TRS-80 PC-6 Troubleshooting Guide
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Faults on the TRS-80 PC-6 are set out below from Tandy's service manual, which has a fault table, memory and printer test programs, and a key-by-key operation check.[1] The PC-6 has two HD61747 processors, an 8 KB HM6264ALFP-50 RAM, the MB64H193 gate array, two circuit boards joined by a heat seal, an LCD on a second heat seal, and three lithium cells. Kaiser reports that most second-hand PC-5 and PC-6 units have broken hinges or latches; see TRS-80 PC-6 General Maintenance for those.[2]
Tandy's fault table
[edit | edit source]| Symptom | Cause | Solution |
|---|---|---|
| No display at all | Weak cells | Replace the cells |
| Poor battery contact | Clean the cell and the contact spring; lift the battery spring | |
| Peeled heat seal | Replace the heat seal | |
| Broken heat seal | Replace the heat seal | |
| LCD (CA51-TS) | Replace the LCD | |
| LSI (HD61747B) | Replace the LSI | |
| One or more segments cannot be displayed | Peeled heat seal | Replace the heat seal |
| Broken heat seal | Replace the heat seal | |
| LCD (CA51-TS) | Replace the LCD | |
| LSI (HD61747B) | Replace the LSI | |
| Calculation impossible | LSI (HD61747B) | Replace the LSI |
| A key does not work | Poor key contact | Clean the contact pad on the board and the key contact rubber |
| Broken PC line (a track on the circuit board) | Reconnect it | |
| Peeled heat seal | Replace the heat seal | |
| LSI (HD61747B) | Replace the LSI |
The heat seals are part T-07-12 between the LCD and the board, and CAX-12-3 between the two boards PCB-G707-1 and PCB-G707-2.[1]
Which processor?
[edit | edit source]The two processors split the keyboard and the display, so the pattern of a fault points to one of them.[1]
| Chip | Display characters | Keyboard | Other functions |
|---|---|---|---|
| IC1, HD61747B20 (main) | 1–6 and 13–18 | Lower keyboard | BASIC |
| IC2, HD61747B25 (sub) | 7–12 and 19–24 | Upper keyboard | Calculation, input and output |
Kaiser's description gives the Data Bank, the assembler and the option port to the second processor as well.[2] A display blank in two blocks of six characters, or one half of the keyboard dead, therefore points first at one chip and its connections.
Test programs
[edit | edit source]Tandy's memory check program is two short BASIC programs, in P0 and P1, listed on page 11 of the service manual. P0 fills 904 array elements and a string; P1 reads them back and prints CHECK**OK** or CHECK**NG**. The manual advises typing them in once and keeping them on tape. The procedure on page 12 loads the programs from tape through the cassette interface, switches the computer off and on with the interface removed, and runs them: a good unit shows INPUT OK* and, after the inputs listed in the manual, finishes with CHECK**OK**A=5 and a beep, while a fault gives CHECK**NG**.[1]
The printer check program, in P1, prints a test pattern of every character on the 26-3652 printer; the expected print-out is shown on page 13. The memory and printer programs cannot be in memory at the same time.[1]
Operation check
[edit | edit source]Pages 14 to 18 of the manual are a key-by-key operation check: a list of key sequences, each with the display expected after it, covering every key, the modes, the memo and assembler functions, and the contrast control. It ends by switching off and on three times, checking that the display clears and returns to Ready P0, and turning the contrast control to see the display change. If a step goes wrong, go back to the nearest step marked with an asterisk and continue from there.[1]
Switch the computer off before connecting the printer or the cassette interface.[1]
Buzzer
[edit | edit source]The MB64H193 gate array drives the buzzer.[1]
| Source | Frequency | Duration |
|---|---|---|
| BEEP 0 | 0.9 kHz | 110 ms |
| BEEP 1 | 4.55 kHz | 110 ms |
| Key press | 4.55 kHz | 11 ms |
Supply and signal reference
[edit | edit source]| Line | Purpose |
|---|---|
| VDD1 | Supply for the CPUs, RAM and options |
| VDD2 | Equal to VDD1 when switched on; goes to GND at power-off or auto power-off |
| VRAM | Holds the RAM's supply |
| VSS1 | Supply for the TC4071BF |
| V1–V5 | LCD drive voltages, from resistors connected to VDD2 |
| Pin | Signal | Function |
|---|---|---|
| 19 | SW | Power switch signal: GND when switched off, VDD1 when on |
| 20–24 | V5–V1 | Display voltages divided from VDD1 |
| 25, 26 | PG2, PG1 | CPU internal clock |
| 73 | GND | Ground |
| 74, 75 | CE2, CE1 | Chip selects for the RAM and other devices |
| 76 | VDD1 | Supply |
| 77 | VDD2 | VDD1 output from the LSI when switched on; GND when switched off |
| 78, 79 | φ1, φ2 | Clock pulses |
| 80 | OP | Operation signal |
At power-off the interrupt circuit stops the clock reaching the CPU as VDD2 drops, and takes the gate array's supply and VDD4 to GND, stopping the RAM and options.[1] If programs are lost when the main cells are changed, check the CR1220 backup cell; Tandy rated it at about two years with the main cells fitted.[1]
Related pages
[edit | edit source]- TRS-80 PC-6
- TRS-80 PC-6 General Maintenance
- TRS-80 PC-6 Capacitor Replacement Guide
- TRS-80 PC-6 Documentation
- TRS-80 PC-5 Troubleshooting Guide
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Tandy, Service Manual: Pocket Scientific Computer PC-6, catalogue number 26-3672. Hosted as File:Tandy PC-6 Pocket Scientific Computer Service Manual (26-3672).pdf. Specifications p. 3; block diagram p. 4; LSI descriptions p. 5; buzzer, interrupt and keyboard circuits p. 6; power supply p. 7; LSI pin functions p. 8; display p. 9; troubleshooting guide p. 10; test programs p. 11; program save, load and memory test p. 12; printer check p. 13; operation check pp. 14–18; parts lists pp. 20–22; schematic p. 24.
- ↑ 2.0 2.1 Cameron Kaiser, "Tandy Radio Shack Pocket Computer PC-4 and Tandy Pocket Scientific Computer PC-5, PC-6, PC-7", Floodgap Retrobits, Tandy! Pocket! Mania!. Retrieved 2026-10-21. PC-5 and PC-6 section: broken hinges and latches on most second-hand units; the two-CPU design; the memory backup battery; the membrane keyboard; the 26-3671 link cable.