TRS-80 PC-4 General Maintenance

Exploded view of the PC-4 (26-3650) from Radio Shack's service manual. Items 3 to 5 are the battery springs, 15 and 22 the rubber key contacts

The TRS-80 PC-4 has no mains supply. It runs from two CR2032 lithium cells, so routine care comes down to the cells and their contacts, the display, the keyboard and the case. Radio Shack's owner's manual and service manual are the sources for everything below that is not credited to someone else.

Cells

When to change them

Radio Shack told owners to replace both cells every two years whatever the use, because worn cells may leak and cause a malfunction, and never to leave flat cells inside the computer. The other sign is the display: if the contrast is still weak with the contrast control turned fully up, change the cells. The service manual rates the two cells at about 360 hours of continuous use, at a maximum consumption of 0.02 W, and the computer switches itself off about seven minutes after the last key press.

The PC-4 has no backup cell, so removing the cells clears the programs and variables in memory. Cameron Kaiser notes that the PC-5 and PC-6 were the only Tandy pocket computers that kept their memory while the main cells were out. Save anything worth keeping to tape through the 26-3651 cassette interface first.

Changing the cells

The owner's manual gives this procedure.

1. Switch the computer off.
2. Loosen the two screws on the back and remove the rear panel.
3. Press on the marked point of the battery compartment lid, slide the lid in the direction of the arrow and lift it off.
4. Remove the old cells. Tapping the unit lightly with the compartment facing down helps.
5. Wipe the new cells with a dry cloth and fit them with the positive side facing up.
6. Slide the lid closed while pressing down on the cells.
7. Refit the rear panel and tighten the screws.
8. Switch on and press the ALL RESET button on the back with a pointed object.

Replace both cells together, fit them the right way round, and do not put old cells in a fire. The cell is a CR2032; Radio Shack sold it as catalogue number 23-162.

Contacts and leakage

The cells sit in a holder against three battery springs and an insulating plate (items 3 to 7 in the exploded view). If a cell has leaked, take the board out as described below, clean the springs and holder, and check the board around them; the cross-cutting page Battery Explosion, Capacitor or Corrosion Damage covers cleaning up after a leak. The service manual's part numbers for these and the other parts that maintenance touches are below.

PC-4 parts that maintenance touches (service manual parts list, 26-3650)

Exploded-view item	Description	Manufacturer part number
3	Spring, Battery A, G272 A33938A-1	63297621
4	Spring, Battery B, G272 A33939-1	63297630
5	Spring, Battery C, G272 A33940-1	63297640
6	Plate, Battery Insulation, G272 A45154-1	63297660
7	Holder, Battery, G420 A45884-1	63327190
15	Key Contact, Rubber A, G420 A35389-1	63326970
22	Rubber, Key Contact B, G420 A35390-1	63327030
18	Film, Common, G420 A21302-1	63327290
–	Seal, TH heat, TH-07-1 (LCD heat seal)	56004397
–	LCD, LD-B8105A	33306768

Cleaning and handling

• Wipe the case with a soft, dry cloth, or one dampened with a neutral detergent.
• Keep the computer away from shocks, drops, sudden temperature changes, and places that are hot and humid or dusty. In the cold the display responds slowly or goes blank; it recovers when the computer warms up again.
• Connect nothing to the adaptor socket except the PC-4's own options.
• While a dash shows on the display the computer is busy and ignores most keys; wait for it to clear.

Display contrast

The contrast control is on the right-hand side of the case. Turn it in the direction of the arrow for more contrast. Inside, it is a 150 kΩ variable resistor (EVL-AHAA00BC5) at the bottom of the resistor chain that sets the LCD drive voltages V1 to V5. A display that stays faint at full contrast points first to the cells.

RAM module

The 26-3653 RAM module (also sold as 26-3653A) plugs into a socket and a row of contact pads under the rear panel. The owner's manual gives these precautions:

• Discharge static from your body before handling the module, by touching a metal object.
• Do not touch the module's connector or the pads on the computer's board, and keep both free of dust and finger marks; dirty pads cause poor contact.
• After fitting or removing the module, switch on and press ALL RESET. Without the reset the memory contents may change or the display may show nonsense.
• Keep a removed module in its case.

After fitting it, MODE 1 should report 1,568 free steps; 544 means the module is not being seen. The TRS-80 PC-4 Troubleshooting Guide has the test for the module itself.

Opening the case

Radio Shack's disassembly procedure for the 26-3650 is short.

1. Remove the two screws from the back.
2. Lift and slide down the lower case.
3. Remove the battery cover.
4. Take out the cells.
5. Remove the eight screws from the guide plate.
6. Remove the guide plate.

Under the guide plate are the board, the two rubber key contact sheets (items 15 and 22) and the LCD, which connects to the board through a heat seal (TH-07-1). Faults in both are covered in the TRS-80 PC-4 Troubleshooting Guide. The HD61913 is a CMOS part and the owner's manual warns that static can damage the RAM module, so work on a static-safe surface.

Tools

• A small cross-head screwdriver for the case and guide plate screws
• Fresh CR2032 cells, two at a time
• A pointed object for the ALL RESET button
• An earthing strap or other static protection when the case is open

See also Recommended Tools.

Related pages

• TRS-80 PC-4
• TRS-80 PC-4 Troubleshooting Guide
• TRS-80 PC-4 Capacitor Replacement Guide
• TRS-80 PC-4 Documentation