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Defective Head Park Switches


  • One possible failure of an ATARI 1050 disk drive that will cause endless boot errors is a failure of the sensor that detects if the read head is parked. This sensor is currently NOT AVAILABLE as a repacement part, but MUST be in working order for the drive to operate. This article will help you to replace it with more commonly available parts.


    #2 Phillips head screwdriver
    #1 Phillips Head screwdriver
    30 watt maximum soldering iron
    Small flat blade screwdriver
    Small needle nose pliers
    Epoxy or equivalent glue
    Xacto Knife


  • Turn the 1050 on its back and remove the 6 phillips head screws. Carefully turn the drive back onto its feet and set it down. Gently lift the back of the cover and slide it forward. The front bezel will come off with it.


  • The head assembly slides on tracks and is driven by a stepper motor located to the right of the disk platter. The head assembly has an arm that sticks out to the left. This arm slides into a u shaped sensor when the head is retracted.

  • Connect power to the drive but do not attach it to the computer. Turn the drive on (do not put a disk in the drive). As the drive powers up it must find the location of the read head. If the head is parked, it will index the head forward until it clears the sensor and then repark it. If the head was not parked, it will retract the head until it is parked.

  • If the sensor is bad, the head will index forward 1/4 of inch and stop. Every time the drive is turned on the head will index forward. Eventually the head runs out of travel and will bang repeatedly on the forward stop. When connected to a computer, you get a boot error because the drive cannot find the boot sectors.

  • As a final test, disconnect the J10 connector on the circuit board. Do not pull on the wires!!!!! Use the needle nose pliars. It is the next to last connector on the back left of the circuit board. When the connector is removed, 4 pins will be exposed on the circuit board. Jump the front two pins on the board together and turn on the drive. If everything else is alright the head will begin to retract. As soon as the head begins moving, turn the drive off and remove the jumper. If the head did not move backwards then the problem is in the circuit board or the stepper motor. That is beyond the scope of this article.


  • The sensor is composed of an infrared LED and an infrared photo transistor. Either one of the pair could be bad. I have not found direct replacements for them, but, a pair made by Radio Shack will work. The LED is part number 276-143A and the receptor is part number 276-145. Please note, these parts are larger than the original parts and will not fit into the existing sensor housing. You will have to fabricate a housing for them.


    1. Remove the sensor from the drive. The wire tie to the frame must be cut.

    2. Cut the sensor from the wires. Orient the plug as it goes in the drive. Place shrink tubing over the wires and then solder the parts to the wires as follows. The order is from front to back.

    1st wire-emitter of part 276-145

    2nd wire-collector of 276-145

    3rd wire-cathode of 276-143A

    4th wire-other lead of 276-143A

  • After soldering, position the shrink tubing over exposed connections and heat it.

    3. Plug the J10 connector back into the circuit board. 4. Fabricate a mounting that positions the LED vertically looking down. The mounting must have fore and aft adjustment and position the LED above the arm on the head unit. The arm on the head unit must pass under the LED. Ideally,the photo transistor pair should face each other. Unfortunately, there is not enough room under the arm for thr receptor. By trial and error I discovered that the receptor can be placed on its side. The LED though must shine directly into the receptor.

    5. Fabricate a mounting that positions the receptor horizontally looking toward the head mechanism. The mounting must have fore and aft adjustment and allow the arm on the head unit to pass over it. I cut the original housing into pieces and glued the LED and receptor to halves of the mounting. I then used standoffs to set the parts to the correct height.

    6. Turn the drive on. As the drive powers up it will position the head over the sensor pair. If the head will not position, check to make sure that the wires are properly connected.

    7. Now that the head is being parked, make sure that it is parked in the proper place. As a starting position the back end of the head mechanism should be 3/8 of an inch from the inside boss that holds the arms that the head slides on. If the location is wrong, turn the drive off and move the sensor pair as required. When the drive is turned back on it will repark the head. Continue moving the sensor pair until you get the 3/8 dimension. The sensor pair is only moved with the drive powered off.

    8. Connect the drive to the computer and attempt to boot a disk. SPARTADOS is less sensitive to head location than DOS 2.5. If the disk boots, run some programs to check that the drive can read all disk sectors. If the drive can not read all sectors or will not boot then move the sensor pair a little forward or backwards. My two drives both held a dimension of 7/32 of inch on the gap referenced above.

    9. Be patient. Eventually you will get the correct gap. Reassemble the drive cover and you are finished.

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