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Sharp PC-1251 Troubleshooting Guide

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Sharp PC-1250 and PC-1251 power and LCD bias circuit (service manual)
The PC-1250/PC-1251 power and LCD bias circuit from Sharp's service manual.

Fault-finding for the Sharp PC-1251 (and the PC-1250, which Sharp's service manual covers with it). The instruction manual has a short troubleshooting table and the service manual gives the supply circuit, the current consumption limits and several service precautions, which together cover most faults on a machine with no mains supply and only a handful of discrete parts.[1][2][3]

Preliminary checks

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  • Fit two fresh CR2032 cells, both new. Sharp says not to mix new and used cells.[4]
  • Undock the computer from the CE-125 and try it alone.
  • A cell change, or a reset without a key held, clears the memory. Save programs to tape first if the machine still works.[5][6]
  • Use an anti-static strap if the case is opened.

Sharp's troubleshooting table

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From the PC-1251 instruction manual[1]
Symptom What to do, in order
Nothing on the display when switched on Check that the slide switch is at RUN, PRO or RSV; press ON in case auto power-off has operated; replace the cells
A display but no response to keys Press CL; press CA (SHIFT CL); turn off and on again; hold any key and press RESET; press RESET without a key
No response from any key Hold any key and press RESET; if that fails, press RESET without a key, which clears the program, data and reserve area

Auto power-off operates after about 11 minutes without a key press, except while a program is running. Pressing ON brings the machine back exactly as it was.[7]

Power and current

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The two CR2032 cells are in series and feed the VGG supply through a 22 Ω resistor and a diode. A 3.3 µF capacitor and an RD6.2E8 zener sit across the supply. Sharp gives VGG as about 6 V.[3][8] The circuit diagram marks the 3.3 µF capacitor's positive terminal on GND and its negative terminal on VGG, and Sharp's LCD drive example gives the bias levels as negative voltages (−4.2, −5.6 and −7.0 V for a VDIS of 7.0 V). Note the polarity before replacing anything on the supply.[3]

Sharp's service manual gives a current check that works well on the bench:[2]

Current consumption (Sharp service manual)[2]
Condition Supply Maximum current
On, after pressing ON 5.8 V 400 µA
Off, after pressing OFF 5.8 V 45 µA

Press the ALL RESET switch before measuring, as Sharp instructs. A machine that draws more than these figures from a bench supply has a fault on the board. One that drains its cells quickly while switched off should be checked against the 45 µA limit.

Display faults

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Display symptoms
Symptom Check
Display dim and hard to read even with the contrast control turned fully anticlockwise Cells exhausted; replace both[4]
Flickering display A cell not seated properly or making poor contact; Sharp warns this can also damage other parts[2]
Poor contrast at some angles Set the contrast control. Sharp's service check is no blurring at 30° and no irregular contrast at 10°[2][9]
Contrast control has no effect The control is a 20 kΩ variable resistor setting VDIS; check it and the 1 µF and 0.1 µF capacitors of the LCD bias divider[3]

The LCD (Sharp LF8126E) is driven by an SC43536 driver chip.[10]

Keyboard faults

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The keys are scanned by the processor's IA1 to IA8 input and strobe lines and IB1 to IB3 strobe outputs; the ON key has its own input pin.[11] The block diagram shows a 51-key matrix plus the ON key.[12]

  • One dead key: clean the contacts.
  • A row or group of keys dead together: trace the strobe or input line concerned on the circuit diagram. The parts list includes a flexible printed cable (FPC, item 43, QCNW-1236CCZZ); check it and its joints.[3][10]
  • All keys dead after static or electrical noise: hold any key and press RESET.[13]

Memory and module faults

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  • Improper operation after the memory module board is taken out and refitted: Sharp says the play in the screw holes can leave the board slightly misaligned. Refit it and press ALL RESET.[2]
  • The PC-1250 and PC-1251 take different memory module boards (DUNTK6915CCZZ and DUNTK6917CCZZ), which carry the LH532916 ROM and TC5517AF RAM.[10]

CE-125 and error 8

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  • ERROR 8 is an I/O device error between the PC-1251 and the CE-125's printer or tape. Check the CE-125's low battery indicator and all the connections.[14]
  • A low CE-125 battery or strong electrical noise can stop the CE-125 and hang the PC-1251, as can sending LPRINT or LLIST with the CE-125 switched off. CL usually clears it; otherwise use the ALL RESET. Recharge the CE-125 before using it again.[15]
  • Docking the PC-1251 with its power on can hang it. Switch both units off before connecting them.[16]

Working on the board

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Sharp's production chassis sits on top of the main board and makes it hard to work on. Sharp's service tool chassis (LCHSS1134CCZZ) is smaller and is meant to be fitted only while troubleshooting; refit the original chassis afterwards.[2]

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References

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  1. ↑ 1.0 1.1 Sharp, PC-1251 instruction manual, "Troubleshooting", printed page 152 (PDF page 154). Hosted as File:Sharp PC-1251 Instruction Manual.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Sharp, Service Manual, Model PC-1251, PC-1250, section 6 "Service precautions", printed page 9 (PDF page 10): service tool chassis; memory module board alignment; cell installation; current consumption at 5.8 V, 400 µA on and 45 µA off maximum, after ALL RESET; display quality check at 30° and 10°.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Sharp, Service Manual, Model PC-1251, PC-1250, section 7 "Circuit diagram", printed page 10 (PDF page 12).
  4. ↑ 4.0 4.1 Sharp, PC-1251 instruction manual, chapter 2, "Cell replacement", printed pages 13 to 15 (PDF pages 15 to 17).
  5. ↑ Sharp, PC-1251 instruction manual, chapter 3, printed page 26 (PDF page 28).
  6. ↑ Sharp, PC-1251 instruction manual, chapter 2, "ALL RESET", printed page 12 (PDF page 14).
  7. ↑ Sharp, PC-1251 instruction manual, chapter 3, "Auto off", printed page 16 (PDF page 18).
  8. ↑ Sharp, Service Manual, Model PC-1251, PC-1250, section 5 "LCD drive circuit and timings", printed pages 7 to 8 (PDF pages 8 to 9): example levels VA −4.2 V, VM −5.6 V, VB −7.0 V with VDIS = 7.0 V; VGG about 6 V.
  9. ↑ Sharp, PC-1251 instruction manual, chapter 2, "Contrast control", printed page 13 (PDF page 15).
  10. ↑ 10.0 10.1 10.2 Sharp, Service Manual, Model PC-1251, PC-1250, section 9 "Parts list" (PDF page 18).
  11. ↑ Sharp, Service Manual, Model PC-1251, PC-1250, section 4, CPU terminal descriptions, printed pages 4 to 6 (PDF pages 5 to 7).
  12. ↑ Sharp Corporation, Service Manual, Model PC-1251, PC-1250, December 1982, section 3 "System block diagram", printed page 3 (PDF page 4). Hosted as File:Sharp PC-1251 Service Manual.pdf.
  13. ↑ Sharp, PC-1251 instruction manual, "Maintenance", printed page 154 (PDF page 156).
  14. ↑ Sharp, PC-1251 instruction manual, appendix A "Error messages", printed pages 155 to 156 (PDF pages 157 to 158).
  15. ↑ Sharp, PC-1251 instruction manual, chapter 7, CE-125 care and hang-ups (PDF page 64).
  16. ↑ Sharp, PC-1251 instruction manual, chapter 7, "Connecting the PC-1251 to the CE-125" (PDF page 57).


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