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IBM System/23 Datamaster Capacitor Replacement Guide

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IBM System/23 Datamaster CPU planar board, European version. The read/write storage cards and I/O feature cards plug into sockets on this board.

IBM's service manual for the IBM System/23 Datamaster 5322 treats the power supply card, the CPU planar board, the storage cards and the display unit as field-replaceable units, and it gives no capacitance value for any part in the machine. It does give a voltage tolerance and a ripple limit for every DC output, and a way to measure ripple with an ordinary meter, so the condition of the supply's filtering can be checked before anything is replaced.[1]

Power supply types

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A 5322 contains either a ferroresonant power supply or a transistor-switching-regulator (TSR) supply. IBM describes the two as functionally interchangeable, with the same output voltages. In the ferroresonant supply the mains feeds transformer T1, and card A1 converts five of its outputs to -5, +5, -12, +12 and +24 V DC and filters them; the sixth output, 120 V AC, runs the diskette drive motors, the CRT display unit and the cooling fan. The TSR supply has a transistor-switching-regulator card in place of card A1.[2]

The manual documents three ferroresonant supplies: PN 6838279 (60 Hz, Canada and US), PN 6838280 (60 Hz, GBG/I) and PN 6838281 (50 Hz, GBG/I). Each diagram applies to its own part number only; for a supply with any other part number, IBM says to use the labels attached to that supply.[3]

Safety

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IBM's DANGER notice: up to 800 V AC is generated in the power supply. Before removing the supply or any part of it, switch off the 5322 and disconnect the power cable from the outlet.[4]

  1. Switch off and unplug the 5322.
  2. The supply slides out of the rear into a service position, where the fuses, voltage test points, supply parts and power switch are accessible. To switch on with the supply in that position, IBM first has the engineer check grounding continuity between the 5322 main base plate and the power supply frame, with the meter on the ohms RX1 range.[4]
  3. Follow the CRT Discharge Procedure before working near the display unit.
  4. Discharge each diskette drive motor capacitor before touching its terminals (see below).

See also Capacitor Failure Symptoms and Recommended Tools.

Capacitors IBM identifies

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Two capacitors appear in the manual by name. Neither has a value.

Capacitors named in the 5322 service manual
Part Where What IBM says
C6, resonant capacitor Ferroresonant supply, beside transformer T1 Shown on the supply layout and wired across transformer terminals 23 and 24 on the AC distribution diagrams. It is part of the ferroresonant circuit, not a DC filter capacitor.[3]
AC drive motor capacitor (PN 4240681) Each internal diskette drive, clamped to the left side of the drive casting above the AC drive motor Item 1551. "High voltage may be present at the capacitor terminals."[5] The parts list calls it "CAPACITOR ASM" and gives a part number but no value.[6]

IBM's procedure for the drive motor capacitor:[5]

  1. Switch off the 5322 and disconnect the AC power cable.
  2. Remove the insulator caps from the capacitor terminals.
  3. Discharge the capacitor by shorting its terminals with an insulated screwdriver.
  4. Disconnect the motor leads, loosen the clamp screw and remove the capacitor.
  5. To refit, clamp the capacitor with its terminals facing the pulley side of the drive, connect motor leads 2 and 3 to the upper terminal and lead 1 to the lower terminal, and refit the insulator caps.

The AC drive motor procedure, item 1550, has the same DANGER notice and discharge step, because the motor leads come off the capacitor terminals.[7]

Everything else (card A1, the CPU planar, the storage cards, the display unit and the diskette drive control cards) is replaced as an assembly. IBM notes that display units from several manufacturers were used in the 5322 and that they are interchangeable, so a replacement unit may look different from the original.[8] The parts list for the 73 W US and Canada supply (PN 6838279) gives card A1 no part number ("for part number see card") and lists its fuses, fuse holder, switch, labels and hardware, but no capacitor.[9]

Checking the supply

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Measure at the power supply A1 card; IBM's tolerances apply there.[1]

IBM 5322 DC outputs: tolerance, ripple limit and fuse
Output Tolerance Ripple, peak to peak Fuse
+5 V +4.6 to +5.5 V 0.12 V F5, 10 A
-5 V -4.6 to -5.5 V 0.05 V F1, 0.5 A
+12 V +11.0 to +13.2 V 0.175 V F2, 1.5 A
-12 V -11.0 to -13.2 V 0.12 V F3, 0.5 A
+24 V +22.0 to +26.4 V 0.325 V F4, 1.5 A
Power good +2.4 to +5.5 V 0 -

The fuse ratings are the same on all three documented supplies.[10]

Ripple without an oscilloscope

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IBM's method uses the CE meter:[1]

  1. Put a capacitor of 0.22 µF or more, rated 50 V DC or higher, in series with one meter lead.
  2. Set the meter to the 2.5 V AC range.
  3. Connect one lead to the DC output being checked and the other to frame ground.
  4. If the pointer moves off zero, excess ripple may be present. Confirm against the peak-to-peak limits above.

