The 1541 Disk Drive

---

There are at least three 1541 models and some variations of these models. Commodore numbered them 1540, 1541, and 1542, although they usually carried the number 1541. The first two had the Alps mechanism, with the push-down disk eject, while the latter model typically used a Neutronics drive, the one with the lever-type release. The PC boards in all these drives are referred to as either "long" or "short", with later versions having fewer chips to save costs. Be aware that the Alps and Neutronics drive mechanisms are interchangeble, except in the very earliest machines. The electronics don't care which drive they control. Having never seen the inside of a 1541 II, I couldn't say what's there.

From the comp.sys.cbm FAQ:

The most common problem facing the 1541 disk drive is alignment. If your 1541 has trouble reading commercial disks, or reading disks written some time ago, but has less trouble reading recently wless trouble reading recently written disks, chances are that your 1541 is out of alignment. C= service centers (if you can FIND one) will typically align a 1541 for anywhere from $20 to $45. There are also 1541 alignment programs, (e.g. Free Spirit's "1541/71 Alignment System") which allow you to align a 1541 yourself. There are those who claim that this does not produce good results, but there are others who claim to have had satisfactory results with these programs. There were a couple of articles in COMPUTE Gazette and I think RUN on how to do this. The real problem is mechanical in nature and can be overcome. Other products are PHYSICAL EXAM 1541 and 1571 versions.

Cleaning the 1541 Drive

Take the outer plastic case off by removing the four screws on the bottom of the unit. Remove the two screws holding the metal shield and remove it. Disconnect the five wire connectors on the PC board, making sure to note their positions and orientation. Take out the screws holding the PC board in place and remove it. Now you have access to the drive head and the rails.

I usually wash the plastic outer case with detergent and dry it thoroughly. This way it looks as good as it can be on the outside too. Clean any dust off the PC board with canned air or a dry brush.

The drive head can be cleaned by lifting the pressure pad cover and wiping with a cotton swab wet with alcohol. The head assembly travels back and forth on two metal rails that should be very lightly oiled. Over time they get gummy, affecting the head movement. Clean the rails first and then re-lubricate sparingly with a light oil. Slide the head assembly back and forth a few times to spread the oil evenly.You can also lubricate the stepper motor by putting a drop of oil on the top of the shaft. There may be a paper dot there that you need to peel aside.

Adjusting the drive speed

Basically the speed adjustment is a simple screwdriver tweak. The Alps drives had a paper timing disk stuck on the flywheel. With the drive motor running at the correct speed, the marks on the disk appear to be stationary when illuminated by fluorescent light. If no fluorescent source is available you can use the light from your TV or monitor screen! Change the colors of the screen with these pokes to get the brightest light to use for adjusting:

The Neutronics drives probably won't have the handy timing marks pasted on the flywheel. There are three ways to deal with this. You can use a frequency counter connected to pin 2 of the governor IC, the chip near the front of the daughterboard, which is mounted above the drive in this unit. The correct frequency should be 367 Hz. You can print your own timing disk from a geoPaint PhotoScrap. It should measure 1.4" in diameter when printed correctly. Or you can download this alignment program that will measure speed for you.

Prop the drive on it's side with the outer covers off. Connect the serial cable between the drive and your C64/C128. Plug in the AC cord. Turn on the drive, monitor, and computer. Load the utility program and start the drive motor by pressing F5, or type this command in direct mode:

If you are not using a computer, short out pin 3 to pin 1 on P5 to run the motor.

By the light of your monitor, or fluorescent lamp, adjust VR1 until the timing marks stop or the frequency counter reads 367 Hz, deounter reads 367 Hz, depending on which method you are using. VR1 is the small variable resistor found on the daughterboard on the underside of the Alps model, or above the Neutronics mechanism.

Alignment - Simple Method

With the PC board lying on some non-conductive material, replace the connectors except for the green LED. Connect the serial cable and the power cord also. Be careful, the AC can give you quite a bite!

Ground the PC board to the metal frame with a wire with alligator clips on both ends. Clip one end to the frame and the other to the ground trace around on of the PC's screw holes.

Load one of the alignment programs. If the drive is too far out of alignment you may have to type it in. Put an alignment disk, or a commercially prepared disk in the drive. The Commodore 1541 Demo disk is a good one to use. Run the alignment program and have it read track 17. The red LED should flash if the drive is out of alignment. This signifies a read error.

Find the two screws that anchor the stepper motor to the frame. Their heads may have a spot of lacquer on them. This needs to be scraped off so you won't strip the heads. Loosen the screws and rotate the motor slightly, keeping an eye on the red LED. When the drive head is directly over the track, the LED will remain on. If you rotate to the left or right of dead cen left or right of dead center, the LED will blink. Using the alignment program, "bump the head" every time you make an adjustment. This makes for a more accurate reading.

