Macintosh Plus Low Voltage Repair

The power supply section of a Macintosh Plus analog board, component side. The multicoloured harness to the logic board runs down the left of the picture. Photo: Cody Logan, CC BY-SA 4.0, via Wikimedia Commons.

A Macintosh Plus whose +5 V supply is low resets at random, falls into a start-up loop, shows a shrunken picture or gives repeated system errors. Pina finds two causes when the voltage control R56 cannot put it right: a failing zener diode in the regulator, and resistance in the harness between the analog board and the logic board. Lee finds the harness contacts at J4 responsible in nine cases out of ten, and warns against raising R56 to compensate.

The Macintosh 128K, Macintosh 512K and Macintosh 512Ke use the same analog board circuit. Pina's references below are for the U.S. (120 V) board, with the International board's zeners noted where they differ.

Symptoms

• The machine resets from time to time, then begins to loop: start-up sound, reset, start-up sound.
• R56 will not bring +5 V up to 5.00 V; or it reaches 5.00 V but the supply cannot be pushed to its crowbar level.
• The adjustment is right one day and as much as 0.10 V low the next.
• Below about 4.95 V Pina expects a badly shrunken picture and system errors.

A machine that is completely dead, or "flups" at switch-on, has a different fault; see Macintosh Plus Power Supply Repair. The adjustment itself is described in Macintosh Plus Video Adjustment and Macintosh Plus General Maintenance#Voltage check.

How the regulation works

R56 sets the +12 V output directly; the +5 V and −12 V outputs follow it through the transformer turns ratios. The control circuit compares a divided-down +12 V with a 6.2 V reference from the zener diode CR19. CR18, a 12 V 5 W zener, is the overvoltage reference that fires the crowbar SCR Q8. Because only +12 V is regulated, a voltage drop in the +5 V wire to the logic board is never corrected. Turning R56 up to cure it raises +12 V as well, possibly out of specification.

Safety

Warning: the zener readings below are taken on a running machine with the cover off, with mains voltage on the analog board and the CRT at about 13 kV. Touch only the meter probe to the solder side, keep the other hand away, and unplug the cord before any soldering or before handling the harness. Pina wears goggles when pulling the harness, because a slipping tool can strike the glass CRT. See CRT Discharge Procedure.

Tools and parts

• T15 Torx driver, case-cracking tool, multimeter, inspection lamp.
• Soldering iron, solder and desoldering pump; curved 6 in hemostats help to place CR19.
• Contact cleaner: Pina uses liquid metal polish on a cotton swab and then a contact stabilant; Lee uses a pink pencil eraser or a cleaner such as Cramolin.

Regulator zeners (Pina, chapter 4 and Appendices C and D)

Board	Position	Part	Rating
U.S.	CR18	1N5927 (1N5349B accepted)	12 V 5 W zener
U.S.	CR19	1N5234B	6.2 V ½ W zener
International	CR15, CR17, CR18	1N5234B	6.2 V ½ W zener

• For the harness: Pina's upgrade is a heavy-duty 18 AWG J4-J7 harness, the same length and colours as the original. Original harnesses were 20 AWG or, on some older sets, 22 AWG; he says the 22 AWG type should not be used with memory upgrades.

Procedure

Opening up

1. Switch off, unplug and remove the rear housing.
2. Measure +5 V at socket 6 of the logic board connector, or at pin 6 of the external drive port, and try to set it to 5.00 V with R56 as described in Macintosh Plus Video Adjustment#1. Supply voltage.

Harness and J4 contacts

Do this first. It is the commonest cause and costs nothing.

1. Unplug the power cord and put on goggles.
2. Pull the harness off the analog board connector J4 by grasping the wires and rocking it to release the locks, pulling straight off. Pina does not use pliers on these connectors: a slip can drive the tool into the CRT. The harness is tie-wrapped and strong enough to be pulled by its wires.
3. Look at the board's connector pins. Bright, shiny pins are good; a uniform dull grey means tarnish.
4. Clean the pins. Pina polishes them with liquid silver polish on a cotton swab, then treats them with a contact stabilant; he does not try to clean the sockets in the cable's own connectors, which are too small. Lee rubs the pins with a pink pencil eraser, clears every bit of residue, and then plugs and unplugs the harness several times to wipe the sockets.
5. Lee suggests resoldering the J4 joints on the board as well, as for J1.
6. Refit the harness, or fit an 18 AWG replacement. Pina's measurements show what the gauge costs (Table 4-2):

