Twentieth Anniversary Macintosh Troubleshooting Guide

A working Twentieth Anniversary Macintosh with the green power-on light lit (photo: stiefkind, CC0)

Apple published no troubleshooting charts for the Twentieth Anniversary Macintosh (TAM): its Service Source volume says that any repair needing the enclosure screws removed went to a central repair centre, and reproduces the owner's manual in place of the usual service chapters. Chapter 12 of that manual lists the symptoms Apple expected owners to meet and the first checks for each. Owners and restorers have since traced the umbilical cable and recapped the boards.

Preliminary checks

Power-up symptoms

Symptom	Probable cause	Action
Nothing on screen, power-on light on the front is off	No mains to the bass unit, or the umbilical not connected	Look at the green status light in the well under the bass unit. If it is off, try another outlet and check any power strip. If it is on, check that the computer's cable is coupled to the bass unit's cable.
Power-on light on, screen dark	Sleep, a screen saver, or the brightness turned down	Press a key or the Power key; check the brightness button at the left under the screen.
Four tones at startup	Hard disk problem	Start up from the system software CD (hold C), test the disk with Drive Setup, and reinstall system software if needed.
Sad Mac icon	System software or hardware fault	Hold the trackpad button at power-on to eject any floppy, then try starting from the CD. If the Sad Mac returns, Apple refers the machine for service; see Sad Mac Error Codes.
Disk with a flashing question mark for more than 15 seconds	External SCSI conflict, or no usable system on the hard disk	Shut down, disconnect the first external SCSI device and restart; then test the hard disk.
Keyboard or trackpad dead	Cable loose	Shut down with the power button on the back, check the cables, then restart. Never connect them with the power on.

Bass unit and power supply

Power supply board from the bass unit, with fuse F1 at lower left and the two 200 V reservoir capacitors (1 and 2) at upper left (MacDat)

The bass unit holds the power supply board, the Bose amplifier board and the subwoofer. Apple's first checks for a dead machine are the green status light in the well under the bass unit, the outlet, and free air flow round the bass unit. Then check the voltage switch in the same well: "115" for 100–130 V, "230" for 200–270 V.

Warning: the supply board carries mains voltage. Its reservoir is a pair of 470 µF 200 V capacitors, and the board is marked with a warning that heatsinks HS1 and HS2 are live. Unplug the bass unit, discharge both capacitors through a resistor and confirm with a meter before touching the board. Fuse F1 on the board in MacDat's photograph is marked F6.3AH 250V.

Rails and connectors

Apple gives the supply's maximum currents but no voltage tolerances.

DC outputs (Apple)

Output	Maximum current
+5 V	8.5 A
+5 V trickle	0.1 A
+3.3 V	5 A
+12 V	2.3 A
−12 V	0.25 A

The total output may not exceed 98 W, with no more than 10.7 A on +5 V and +3.3 V together.

J2 (audio) and J16 (power) on the main unit board where the umbilical lands (Stephen Arsenault, CC BY 4.0)

Stephen Arsenault traced the umbilical cable. Inside the main unit it ends at two board connectors: J16 carries the power and J2 carries the sense lines, the −12 V and trickle supplies and the audio.

J16 power connector (Arsenault)

Pin	Signal	Pin	Signal
1	+12 V	5	+12 V
2	+5 V	6	Ground
3	+5 V	7	Ground
4	+5 V	8	Ground

J2 audio connector (Arsenault)

Pin	Signal	Pin	Signal
1	+5 V (sense)	9	Left speaker return
2	+5 V trickle	10	Right speaker
3	Ground (sense)	11	Right speaker return
4	PFW	12	Amplifier return
5	+12 V (sense)	13	Amplifier left
6	−12 V	14	Amplifier right
7	Amplifier OE	15	Not connected
8	Left speaker	16	Not connected

Pin numbers on the 23-pin umbilical plug (Stephen Arsenault, CC BY 4.0)

23-pin umbilical plug (Arsenault)

Plug pin	Board pin	Signal	Plug pin	Board pin	Signal
1	J2-7	Amplifier OE	13	J16-2	+5 V
2	–	–	14	J16-6	Ground
3	J2-14	Amplifier right	15	J2-3	Ground (sense)
4	–	–	16	–	–
5	J2-10	Right speaker	17	J2-1	+5 V (sense)
6	J2-9	Speaker return	18	J16-3	+5 V
7	J2-8	Left speaker	19	J16-7	Ground
8	J2-12	Amplifier return	20	–	Ground
9	J2-13	Amplifier left	21	J16-8	Ground
10	J2-4	PFW	22	J16-1	+12 V
11	J2-6	−12 V	23	J2-5	+12 V (sense)
12	J2-2	+5 V trickle

The plug's outer ring and braided sheath are also grounded. Arsenault suggests that a 12 V and 5 V supply such as a PicoPSU connected to J16 will run a TAM whose bass unit is missing. J2 carries the −12 V and +5 V trickle lines and all the audio, so a machine run that way has none of those.

