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Sharp PC-1251 Capacitor Replacement Guide

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Sharp PC-1250 and PC-1251 main board capacitor positions (service manual)
Capacitor positions on the PC-1250/PC-1251 main board: the 3.3 µF capacitor beside the RD6.2EB zener at top left and the 1 µF capacitor on VDIS (Sharp service manual).

Capacitors in the Sharp PC-1251 and PC-1250. Sharp's service manual lists only two polarised capacitors on the board: a 3.3 µF 10 V capacitor across the VGG supply and a 1 µF 6.3 V capacitor in the LCD bias circuit.[1][2] Both are drawn as two-pad surface-mount parts on the board drawing.[3] Everything else is chip ceramic or small leaded ceramic.

On a machine this simple, the cells do more damage than the capacitors. A leaking CR2032 or a poorly seated cell is the more likely cause of trouble; see Sharp PC-1251 General Maintenance and Battery Explosion, Capacitor or Corrosion Damage.

PC-1251 capacitor list

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Sharp's parts list gives part codes and values without circuit designators, so the capacitors are described by position.[2]

PC-1250/PC-1251 polarised capacitors (Sharp service manual)
Value Rating Sharp part code Circuit Position on the main board
3.3 µF 10 V RC-SZ1021CCZZ Across the VGG supply from the two CR2032 cells, beside the RD6.2E8 zener; the circuit diagram marks its positive terminal on GND[1] Top left, beside the RD6.2EB zener[3]
1 µF 6.3 V RC-SZ1007CCZZ LCD drive voltage VDIS, between the 20 kΩ contrast control and the bias divider[1] Below the VA pad, with VDIS beside it[3]

Sharp's part codes for the aluminium electrolytics in the Sharp PC-1500 begin RC-EZ; these two begin RC-SZ, and the PC-1251 manual does not say what type they are.[2][4] Identify the parts on your own board before ordering replacements.

Other capacitors in the parts list[2]
Sharp part code Description
RC-CZ1021CCN1 Chip, 0.1 µF
RC-CZ1035CCN1 Chip, 100 pF
RC-CZ1037CCN1 Chip, 0.01 µF
RC-CZ1000QCZZ 0.1 µF 12 V
VCKYPU1HB101K 100 pF 50 V
VCTYPU1EX333M 0.033 µF 25 V

The circuit diagram places the 0.033 µF capacitor at the reset switch, two 100 pF capacitors at the 576 kHz crystal, a 0.01 µF capacitor at the ON/BRK key and two 0.1 µF capacitors on the LCD bias divider.[1] These do not need routine replacement.

Signs of trouble

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  • Corrosion or residue round either polarised capacitor, often from a leaking cell nearby.
  • A flickering display with good, well-seated cells.
  • No usable contrast range from the contrast control, which points at the VDIS circuit and its 1 µF capacitor.[1]
  • Current above Sharp's limits: 400 µA on and 45 µA off at 5.8 V, measured after an all reset.[5]

Recording your own board

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  1. Take out the cells and undock the computer from the CE-125.
  2. Open the case. Sharp's service tool chassis (LCHSS1134CCZZ) gives better access than the production chassis, which sits on top of the main board.[5]
  3. Photograph the board and find the two polarised capacitors beside the zener and on VDIS.
  4. Note the polarity marks on each part and on the board before removing anything.

Replacement

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  1. Use an anti-static strap; the processor and LCD driver are CMOS.[6]
  2. Remove each part with two irons or hot tweezers, or by adding fresh solder to both pads and lifting it.
  3. Fit a 3.3 µF part rated 10 V or more and a 1 µF part rated 6.3 V or more, in the same package size, observing the polarity noted.
  4. If the memory module board was disturbed, reseat it carefully and press ALL RESET; Sharp warns that a misaligned board causes improper operation.[5]

Checks after replacement

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After the work (Sharp)[5][7]
Check Expected
Current with the machine on, 5.8 V supply, after ALL RESET 400 µA maximum
Current with the machine off 45 µA maximum
Switch on and press RESET after fitting cells The prompt > appears
Contrast control Clear display; no blurring at 30° and no irregular contrast at 10°
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Sharp, Service Manual, Model PC-1251, PC-1250, section 7 "Circuit diagram", printed page 10 (PDF page 12): 3.3 µF across VGG with its + side to GND; 1 µF on VDIS; 0.1 µF × 2 on the divider; 0.033 µF at the reset switch; 100 pF × 2 at the 576 kHz crystal; 0.01 µF at ON/BRK.
  2. ↑ 2.0 2.1 2.2 2.3 Sharp, Service Manual, Model PC-1251, PC-1250, section 9 "Parts list" (PDF page 18): RC-SZ1021CCZZ capacitor 3.3 µF 10 V; RC-SZ1007CCZZ capacitor 1 µF 6.3 V; ceramic and chip capacitors; no designators.
  3. ↑ 3.0 3.1 3.2 Sharp, Service Manual, Model PC-1251, PC-1250, section 8 "Parts & signal position", printed page 11 (PDF page 13).
  4. ↑ Sharp, Service Manual, Model PC-1500 & Option, February 1982, section 11 "Parts list & guide" (PDF page 33): RC-EZ227ACC1A 10 V/220 µF and RC-EZ105ACC1H 50 V/1 µF. Hosted as File:Sharp PC-1500 Service Manual.pdf.
  5. ↑ 5.0 5.1 5.2 5.3 Sharp Corporation, Service Manual, Model PC-1251, PC-1250, December 1982, section 6 "Service precautions", printed page 9 (PDF page 10). Hosted as File:Sharp PC-1251 Service Manual.pdf.
  6. ↑ Sharp, PC-1251 instruction manual, appendix G "Specifications", printed page 173 (PDF page 175): 8-bit CMOS CPU. Hosted as File:Sharp PC-1251 Instruction Manual.pdf.
  7. ↑ Sharp, PC-1251 instruction manual, chapter 2, "Cell replacement", printed pages 13 to 15 (PDF pages 15 to 17).


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