Macintosh Plus Troubleshooting
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Fault finding on the Macintosh Plus starts with Apple's symptom charts, which list cures in order of likelihood, one module at a time, and Apple asks the technician to refit the original module before trying the next cure.[1] Component-level entries come from The Dead Mac Scrolls, whose "Early Macs" chapter covers the analog (power/sweep) board and logic boards of the 128K, 512K, 512Ke and Plus by board location.[2]
The Plus has a CRT and a live analog board. Follow CRT Discharge Procedure before working inside. Apple says never to wear a grounding wrist strap until the CRT has been discharged, and to make live adjustments from the rear while watching the screen in a mirror.[1]
Before you start
[edit | edit source]Many faults on this machine come from the supply voltage, the cables between the boards, or both. Before replacing parts:
- Reseat the logic board cable, the yoke cable and the neck cable on the power/sweep board, and the floppy drive cable on the logic board.[1]
- Run Apple's voltage check with the 077-0135 test cable: +12 V between 11.90 and 12.75 V, +5 V between 4.85 and 5.15 V, measured at the external drive port. The procedure is on Macintosh Plus General Maintenance#Voltage check.[1]
- The voltage trimmer R56 ("Voltage") sets both rails together. Do not turn it up to cure a low +5 V reading until the cables are clean and seated; doing so pushes +12 V out of tolerance.[3]
No power, no start-up sound, or noises
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| No video, no start-up sound, no drive activity | Power cord, fuse, power/sweep board, logic board | Apple's order: connect and switch on; replace the power cord; check the fuse; replace the power/sweep board; replace the logic board, keeping the customer's SIMMs.[1] Recap-a-Mac also suggests checking the RIFA capacitors for cracks, the fuse, cracked solder joints on the connectors, and the resistors and diodes along the bottom of the analog board.[4] |
| Smoke or smell | Power/sweep board | Apple: replace the power/sweep board.[1] The usual culprit is the RIFA 0.1 µF mains filter capacitor in the supply, which cracks with age and fails with acrid smoke; replace it with a modern X2 part.[5] |
| Beeps continuously and keeps trying to power up | Voltage out of adjustment; internal drive; logic board | Apple's order: voltage adjustment; disconnect the internal drive and replace it if that cures the fault; replace the logic board; finally replace power/sweep board, logic board and internal drive together. Apple warns that replacing only the power/sweep board in this case may damage the new board.[1] |
| Clicking or chirping sound | Logic board cable; voltage; power/sweep board | Apple's order: connect the logic board cable; voltage adjustment; replace power/sweep board; replace logic board.[1] |
| No start-up sound, low "flup flup flup" noise, dark screen, right after service | Voltage set too high, or power/video cable not connected | Check both ends of the power/video cable. Turn R56 fully anticlockwise, restart, and set it for 5.0 V at pin 6 of the power/video cable.[6] |
| As above, out of the blue | Schottky rectifier CR20; leaky filter capacitors | With the power off, measure across CR20: about 31 Ω and 30 Ω is normal, 0 Ω both ways means it is shorted (fit an MBR1045). If CR20 is good, check C24 and C29 (2,200 µF) and C25, C26 and C31 (1,000 µF).[7] The Dead Mac Scrolls gives all five as 10 V parts, but Pina's own parts list for the U.S. board gives C25, C26 and C31 as 16 V (25 V acceptable) and C24 and C29 as 10 V or 16 V; fit 16 V or higher.[8] See Macintosh Plus Capacitor Replacement Guide. |
| "Bong ... flup, bong ... flup", no display | Rectifier CR29 | Replace CR29 (1N4001).[9] |
| Dead, groaning or screeching, or AC hum from the speaker | Flyback transformer T1 | With the power off, measure across CR5. Normal is 3.6 kΩ one way and open the other; 44-45 Ω or 11-12 Ω means T1 (Apple 157-0026-B) has failed.[10] |
| Dead, CR5 reading normal | Switching supply parts at the bottom of the analog board | Check R46, R47, R49, R50, Q9 (2N3906), Q10 (CR400Y), Q11 and the fuse FU1.[11] |
| High-pitched whine; ozone smell; herringbone pattern | Flyback transformer | Replace T1 with the later type 157-0042-C. With a burning smell as well, check the horizontal output transistor Q3 (BU406, heavy-duty BU406D preferred).[12] |
| Loud burp at shutdown | Opto-isolator U3 | Replace the 4N35 at U3.[13] |
Start-up sound but no or bad video
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| No video, start-up sound present, drive works | Contrast, cables, power/sweep board, neck cable, logic board | Apple's order: contrast control fully clockwise; check video cable connections; replace power/sweep board; replace neck cable; replace logic board.[1] Check the brightness control on the bottom front of the case first.[4] |