+5 V lost across the harness (Pina, Table 4-2)

Harness	Analog board end	Logic board end (J7)	Drop
22 AWG	5.10 V	5.00 V	0.10 V
20 AWG	5.03 V	5.00 V	0.03 V
18 AWG	5.01 V	5.00 V	under 0.01 V

7. To measure the drop on your own harness, set +5 V to 5.00 V at socket 6 of J7, then measure the middle (+5 V) wire at the analog board end. Pina's text calls this end J3; his connector table names the analog board end J4.
8. Reset +5 V to 5.00 V at the logic board end.

J4 (analog board) to J7 (logic board) harness (Pina, Table 5-7)

Pin	Wire	Function
1	White	Composite video
2	None	Key
3	Orange	Horizontal sync
4	Yellow	Internal speaker
5	Green	Vertical sync
6	Blue	+5 V
7	Violet	Ground
8	Red	−12 V
9	Orange	Ground
10	Yellow	+12 V
11	Green	+4.5 V battery

Testing the zener diodes

Approximate positions of CR18 (left) and CR19 (lower right), seen from the solder side of the U.S. board, with Pina's readings marked at the test points (Figure 4-8).

1. Set the inspection lamp to shine through the board from the component side, and find the joints of CR18 and CR19 on the solder side (Pina's Figure 4-8).
2. Clip the meter's black lead to the chassis, set it to DC volts, plug in and switch on.
3. Touch the red probe to the upper joint of each diode. Pina's typical readings on a good supply are +12.25 V at CR18 and +6.09 V at CR19.
4. Read the results. If the supply cannot get above about 4.80 V, replace CR18. If it reaches 5.00 V but cannot be pushed to the crowbar level, CR19 is the suspect; leave CR18 alone. CR19 often reads about 5.86 V, inside its tolerance but too low for the supply to deliver full voltage.
5. On the International board, the suspects are CR15, CR17 and CR18, all 1N5234B.

Pina notes that the parts are so cheap they can be replaced on suspicion, without testing.

Replacing a zener

Pina's general procedure for replacing any analog board part, using CR19 as his example:

1. Switch off and unplug the cord. Lay the machine down with the solder side of the analog board up.
2. Heat the joint and, as the solder melts, lever up any bent-over lead with the iron tip; then clear the hole with the desoldering pump held perpendicular to the board. Empty the pump into a can, never onto the bench.
3. Check that the leads move freely, then pull the old diode out.
4. Stand the machine up. Bend the new diode's leads to match the old one. Fit it with the striped end in the same place; on the U.S. board the stripe of CR18 and CR19 goes to the top. If in doubt, check the symbol printed on the component side.
5. CR19 is awkward to reach: insert it from between the disk drive and the CRT with curved hemostats, or loosen the board's screws and slide the board back a little.
6. Bend the leads about 45° on the solder side so the part stays put; refit the board screws and any harness you unplugged.
7. Lay the machine down again and solder each lead, touching the iron to one side of the lead and the solder to the other, just long enough for the solder to flow round the joint.
8. Clip the leads close to the joints and pick up the offcuts.

Measuring +5 V at socket 6 of the logic board connector (Pina, Figure 3-5).

Testing

1. Plug in, switch on and measure across the new diode: a new CR19 typically reads 6.01-6.09 V.
2. Measure +5 V at socket 6 of J7 and adjust R56. Pina checks that the supply now reaches the crowbar point (5.35-5.40 V, with a bong and a "flup, flup, flup"), backs off at once, and sets 5.00-5.01 V. Do not let the overvoltage continue.
3. Check +12 V: Apple's limits are 11.90-12.75 V with +5 V at 4.85-5.15 V.
4. Refit the vinyl jacket, then readjust height and width, which change with the supply.
5. Run the machine for some hours; the resets or loop should not return.

Related pages

• Macintosh Plus
• Macintosh 128K, Macintosh 512K and Macintosh 512Ke
• Macintosh Plus Video Adjustment
• Macintosh Plus Power Supply Repair
• Macintosh Plus General Maintenance
• Macintosh Plus Troubleshooting
• Macintosh Repair & Upgrade Secrets