Macintosh Plus Power Supply Repair

The power supply section of a Macintosh Plus U.S. analog board. Photo: Cody Logan, CC BY-SA 4.0, via Wikimedia Commons.

The Macintosh Plus has no separate power supply: a switching supply occupies the lower part of the analog board, beside the CRT circuits. Thomas Lee calls it "another Achilles heel" of these machines, under-designed for the demands a switcher places on its filter capacitors and rectifiers and made worse by the lack of a fan. It fails in two ways. A dead set does nothing at all when switched on. A "flupping" set makes a repeated "flup, flup, flup" as the overvoltage crowbar shorts the supply and it tries to restart.

The Macintosh 128K, Macintosh 512K and Macintosh 512Ke use the same supply. Designators below are for the U.S. (120 V) board; the International board's switching section has more parts and different references, and Pina gives the International equivalent where he knows it.

Symptoms

Pina's four cases (Macintosh Repair & Upgrade Secrets, p. 109)

Case	Suspects
Dead set, video area looks undamaged	Q11 (International Q10), R52 (International R55), Q8, CR21, the power switch SW1, the line cord and the outlet
Dead set, video area burned	C1 and T1; inspect J1 and L2; Q3 (International Q2); test R46-R51 (International R48-R55) and Q9-Q10 (International Q11-Q12)
Flupping set, logic board good	The harness connections; the setting of R56 (International R59); capacitors C3, C24-C26 and C29-C31 (International C3, C24, C27, C28, C30-C32); rectifiers CR1, CR5, CR20 and CR21; the crowbar SCR Q8
Flupping set, analog board good	The TSM chip and the 7905 regulator on the logic board

The Dead Mac Scrolls adds these cases:

• Flupping and a dark screen right after service: the power/video harness is unplugged at one end, or its blue +5 V wire is broken; or R56 is set too high.
• Flupping out of the blue: CR20 shorted; if CR20 is good, the filter capacitors C24 and C29 (2200 µF) and C25, C26 and C31 (1000 µF).
• "Bong ... flup, bong ... flup" and no picture: rectifier CR29 (1N4001).
• A 512Ke with a memory upgrade that flups on a hot restart but starts cold: CR21.
• Completely dead with a normal reading across CR5: CR21, then R46, R47, R49, R50, Q9, Q10, Q11 and the fuse.

If the machine is dead and also shows signs of a failed flyback, or reading across CR5 is about 11-12 Ω or 44-45 Ω, start with Macintosh Plus Flyback Transformer Replacement. If the machine starts but resets or loops, see Macintosh Plus Low Voltage Repair.

How the supply protects itself

The +12 V output is the only one regulated directly; the others follow it. If +12 V rises about 15% too high, the 12 A crowbar SCR Q8 short-circuits it, the supply goes into current limiting, and it keeps trying to restart, giving the "whup-whup" or "flup" noise. A shorted rectifier or a failed filter capacitor draws enough current to have the same effect. If the overvoltage goes on long enough, Q8 can fail open, and a set that was flupping becomes dead.

Safety

Warning: the primary side of this supply works at mains potential, with its reservoir capacitors charged to about 170 V DC on the U.S. board, rectified from the 120 V line. Lee describes a bleed resistor that discharges them at switch-off, but measure across them before touching the primary side. Everything between the mains inlet and the power switch is live while the cord is connected, even with the switch off. All the tests below except the capacitor test are made with the cord unplugged. The CRT anode runs at about 13 kV; see CRT Discharge Procedure.

Tools and parts

• T15 Torx driver, case-cracking tool, inspection lamp and magnifying glass, soldering iron, desoldering pump.
• An analog multimeter for Pina's transistor and SCR tables, which use the R×100 and R×10K ranges; a digital meter on its kΩ range for resistors and rectifiers.
• Replacement parts as found. Pina recommends heavier parts than the originals:

Supply parts and Pina's replacements (U.S. board)

Position	Original	Pina's replacement
R52	10 kΩ ¼ W carbon film	10 kΩ ½ W carbon film
R46	22 Ω 1 W metal oxide	22 Ω 2 W metal oxide
R47, R49, R50	1.5 Ω ¼ W carbon film	1.5 Ω ½ W carbon film
R51	15 Ω ¼ W carbon film	15 Ω ½ W carbon film
CR1, CR5	GI854 barrel rectifier, 600 V 3 A	MR824, 400 V 5 A
CR20	IR31DQ Schottky, 40 V, or MBR1035 35 V 10 A	MBR1045, 45 V 10 A, TO-220
CR21	IR31DQ Schottky, 40 V	MBR1045, 45 V 10 A, TO-220
Q8	2N6394 or S1010BH SCR	Same
Q9	2N3906 or 2N4403 PNP	Same
Q10	E0122, E0121, E0102, SD8335 or CR400Y	Same
Q11	2SC2827, 2SC2810 or 2SC3039 NPN, 12 A 100 W, TO-220	Same

