1050 Repair: Defective Head Park Switches

From: Michael Current (aa700@cleveland.Freenet.Edu)
Date: 02/12/92-10:29:48 AM Z

From: aa700@cleveland.Freenet.Edu (Michael Current)
Subject: 1050 Repair: Defective Head Park Switches
Date: Wed Feb 12 10:29:48 1992

Reprinted from the A.C.E.C. BBS (614)-471-8559

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


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

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.
 Michael Current, Cleveland Free-Net 8-bit Atari SIGOp   -->>  go atari8  <<--
   The Cleveland Free-Net Atari SIG is the Central Atari Information Network
      Internet: currentm@carleton.edu / UUCP: ...!umn-cs!ccnfld!currentm
     BITNET: currentm%carleton.edu@{interbit} / Cleveland Free-Net: aa700

Return to message index