| Screen bright, sound present, no picture information | Power/sweep board or logic board | Apple: replace power/sweep board, then logic board.[1] The Dead Mac Scrolls gives the 74LS38 at U2 on the analog board for a very bright raster with visible scan lines and no desktop.[14] |
| Display blinks to black, white lines flash, tapping the case changes it | Cracked joint at J4 pin 1 (composite video) | Resolder J4 pin 1.[15] |
| Display collapses to a vertical line after a while | Cracked joints at J1 pin 4 and the width coil L2 | Resolder them; the joint may be under a foam pad.[16] |
| Display collapses to a horizontal line or rolls | Cracked joints at J1 pin 1, J4 pin 5, C5 | Resolder them.[16] |
| Bright vertical line or diagonal lines, then smoke; earlier poor linearity | Horizontal sweep capacitor C1 | Replace C1 (3.9 µF non-polarised) with a 100 V part and check the yoke plug J1 for burning.[17] |
| Bright white dot in the centre after service | Yoke cable disconnected | Reconnect it to J1.[18] |
| Loud sizzling, no display, after service | HV lead off the anode | Discharge the CRT and reconnect the flyback lead to the anode.[19] |
| Occasional bright flash | Cracked joint at J2 pin 1 | Resolder J2 pin 1.[20] |
| Top half squashed, white line, bottom black (or the reverse) | Vertical sweep | Top: R3 and Q1 (MPSU01). Bottom: R2 and Q2 (MPSU51).[21] |
| One half of the picture stretched off screen; Height control affects only the other half | Vertical op amp | Replace the LM324.[22] |
| Bottom of the picture shakes, or right side moves in and out, when the disk drive runs | Zener CR15; rectifier CR5; Q3 | Bottom: zener CR15 (1N5234B). Right side: barrel rectifier CR5 (a 400 V 5 A MR824 is the heavy-duty replacement), then Q3.[23] |
| Picture too small, too large, off centre or tilted | Adjustment | Apple sets the raster to 7 in (177.8 mm) wide with the width coil and 4.7 in (119.4 mm) high with the height control; centring and tilt are yoke adjustments.[1] The Dead Mac Scrolls gives 7.11 × 4.75 in (R55 Height, L2 Width).[24] |
For a machine with no start-up sound and no video whose analog board works in another Mac, The Dead Mac Scrolls points at the PALs on the Plus logic board, including the LAG PAL (342-0515-A) at E1 and the BMU2 PAL (342-0520-A) at E2.[25]
Sad Mac codes
[edit | edit source]The Plus shows a six-digit code under the Sad Mac and plays no error sound. The first two digits are the test class; see Sad Mac Error Codes and the Sad Mac Decoder for the full scheme.
| Code | Meaning | Action |
|---|---|---|
| 01xxxx | ROM test | Reseat or replace the ROMs at D6 and D8. After a ROM upgrade, codes such as 010601 or 01FE01 mean the two ROMs are from different versions.[26] |
| 030001 to 030080 | RAM, low byte | Check SIMMs 1 and 3.[27] |
| 030100 to 038000 | RAM, high byte | Check SIMMs 2 and 4.[27] |
| 0F000D, no start-up sound, after fitting a programmer's switch | Interrupt lever pressing the logic board's interrupt button | Reposition the switch; trim its shafts.[28] |
| 0F0002, 0F0003 or 0F000A near the end of start-up, only with an external hard disk connected | Bad partition map on the hard disk | Boot from floppy, then switch the drive on and repair the partition map with third-party formatting software.[29] |
Memory rules and the R8/R9 resistors are on Macintosh Plus Memory Upgrade Guide. If Apple's MacTest reports a 1 MB Plus as a 128K/512K, Apple replaces the logic board.[1]
Floppy drives
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Disk ejected, disk icon with blinking X | Disk, cable, drive, logic board | Apple's order: try another disk; replace the floppy cable; replace the drive; replace the logic board.[1] |
| Disk will not go all the way in | Drive has not finished an eject cycle | Push a straightened paper clip into the hole beside the slot, or hold the mouse button down while switching on; insert the disk metal end first with the write-protect tab underneath.[1] |
| Internal or external drive does not read disks | Disk, cable, Mylar RFI shield, drive, ROMs, logic board | Apple's order: another disk; floppy cable; Mylar RFI shield; the drive; verify the ROMs; the logic board.[1] |
| External 800K drives call every disk unreadable; internal drive fine | RC filter network at A11 | Replace the filter (Apple 115-0002) at A11.[30] |
| Start-up sound and video, drive does nothing | Cable, drive, logic board | Apple's order.[1] After service, check both ends of the drive cable.[31] |
| Drive runs continuously | Disk, cable, drive, logic board | Apple's order.[1] |
| 400K external drive will not boot | Rev. A ROMs with a drive whose stepper motor serial is F518 or higher | Upgrade to Rev. B ROMs, or replace the drive.[1] |
| Disks rub on the case opening | Drive not centred | Loosen and reposition the drive.[32] |
Drive cleaning and mechanism service are on Macintosh Floppy Drive Maintenance.
