 and J2 at the board edge. Like the 1541, the 1541C uses two on-board power supply sources: +5VDC for the chips and +12VDC for the motors. THE DRIVE MECHANISM I've only seen one mechanism in a true C drive, Mitsumi (actually a Newtronics chassis). The Mitsumi reminds me of a 1571 mechanism but the single fixed head in this drive of course eliminates one of the weak points of the 1571, namely the flimsy top head mount. It does share one other failure point however... the door latch pin that works loose and falls out. When that happens, the door latch doesn't work and the lever just flaps loosely back and forth. If the pin is heard rattling around inside the drive (hopefully it hasn't shorted anything out), it can be reinserted in the door latch and secured with a dab of superglue. The Mitsumi mechanism has a spindle drive belt like earlier 1541 types and has a motor speed control on the drive PC board. Maintenance for any drive should begin with cleaning of the head (use alcohol or other solvent on a cotton swab) and stepper rails if they appear to be sticky. Before any other maintenance, if there is a buildup of dust inside the drive, blow it out with compressed air. Pay attention to the IR sensor near the front left side of the drive. That sensor signals the drive that a disk has been changed, and if clogged with dust, it may not report a disk change. That could corrupt the next disk inserted and written to in the drive. The head gap is so tiny, you may not be able to see the residue that keeps your drive from reading disks reliably. It's best to just clean it every few months if the drive is used regularly and more often if your disks have seen a lot of use. It is not dirt that contaminates a head but oxide from worn disks. Make sure the felt pressure pad is intact on the upper spring loaded plate. If missing, you can replace it with a pad from an old audio cassette that has been trimmed to fit. Glue it with contact cement. The stepper rails are the two metal bars that the head assembly slides back and forth on. Swab the rails with solvent and, either run them dry or apply a very light coat of sewing machine oil. Don't use any kind of spray lube inside the drive. Spray instead on a cotton swab, then apply to the rails. Plain oil tends to collect dust and gets sticky over time. A very light application of silicone spray on a Q-tip applied to the rails would work better and last longer than any kind of grease or oil. Suspect a drive alignment problem -only- if the drive can't load known good programs (commercial disks) but it works fine with its own recently formatted disks. A slightly misaligned drive will chatter when loading programs as it encounters disk read errors, and it will fail to load if severely misaligned. Note that some commercial programs have intentional errors as copy protection on the disk(s) that will make a normal drive chatter and flash the green LED while loading. The 1541C can be "sluggish" and look like it's out of alignment but it's usually caused by sticky head rails. Any 1541 drive can become sluggish if it sits for a long time without being used. The head assembly should slide back and forth easily (drive turned off, of course). If rails are sticky, the head has trouble finding the correct track quickly enough and you end up with intermittant read errors, especially when the drive is cold and seeks are at the outer tracks. If your computer setup or components have been moved recently, take note: disk drive and serial cables too close to a tube TV or monitor can sometimes pick up interference from the flyback transformer in the display 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 route the cables away from the source of the interference. IMPORTANT NOTICE: If you remove the mechanism from the drive, mark the connectors so you will be sure to put them back correctly. Pin 1 is indicated on the board for each one. If accidentally reversed, you can do serious damage to the drive board or mechanism. Even though the black head connector is "keyed", if connected incorrectly, the hard-to-get R/W head amplifier chip may be destroyed!!! CHIP FAILURES AND POSSIBLE SYMPTOMS - 251854 SHORT BOARD UA1 7406 LOGIC No drive reset from computer or "Searching for..." but no further response from drive. See also UB1 UA2 251968-01 OR -02 DOS ROM, (reads as 27128 EPROM) Spindle motor runs continuously and red LED stays on or blinks. UA3 LC-3517A SRAM (generic 6116 2K SRAM) When drive powered up, motor runs continuously and red LED flashes slowly (about 1 flash every 2 seconds). UB1 74LS14 LOGIC No response from computer UC1 6522 VIA Drive appears to start up normally but does not respond to commands from computer or "Searching for..." but no further response from drive. UC2 6502 MICROPROCESSOR Spindle runs continuously with red activity LED