Fuses and power good

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Each DC output has its own fuse, so a blown fuse shows which rail is loaded. IBM checks fuses for continuity on the ohms RX1 range after removing them with a fuse puller, and warns never to fit a fuse of a higher rating.[10] On the US and Canada supply the AC input is protected by fuse F6, 5 A slow-blow;[11] the GBG/I supplies have circuit breaker CB1 in its place.[2]

The power-good signal goes active when the -5, +5, -12 and +12 V outputs are within about 80 % of nominal, and an LED visible from the rear of the supply is lit while it is active.[2]

The power-on test checks the +24 V feed to the internal diskette drives in routine 39. IBM classes it as not critical, so a failure is shown in reverse video and pressing ERROR RESET lets the machine carry on.[12] A 39 in reverse video points to fuse F4 first, then to the +24 V output at the A1 card.

Finding a card that loads a rail

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The read/write storage card sockets carry +5 V (pin B15), -5 V (pin A10) and +12 V (pin A11) from the planar.[13] The planar board drawer slides out of the rear after the rear access cover and two screws are removed, giving access to the attachment cards and storage cards in the service position.[13] IBM's own repair method is card exchange,[14] so a rail that is pulled down or blows its fuse can be traced by removing the feature storage card and the I/O feature cards and trying again. The machine update card, which holds microprogram updates read from the Customer Support Function diskette, also plugs into an I/O card socket on the planar; jumper A3 on the planar marks it as fitted.[15] One owner found that this card was what made his System/23's power supply blow its 12 V fuse. A reply suggested checking it for a failed tantalum capacitor; the thread does not record what the fault turned out to be.[16]

Replacing capacitors

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With no IBM values to work from, take every value from the part being replaced.

  • Photograph both sides of the board before desoldering, and read the value, voltage and polarity off each part.
  • Fit the same capacitance with an equal or higher voltage rating.
  • Leave C6 and the drive motor capacitors alone unless one has failed. Both are AC parts in mains-derived circuits.
  • Check each output against IBM's tolerance and ripple limits before refitting the supply.
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References

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  1. ↑ 1.0 1.1 1.2 IBM, IBM 5322 Computer Service Manual, SY34-0171-0 (December 1980), item 1211, p. 2-11, "DC voltage distribution diagram" (hosted here).
  2. ↑ 2.0 2.1 2.2 IBM 5322 Computer Service Manual, SY34-0171-0, p. 2-57, "Power supplies".
  3. ↑ 3.0 3.1 IBM 5322 Computer Service Manual, SY34-0171-0, item 1210, pp. 2-8 to 2-10 (AC voltage distribution diagrams), and item 1240, p. 2-20 (layout diagram, PN 6838279).
  4. ↑ 4.0 4.1 IBM 5322 Computer Service Manual, SY34-0171-0, item 1240, p. 2-21 (power supply removal and replacement).
  5. ↑ 5.0 5.1 IBM 5322 Computer Service Manual, SY34-0171-0, item 1551, p. 5-40 (capacitor removal and replacement).
  6. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, item 1501, p. 5-6 (diskette drive locations, left side: "AC capacitor"), and Appendix A, Figure 7 parts list, p. A-27, index 16 (capacitor assembly, PN 4240681).
  7. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, item 1550, p. 5-38 (AC drive motor removal and replacement).
  8. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, item 1430, p. 4-8 (display unit replacement).
  9. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, Appendix A, Figure 10 parts list, p. A-33.
  10. ↑ 10.0 10.1 IBM 5322 Computer Service Manual, SY34-0171-0, items 1241 (p. 2-22), 1251 (p. 2-29) and 1261 (p. 2-35), F1-F5 removal and service check.
  11. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, item 1241, p. 2-23 (F6 removal, replacement and service check).
  12. ↑ IBM, Diagnostic User Guide 0001, System/23 with Diskette, PN 6841631, EC 327594 (April 1982), section 21, pp. 75-81 (hosted here).
  13. ↑ 13.0 13.1 IBM 5322 Computer Service Manual, SY34-0171-0, item 1230, pp. 2-16 to 2-19 (CPU planar board removal, diagram and connector pin assignments).
  14. ↑ IBM, Diagnostic User Guide 0001, PN 6841631, section 1.0, p. 1.
  15. ↑ IBM 5322 Computer Service Manual, SY34-0171-0, p. 2-46 (machine update microprograms) and p. 2-17 (planar jumper A3, "Machine update card installed").
  16. ↑ Dennis Boone, "S/23 machine update card", cctech mailing list, 18 August 2019, and the reply from Jon Elson the same day.


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