Keyboard, mouse, printer and network
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Pointer does not move | Mouse or logic board | Apple's order: connect, clean, replace the mouse; replace the logic board.[1] |
| Pointer stuck against one edge, or no pointer at all | Broken wire in the mouse cable | Brown wire: stuck right; orange: stuck left; green: stuck top; blue: stuck bottom; black and red: dead.[33] |
| Everything the pointer touches is selected with the button up | Mouse microswitch, or the A11 filter if every mouse does it | Replace the microswitch; if other mice behave the same, replace the filter at A11.[30][33] |
| No response from the keyboard | Keyboard cable, keyboard, logic board | Apple's order.[1] Intermittent keyboards: bent contacts in the RJ-11 jack. An M0110A that died after a telephone cable was used: replace its 8048.[34] |
| Keyboard erratic after a logic board swap | Ground clip touching solder points under the board | Apple fits the ground clip only to Rev. D and later boards; make sure it touches no solder points.[1] |
| ImageWriter or LaserWriter does not print | Chooser, system software, PRAM, cable, logic board | Apple's order: check the Chooser and Control Panel; use known-good system software; reset PRAM; replace the interface cable; replace the logic board.[1] |
| ImageWriter II on the printer port reset several times at start-up, first print fails; or the Plus does not appear on AppleTalk | 26LS30 line driver at B3 | The same printer works on the modem port. Replace the 26LS30 at B3.[35] |
| Freeze after a complex sound, pointer jumps first | 6522 VIA at D11 | Replace the 6522.[36] |
| No sound at all, otherwise normal, after service | Speaker plug at J3; RFI shield touching the logic board | Reconnect the speaker plug; insulate the top of the RFI shield with tape.[37] |
SCSI
[edit | edit source]The Plus uses an NCR 5380 SCSI controller on a DB-25 port. Apple removed the termination power line from that port, so a terminator or SCSI device that expects termination power from the host must get it elsewhere.[38]
A Plus with ROMs 342-0341-A or -B and 342-0342-A hangs at start-up unless an attached SCSI device is switched on first.[1] The Dead Mac Scrolls gives the cure as the version-3 ROMs, 342-0341-C and 342-0342-B, at D6 and D8.[26]
Programmer's switch
[edit | edit source]If the machine resets intermittently with the programmer's switch fitted, Apple's cure is to remove the switch and file it down by about 1/16 inch.[1] A switch whose reset lever touches the logic board's reset button gives no start-up sound and a raster of close horizontal lines with random dots.[28]
Things that are not faults
[edit | edit source]- A soft ticking from some 800K drives (part number MFD-51W-03) until a disk is inserted is characteristic of that drive.[39]
- The date and control panel settings are lost at power-off when the 4.5 V PRAM battery is flat or missing. The battery holder is on the power/sweep board; see Macintosh Plus General Maintenance#PRAM battery.
Related pages
[edit | edit source]- Macintosh Plus General Maintenance
- Macintosh Plus Capacitor Replacement Guide
- Macintosh Plus Memory Upgrade Guide
- Macintosh Plus Service Source
- Sad Mac Error Codes
- Macintosh Floppy Drive Maintenance
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 Apple Computer, Macintosh Plus Service Source, hosted as File:Macintosh Plus.pdf (see Macintosh Plus Service Source). Troubleshooting chapter, Symptom Charts pp. 2-11; Take Apart; Adjustments, Voltage pp. 1-7.