on. UC3 6522 VIA Spindle motor runs continuously with no red LED, or spindle doesn't turn. UC4 251828-0 GATE ARRAY Spindle motor doesn't turn, or write protect not working, or stepper doesn't move. UC5 251829-01 GATE ARRAY UC6 7406 LOGIC No drive reset, spindle motor run continuously UD1 251853-02 (GENERIC H36A2U57) HEAD AMPLIFIER LSI Read and/or write failure UD2 TD308CD/TD1145C MASTER OSCILLATOR DIAGNOSTICS... WHAT TO LOOK FOR When you are troubleshooting a drive, it is important to know how it works normally, and to observe it closely (with the cover off) for symptoms. For example, note how the drive motors and indicator LED's function when the drive is powered up, reset, and accessed by the computer. When the drive is instructed to LOAD a program, note whether the stepper moves, how much it moves, and if it "chatters". Try various functions like Initialize and Format, and observe the results. Sometimes the clues to a malfunction are subtle and different chips can cause the same apparent symptoms. Such things as a bad serial cable can be swapped out, but if another cable is not available, an ohmmeter check from one end to the other should show all pins connected. It's a "straight-through" cable: pin 1 to pin 1, 2 to 2, etc. with no shorts between adjacent pins. If your drive suddenly goes "dead" and you can't read disks without "FILE NOT FOUND" errors, try Formating a disk or the Initialize command: OPEN15,8,15:PRINT#15,"I0":CLOSE15 (assuming drive is set as device 8). This or formatting a disk will return the heads to track zero and may bring it "back to life". The problem is a quirk in the operating system. It's the symptom the DOS ROM in the 1541C was "upgraded" to eliminate. If the drive encounters certain errors while running a program, or if the drive is turned off before a program is properly closed, the drive head may get "stuck" past the directory track. A computer reset or turning the drive off and back on again will not reset that condition, but an Initialize or disk format will return the heads and restore normal operation. As an alternative, if this happens and you have a program in memory that you don't want to lose, turn off the drive, take the top off and push the head back gently with your fingers. 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 motor turns) for about two seconds, then red LED should go out and motor should stop. With an original 1541C DOS ROM, the stepper will chatter on power up and computer resets. COMPUTER POWER UP (OR RESET): Red LED should come on and go out, and spindle motor should start and stop 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 head should move forward to track 18 and read the directory. The screen will show: SEARCHING FOR $. If it finds it, the screen will say READY within a few seconds (time delay determined by size of directory). If the disk read fails for any reason (drive door open, unformatted disk, bad chips in the drive, etc.), the red LED will flash and an error message: FILE NOT FOUND will appear. If you read the disk error channel, it will say: 74, DRIVE NOT READY,00,00. Note that you must "clear" the error by reading the error channel or resetting the drive, or subsequent read attempts will also fail. 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, it 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 be. 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 say: 21,READ ERROR,18,00. FORMAT OR DISK "NEW": When you format a disk, the spindle motor will turn and the red LED 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 (directory). If the format fails, the red LED will flash, but there will be no error message on the screen. If you read the drive error channel, it will say: 21,READ ERROR,00,00. Format failures can be caused by write protect, 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 that read fails, the format will terminate, and the head will not move from track 1. If it advances a few tracks and stops, suspect a bad disk, an intermittant connection to the head or a faulty IC on the board. Swap out the drive mechanics to check the head. It may test good with an ohmmeter (not open circuit), but if defective, may still fail to format a disk. For write protect problems, check the sensor to see if dust or perhaps a loose write protect tab from an old disk is blocking it. As mentioned above, it is sometimes helpful to read the disk drive error channel when the red activity LED is flashing. Here is a small BASIC program to do that. It reads the channel, displays the error message, and turns the flashing LED off. All of the possible drive error messages are listed in the back of the operators manual. 10 OPEN 15,8,15 20 INPUT#15,EN,EM$,ET,ES 30 PRINT EN,EM$,ET,ES 40 CLOSE 15 Ray Carlsen CET Carlsen Electronics... a leader in trailing-edge technology.