HP-85 Repair Notes

Note HP does not recommend removing cover because of high voltages.  Only knowledgeable technicians should work inside these computers.

1.	Removing Cover
a.	Unplug computer
b.	Pull off brown tape eject button
c.	Remove six screws on bottom
d.	Remove Cover

2.	Changing Paper Advance Belt
Belt specification: 80T80 or 80MXL by 1/4 inch wide, T80 / MXL meaning 0.080 inch teeth spacing, and 80 teeth long. A 78 teeth long belt works fine too.

a.	Loosen two screws holding the stepper motor
b.	Remove closest screw and motor
c.	Replace belt
d.	Remount motor
e.	Adjust tension on belt by moving motor until you can press the belt in 3/16’s inch; tighten screws
f.	Do not over tighten the closest screw or you can break the plastic mount

3.	Removing and cleaning print head
a.	Place small screwdriver horizontally in groove on bottom of print head
b.	Press down gently and the print head will lift off the cloth covering the platen bar
c.	Gently lift the cloth back and remove the plastic platen bar which has an orange rubber coating on front
d.	Place left thumb and forefinger between belts grasp the print head and snap it off the top bar and wiggle it out between the belts
e.	Observe the print head and do not loose the small spring
f.	Clean the platen, upper and lower silver guide bars with isopropyl
g.	Clean the print head using a pencil eraser in an up and down motion parallel to the elements on the head
h.	When replacing make sure the bottom of the print head rides on top of the lower guide bar

4.	
Replacing the print head belt
Belt specification: 175T80 or 175MXL by 1/8 inch wide, T80 / MXL meaning 0.080 inch teeth spacing, and 175 teeth long.
a.	Loosen tension on belt by loosening the tension bolt located to the far right between the belts in a parallel orientation to the belt with an Allen wrench this causes the right pulley to move inwards
b.	When loose slip belt off right pulley
c.	The left pulley will then pull straight out of it’s plastic holder, do not loose it’s small metal axle
d.	Replace belt and left pulley
e.	Adjust tension so that pressing on the bottom belt you just touch the bottom metal guide rail

5.	Removing keyboard
a.	Remove two screws in back of keyboard holding black mounting rails to the aluminium chassis
b.	Pivot keyboard forward and remove from hinges
c.	Gently pull electrical connectors from sockets

6.	Replacing / Rebuilding Capstan
a.	Remove keyboard
b.	Remove 4 screws: two upper and two lower holding the metal frame of the tape assembly; the lower screws attach to the aluminium chassis and the uppers to the print mechanism
c.	Pull out entire tape assembly including aluminium frame tape mechanism motor and small circuit board
d.	Remove 4 screws holding the motor and pull the two small bare wires out of the holes in the circuit board - they are not soldered – unplug motor from board and remove motor/capstan
e.	Clean off gooey capstan with knife (should have a very thin coating layer).
f.	Replace rubber with HP 1500-0538 – impossible to find
i)	Alternate method 1 – best
(1)	Buy a piece of 3/8 inch outer diameter / 1/4 inch inner diameter latex tubing
(2)	Cut off a small approx 3/16 inch high band
(3)	Attach using LockTite or Instant glue to metal capstan
ii)	Method 2
(1)	Cover metal capstan with 2 or three layers of electrical shrink tubing heating each layer gently before adding the next and then trim excess
g.	Clean tape head with Q-tip and isopropyl alcohol

The HP85 service manual is available on the Museum of HP Calculators CD/DVD