- ↑ Larry Pina, The Dead Mac Scrolls, Peachpit Press, 1992, ISBN 0-940235-25-0, chapter 1 "Early Macs", pp. 11-143. Hosted as File:The Dead Mac Scrolls 1992.pdf (see The Dead Mac Scrolls). Page numbers are given in each table row.
- ↑ Pina, The Dead Mac Scrolls, p. 27 (voltage set too high) and p. 68 (voltage too low), both at pin 6 of the power/video cable.
- ↑ 4.0 4.1 Macintosh Plus, Recap-a-Mac—link(accessed 2026-09-30)
- ↑ Vintage Computer Up in Smoke? Here's How to Fix It– Ben Chapman, Cracked the Code, 2020-09-14—link(accessed 2026-09-30)
- ↑ Pina, The Dead Mac Scrolls, pp. 26-27.
- ↑ Pina, The Dead Mac Scrolls, pp. 24-25.
- ↑ Larry Pina, Macintosh Repair & Upgrade Secrets (Hayden Books, 1990), Appendix C, "Parts List - Macintosh Analog Board - U.S. Version", pp. 327-328. Hosted as File:Macintosh Repair & Upgrade Secrets.pdf; see Macintosh Repair & Upgrade Secrets.
- ↑ Pina, The Dead Mac Scrolls, p. 29.
- ↑ Pina, The Dead Mac Scrolls, pp. 18-21.
- ↑ Pina, The Dead Mac Scrolls, pp. 22-23.
- ↑ Pina, The Dead Mac Scrolls, pp. 12-13 and 15-17.
- ↑ Pina, The Dead Mac Scrolls, p. 140.
- ↑ Pina, The Dead Mac Scrolls, p. 54.
- ↑ Pina, The Dead Mac Scrolls, pp. 41-43.
- ↑ 16.0 16.1 Pina, The Dead Mac Scrolls, pp. 48-49.
- ↑ Pina, The Dead Mac Scrolls, pp. 44-46.
- ↑ Pina, The Dead Mac Scrolls, p. 106.
- ↑ Pina, The Dead Mac Scrolls, p. 107.
- ↑ Pina, The Dead Mac Scrolls, p. 109.
- ↑ Pina, The Dead Mac Scrolls, pp. 55-56.
- ↑ Pina, The Dead Mac Scrolls, pp. 57-58.
- ↑ Pina, The Dead Mac Scrolls, pp. 36-38.
- ↑ Pina, The Dead Mac Scrolls, pp. 33-35, 101-104.
- ↑ Pina, The Dead Mac Scrolls, p. 143.
- ↑ 26.0 26.1 Pina, The Dead Mac Scrolls, pp. 112 and 114 (mismatched ROMs), pp. 80-81 (ROM locations D6 and D8).
- ↑ 27.0 27.1 Pina, The Dead Mac Scrolls, pp. 115-116.
- ↑ 28.0 28.1 Pina, The Dead Mac Scrolls, p. 60 (reset lever) and p. 118 (interrupt lever, 0F000D).
- ↑ Pina, The Dead Mac Scrolls, pp. 117, 120 and 121.
- ↑ 30.0 30.1 Pina, The Dead Mac Scrolls, p. 76 (external drive) and p. 83 (mouse).
- ↑ Pina, The Dead Mac Scrolls, p. 105.
- ↑ Pina, The Dead Mac Scrolls, p. 77.
- ↑ 33.0 33.1 Pina, The Dead Mac Scrolls, pp. 84-91.
- ↑ Pina, The Dead Mac Scrolls, pp. 93-94 and 97.
- ↑ Pina, The Dead Mac Scrolls, pp. 73-74.
- ↑ Pina, The Dead Mac Scrolls, p. 70.
- ↑ Pina, The Dead Mac Scrolls, pp. 110-111.
- ↑ Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition, Addison-Wesley, 1990, p. 19 (5380 in the Plus) and chapter 11, p. 378 (DB-25 without termination power on the Plus). Hosted as File:Apple Guide to the Macintosh Family Hardware 2nd Edition 1990.pdf.
- ↑ Pina, The Dead Mac Scrolls, p. 141.