CBM 1541-II - DIAGNOSTICS AND REPAIR latest updates and corrections 7-13-2022 The 1541-II is a cost reduced version of the earlier 1541 disk drive, but is identical in its basic functions although the red power LED and green activity LED are exactly the opposite of the LEDs in the 1541, which used green power and red activity LEDs. This drive uses single sided double density disks. High density disks will NOT work. The motherboards (Assy no. 340503) used in the two versions of the drives I own appear to be nearly identical. For the most part, the chips used, board layout, and chip ID numbers match each other. The design is what Commodore called "cost reduced" because it uses fewer more integrated (and some proprietary) ICs. A reduced chip count simplifies troubleshooting but also makes finding some replacement chips difficult. Fortunately, some of the chips are interchangeble with earlier 1541 models. Exceptions are as follows: The R/W head amplifier chip is a surface mount device (SMD), and the only other Commodore product I know of that uses that IC is a C128DCR (metal case USA version). The DOS ROM code is in one 16K chip as in the 1541C, unlike the earlier 1541 with two 8K DOS ROMs. The 1541-II uses a 251968-03 in a 27128 equivalent ROM, likely the only socketed chip unless the drive has been repaired before. The only other proprietary IC is a motor control device number 251828-03 (rev 4 board has a -01). That IC replaces many individual TTL (logic) chips, and so far, I've never seen a bad one. Like the 1571 and 1581, this drive has device select switches on the rear of the case. That allows the user to easily change the device number without opening the drive. The two tiny paddle switches are configured as follows (leftmost switch is #1 and right switch is #2): Both up = device 8 (the default for all drives), #1 down & #2 up = device 9, #1 up & #2 down = device 10, both down = device 11. Never use a pencil point to press those paddles. The lead may break off in the drive and short something out. Like the 1541, the 1541-II uses two power supply sources: +5VDC for the electronics and +12VDC for the motors. However, the 1541-II power supply is external to the drive. That keeps heat buildup inside the drive to a minimum, but some of those external "bricks" suffered early failures. These same power packs are also used for the 1581 drive and there are apparently two different versions. The first one is a smaller unit that supplies +5 volts DC at 700mA (0.7 Amps) and +12 volts DC at 0.5 Amps. The larger power pack is rated at 1 Amp for the 5 volt source. Apparently the earlier design was marginal to begin with and suffered premature failures. Both early and late design power packs are potted with epoxy and are considered unrepairable. However, I was able to peel the bottom cover off one of the smaller units (it was intermittant) and repair it by resoldering all the connections on the internal PC board. The complaints I've heard when a supply starts failing is that it works when cold but causes disk errors, failure to read, and drive lockup after warmup. Suspect a bad supply if the drive LEDs don't light at all. THE DRIVE MECHANISM I've seen two different 1541-II mechanisms: Mitsumi (actually a Newtronics chassis) and Chinon. They both are simple in design and equally robust, and they appear to be interchangeable. 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 shares 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. The Chinon mechanism has a direct drive spindle motor (no belt) and the speed is not adjustable. 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 the 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 it's 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 (copy protection) on the disk(s) that will make a normal drive chatter and flash the green LED while loading. I should state that I've never encountered a misaligned 1541-II. The 1541-II drive doesn't suffer from the heat related alignment problems that the earlier 1541 does, but it can be "sluggish" and look like it's out of alignment. That'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 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 (four screws and the door latch knob), mark the connectors so you will be sure to put them back correctly. Pin 1 is indicated on the board for each one. If accidently 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 will be destroyed!!! Note: there are two versions (I'm aware of) and their plug wiring is shown in the photo. The easiest way to remove the plastic door knob without breaking it is to pull on the front cover against the backside of the knob. It should slide off easily that way. CHIPS FAILURES AND POSSIBLE SYMPTOMS U1 74LS08 LOGIC No reset when drive powered up or from computer. U2 74LS42 LOGIC U3 6502 MICROPROCESSOR Spindle runs continuously with green LED on. U4 251968-03 DOS ROM, (reads as 27128 EPROM) Spindle motor runs continuously and green LED stays on or blinks. U5 LC-3517A SRAM (generic 6116 2K SRAM) When drive powered up, motor runs continuously and green LED flashes slowly (about 1 flash every 2 seconds). U6 6522 VIA Drive does not respond to commands from computer or "Searching for..." but no further response from drive. U7 7406 LOGIC No drive reset from computer or "Searching for..." but no further response from drive. U8 6522 VIA Spindle motor doesn't turn. U9 74LS04 LOGIC No write protect sense. U10 251828-03 MOTOR CONTROLLER Spindle motor doesn't turn, or write protect not working, or stepper doesn't move, can cause continuous spindle and activity LED on. U11 74LS86 LOGIC Drive does not respond to commands from computer. U12 CX20185 R/W HEAD AMPLIFIER (Surface mount IC) alt: Sanyo LA8200M No read and/or write. Check for clogged drive head first! U13 uPA2003C LOGIC, CMOS, 7 GATE DRIVER, Spindle motor doesn't turn (or runs constantly) and/or stepper doesn't move. U14 74LS14 LOGIC No drive reset from computer. Power supply: Drive may work when system is cold but fails to read after warmup, or spindle will run continuously but the LEDs will be dim or will not light at all. Smaller power "brick" (700mA) is a common failure item. 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 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. 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, take the top off the drive 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: Red power LED comes on and stays on, green activity LED comes on (and spindle motor turns) for about two seconds, then green LED should go out and motor should stop. COMPUTER POWER UP (OR RESET): Green 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 green 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 green 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 green 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 green 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 green 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.