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IBM PC Series 300 Capacitor Replacement Guide

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IBM PC 300PL Type 6892 system board

Capacitor work on the IBM PC Series 300 has little published data behind it. IBM's hardware maintenance manual for the family treats the system board, riser card and power supply as field-replaceable units and gives no component values; its only mention of capacitors is the safety inspection guide's warning that a bulging capacitor is an explosion hazard.[1] Read the values from the parts on your own board.

Reported failures

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One owner on the Badcaps forum found three CHHSI 560 µF 25 V electrolytic capacitors (WG(M) series, 105 °C, date code 9846) on a PC 300PL with intermittent faults. The machine type is not stated. Another member advised replacing every CHHSI capacitor on the board whether or not it looked faulty; the owner replaced the three and reported that nothing changed.[2]

Safety

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The power supply's primary side carries mains voltage, and its large electrolytic filter capacitors hold a high voltage with a large stored charge after the supply is unplugged. Most supplies have a bleeder resistor, but it can fail. Measure the voltage on those capacitors with a meter before touching anything; do not rely on time elapsed.[3] See also CRT Discharge Procedure for a monitor.

Replacement

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  1. Photograph both sides of the board and record each capacitor's value, voltage, temperature rating, polarity and lead spacing.
  2. Remove the system board. The HMM gives the removal procedure for each type, and notes that the processor is a separate FRU that must be moved to a replacement board.[1]
  3. Replace each electrolytic with a part of the same capacitance, the same or higher voltage rating and the same or higher temperature rating. Where the original is a low-ESR type, use a low-ESR replacement.[4]
  4. Match the polarity to the board's marking.
  5. Mains suppression capacitors in the supply must be replaced with the same capacitance and class (X2 for X2, Y2 for Y2) at the same or higher AC voltage rating.[4]

Many failed aluminium electrolytics show no visible sign of failure; a capacitance reading within tolerance does not rule out a high ESR.[4]

Post-recap voltage checks

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IBM's limits, measured with the supply connected to the system board or riser card:[1]

PC 300 Series supply limits (HMM p. 29)
Output Reading
+5 V +4.75 to +5.25 V
−5 V −4.5 to −5.5 V
+12 V +11.4 to +12.6 V
−12 V −10.8 to −13.2 V
Power Good +4.75 to +5.25 V
+3.3 V (system board) +3.3 to +3.46 V
+3.3 V (riser card) +3.17 to +3.46 V

The 20-pin connector pin assignment for the types with the connector on the riser is on HMM p. 30. If the voltages are wrong and the power cord is good, IBM's instruction is to replace the supply.[1]

References

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  1. ↑ 1.0 1.1 1.2 1.3 IBM, IBM Personal Computer 300 Series, 700 Series, IntelliStation Hardware Maintenance Manual, S83G-7789-17, Eighteenth Edition (September 1998): replacing a system board pp. 83 and 84; safety inspection guide p. 190; power supply checks pp. 28 to 30; specifications pp. 195 to 216. Hosted as File:IBM PC 300 700 IntelliStation Hardware Maintenance Manual S83G-7789-17 (Sep 1998).pdf.
  2. ↑ IBM 300PL, Badcaps forum thread. First-hand reports, not a manufacturer document.
  3. ↑ IBM 51xx PC Family Computers – Some Hazards Within, minuszerodegrees.net. Written for the earlier IBM PCs; the power supply hazards apply to any mains switching supply.
  4. ↑ 4.0 4.1 4.2 Commonly Failing Electronic Components (in vintage computers), minuszerodegrees.net, section "Aluminium Electrolytic".
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