Once track 17 is aligned, repeat the proceedure for tracks 1 and 35. When you can get the drive to read tracks 1,17, and 35 with a steady red LED, tighten the stepper motor screws. You may have to go back and do this whole proceedure more than once to get it right. Since track 1 is supposed to be at the inner end stop, that is your reference. If that reference is off, you may actually be aligning with reference to track 0 or track 2. Any disk not formatted on this drive will fail to read. Make sure the drive can read the directories on several commercially made disks before you put your drive back together. Put some nail polish on the stepper screw heads to keep them from slipping.

Checking the End Stop Gap

On the top side of the drive mechanism is the end stop cam. It is mounted on the shaft of the stepper motor. The cam bumps against the end stop to keep the drive head from going below track 1. If the end stop moves, this d stop moves, this can happen. You should be able to slip a .006" feeler gauge between the cam and the end stop when the drive is positioned at track 1. If not, loosen the screw and adjust the end stop. Tighten the screw. Bump the head and check the gap again until you're satisfied.

Once you've completed all this, rerun the program to check all the tracks for read errors. If all is well, your drive is aligned and ready to be reassembled.

Alignment Using a Video Detector and Volt/Ohmmeter

This is a more sophisticated method of alignment using a small video detector circuit with a volt/ohmmeter. If you can't find a second video amplifier chip on your PC board it's because the functions of that chip have been integrated into a proprietary chip in the version you have. You have to align by the Simple Method, or if possible, use a PC board from another drive that DOES have the chip.

Disassemble the drive as in the Simple Method above and reconnect the PC board to the five wire harnesses, making sure the PC board does not contact any metal parts. Orient the drive mechanism to allow access to the bottom. Connect the serial cable to a C64 and plug in the AC cord.

Connect the video detector to the second video amp, black lead to pin 7, red lead to pin 8. Connect the other end of the video detector to the voltmeter.

On the Alps model of the 1541, the second video amp (UH5) is a NE592N 14-pin IC located at the front end of the PC board. Looking at the chip from above, with the notch at the top, the pins are numbered from 1 to 14, counter-clockwise starting at the pin to the left of the notch. On the Neutronics models, the second video amp (UF5) is also a NE592N 14-pin IC located at the front of the PC board. There are two of these there. You are looking for the one furthest from the side of the PC board with all the connectors on it.

Power everything up and load or enter the align1541 program. Insert the formatted disk that will be your reference and send the head to track 6. Loosen the two screws that hold the stepper motor and slowly rotate counterclockwise, keeping an eye on your voltmeter. The voltage should increase, peak, and decrease. Rotate the stepper motor clockwise until the voltmeter peaks, then tighten the two screws.

Bump the head back to track 1 by hitting F2. Hit F1 once and the screen will show track 1.5. If the stepping motor drive pulley cam is not touching the end stop skip the next paragraph.

[Move the head to track 16 by hitting F1. Loosen the stepper motor screws and rotate counterclockwise as above through the peak and then clockwise back to the peak. Note: the voltage reading should not be the same as observed at track 6. Ts observed at track 6. Tighten the screws.]

Bump the head back to track 1 by hitting F2. Note the voltage reading at track 1. Hit F1 once and read the voltage at track 1.5. The voltage should be no greater than 30% of the track 1 reading. Repeat this for tracks 2 through 35 noting that the half track readings should be significantly lower than the full track readings.

Bump the head back to track 1 by hitting F2. Adjust the End Stop Gap as above.

Turn everything off and reassemble the 1541.

The Video Detector

Needed Materials

• pair of meter leads (get the type w/clip leads on one end)
• 0.01 microFarad, 50v ceramic capacitor
• 1 k ohm 1/4 watt resistor
• 1N4148 diode, or equivalent
• knife or wire cutters
• soldering iron and solder
>soldering iron and solder
• heat shrink tubing or electrical tape
• ohmmeter
• string or wire ties

Cut the meter leads in half and strip insulation back 1/4" on all four ends. Tin the ends and solder in the components. This is the circuit:

black clip lead                                              to meter
<---------------------------------------------|--------------------->
                                              | 
                                              |
                                              = 0.01 mF
                                              |
red clip lead                                 |              to meter 
<------------------------>|-----\/\/\---------|--------------------->
                       diode    1k ohm 

Cover the diode and resistor with a piece of shrink wrap or tape before soldering the capacitor. Then cover all the components after all the connections are soldered. I use clip leads to connect to the ICs so my hands are free to turn the stepper motor. The other ends should have straight pins or banana plugs to fit your meter sockets.

Alignment Programs

• align1541- Use this one with the video/voltmeter method. It will invideo/voltmeter method. It will increment by half-tracks.
• 1541ali - I prefer this one because of it's flexiblity, but it won't do half-tracks.
• 1541ali2 - This one requires a joystick.
• 1541align - This one only works with drive 8.
• 1541-tester - This one does not work on a C128 in C64 mode. I haven't tested it on a stock C64.
• head banger - This is a text file about banging the head back into alignment. A short type-in Basic program is included.
• alignall - This program will test alignment of 1541, 1571, and 1581 drives.
• alignmnt - This will test 1541 alignment and motor speed. This one is useful if you do not have the timing marks pasted on the flywheel.