Sound faults

The "Bose buzz"

Owners describe a buzz through the speakers that, in one restoration, made the machine unusable after 10–15 minutes. Restorers attribute it to corrosion on the uncoated spade connectors fitted to the subwoofer at the factory and on the pin headers of the Bose amplifier board. Tinning the leads or rubbing oxide off the pins with a pencil eraser helps, but not always for good. Replacing both spade connectors and both pin headers with coated parts had held for eight to nine months when the owner of that machine reported on it.

Opinions differ. One owner found that cleaning the power contacts inside the subwoofer fixed the buzz only for a while, another suspected the umbilical contacts, and an original 1997 owner reports the buzz on brand-new machines that swapping the bass unit and then the main unit did not cure. The same discussion holds that the capacitors are not the cause. The mac27 base unit steps 1–9 reach the Bose board and the spade connectors without taking out the power supply.

Other sound problems

• No bass: the level control in the well under the bass unit sets the subwoofer level, and turned fully down it silences the bass.
• No sound from the speakers with nothing connected: make sure nothing is plugged into the sound output port.
• Rattling: check that the speaker grille covers either side of the screen are fastened. Regular or melodic noise can be interference from nearby electrical equipment.

Display faults

Apple's first checks for a dark screen are sleep, a screen saver and the brightness button. The backlight inverter is a separate board in the rear housing, and its pink and white cable disconnects on the right side of the front assembly. The flat orange display data cable disconnects near the top on the left. Check both connections when the front is off.

The LCD panel is mounted on the front assembly, which comes off at step 9 of the iFixit teardown. One owner replaced a half-working panel and found a Samsung LT121S4 in their TAM and the same part number in a sealed Apple service replacement panel. Low End Mac says the screen is shared with the PowerBook 3400c, and owners name the PowerBook 3400 and the first PowerBook G3 as donors.

CD-ROM drive

CD-ROM drive symptoms

Symptom	Probable cause	Action
Door will not open	Disc still mounted by the system	Drag the disc icon to the Trash. If no icon appears, restart and press the Stop/Eject button just after the startup sound. If that fails, push a straightened large paper clip horizontally into the small pinhole on the left side of the computer until the door releases. Do not force the door.
Door arm or hinges broken	Fragile plastic; knocked door	Glue the arm (one owner's repair). During disassembly, avoid knocking the black latch bar behind the front fascia, which pops the door open.
Disc ejected with no message	Disc off centre, dirty or damaged	Seat the disc on the spindle label side out, clean it, and try it in another drive.
Drive not seen	SCSI ID clash, or a loose cable	The drive is SCSI ID 3; give external devices other IDs. Inside, check the 50-pin SCSI ribbon, power and analogue audio cables at the back of the drive.
Drive misbehaves with the screen tilted	Angle outside Apple's limits	Keep the screen within 15° back and 5° forward of vertical.
Lock-up while reading a CD	One owner saw this with an SD card adapter in place of the hard disk	Try a CompactFlash card on a passive IDE adapter.

Startup, PRAM and clock

Apple's procedure for resetting PRAM and NVRAM on the TAM:

1. Shut down with the Power key and Shut Down. Make sure Caps Lock is off.
2. Press the Power key, and straight after the startup sound hold Command, Option, P and R.
3. When the startup sound has played twice, release the keys and hold Shift until "Extensions off" appears.
4. If the computer switches itself off, press the Power key again.
5. Drag the Display Preferences file from the Preferences folder in the System Folder to the Trash.
6. Restart normally and reset any control panel settings.

The logic board also has a Cuda reset button; an iFixit answer places it "just to the right of the second RAM slot up from the bottom". A clock that loses time means a flat clock battery; see the general maintenance page.

When a TAM stayed dark with its disk spinning after a Sonnet Crescendo G3/L2 card was fitted, another owner's answer was to start up with the card disabled in the Power Macintosh Card control panel, install Sonnet's software, and then enable the card again.

Keyboard, trackpad and remote control

• Pointer does not move: shut down, check the trackpad and keyboard cables, then restart. Clean the trackpad. If another ADB pointing device works, the trackpad is faulty.
• All ADB devices and the serial devices share 500 mA. The keyboard draws 25–80 mA and the trackpad up to 10 mA, and the keyboard has a second ADB port on its back.
• Remote control dead: replace its battery, move closer and aim it at the receiver on the front, and check that Disable Remote Control is not selected in Apple Video Player.

Things that are not faults

• A CD with a heavy label can wobble and make the computer vibrate. Apple says this does no harm.
• Sound from the sound input continues to play while the machine is asleep, and the volume cannot be changed until it wakes.
• The fans in the main unit and the bass unit can be heard in a quiet room.

Related pages

• Twentieth Anniversary Macintosh
• Twentieth Anniversary Macintosh General Maintenance
• Twentieth Anniversary Macintosh Capacitor Replacement Guide
• Capacitor Failure Symptoms
• Hard Drive Maintenance and Repair
• Floppy Drive Repair