The Dead Mac Scrolls gives an MBR360 for CR21. Lee gives MBR1045 for CR20 and the fast-recovery MUR410 (4 A, 100 V, 35 ns) for CR21, and notes that a 2N6394 at Q8 is a 50 V part where some boards have a 100 V 2N6395, and that either works. Capacitor values and replacements are on the Macintosh Plus Capacitor Replacement Guide; on the International board R55 is an odd 20 kΩ value, and Pina accepts 22 kΩ in its place.

Procedure

Opening up

1. Switch off, unplug the cord and remove the rear housing.
2. Before anything else on a flupping set, check that both ends of the analog-to-logic-board harness are plugged in, and turn R56 down; it may have been left too high after a memory upgrade was removed. Pina and Lee disagree on which way lowers it: Pina turns R56 fully anticlockwise, Lee fully clockwise viewed from the insulator side. Check the direction against a meter on a working set before relying on either.

1. Look first

The supply suspects as seen through the solder side of an illuminated U.S. board: SW1, LF1, LF2, F1, CR21, C35, C36, T3, R51, R52, Q8, Q9, Q10, Q11 and U3 (Pina, Figure 6-1).

1. Look at the vinyl jacket before anything else. A brown, cigarette-like burn on it points at the part beneath: in Pina's example it lay over Q11, the 100 W switching transistor near T3, which had been overheating.
2. Look over the component side with a 2.5 in magnifying glass. Overheated resistors often show it only between the body and the board.
3. In a set where T1 shorted while left running, expect a burned video area and open resistors in the supply: Pina's example had lost R46, R47, R49 and R51, with Q9 and Q10 failed as well.

2. Resistors

1. With the cord unplugged, set the lamp to shine through the board and find each resistor's joints on the solder side.
2. Measure each in circuit and compare with Pina's readings:

In-circuit resistor readings, U.S. board (Pina, pp. 114-115)

Resistor	Value	Typical in-circuit reading
R52	10 kΩ ¼ W	about 10 kΩ
R46	22 Ω 1 W	18-20 Ω
R47, R49, R50	1.5 Ω ¼ W	about 1 Ω
R51	15 Ω ¼ W	about 12 Ω

3. An infinite or much higher reading means the resistor has failed. If the set quit during hot weather, go to R52 first; it sits in the lower right corner of the board under LF2.
4. Replace failed resistors with the higher-wattage parts in the table above.

3. Rectifiers

Approximate positions of CR1, CR5, CR20 and CR21 seen through the solder side, with typical in-circuit readings (Pina, Figure 6-6).

1. With the cord unplugged and a digital meter on its kΩ range (an analog meter on R×10K), measure across CR1 and CR5 in one direction, then the other.
2. Switch an analog meter to R×10 and measure CR20 and CR21 the same way.
3. Compare with Pina's readings. A dead short, 0 Ω both ways, is what you are looking for.

In-circuit rectifier readings (Pina, Table 6-2)

Rectifier	Typical good	Bad
CR1	3.6 kΩ / infinity	0 Ω / 0 Ω
CR5	3.6 kΩ / infinity	0 Ω / 0 Ω
CR20	31 Ω / 30 Ω	0 Ω / 0 Ω
CR21	52 Ω / 53 Ω	0 Ω / 0 Ω

4. Remove any rectifier that reads short and test it out of circuit: a good one reads 2 MΩ to infinity one way and about 10-20 kΩ the other. If it tests good out of circuit, the short is elsewhere on the board, usually close by.
5. CR20 (the +5 V rectifier) fails most often, but CR21, CR5 and CR1 also fail. Fit the MBR1045 as a screw-mounted TO-220 part on the existing heat sink, which must be kept: Macintosh heat sinks are hard to find.
6. If CR21, the +12 V rectifier, was shorted, test Q8 as well; it may have failed open.

Original barrel rectifier soldered at CR20 (left) and the MBR1045 upgrade screwed to the heat sink (right), cathode and anode marked K and A (Pina, Figure 7-1).

4. Crowbar SCR Q8

The crowbar: Q8 shorts the +12 V output (J4 pin 10) to ground when CR18, R32 and R33 sense an overvoltage (Pina, Figure 6-7).

1. Lay the machine with the analog board's solder side up and desolder Q8's three leads.
2. With an analog meter on R×100, test the junctions. The tab is common to the anode.

Q8 (2N6394) out of circuit (Pina, Tables 6-3 and 6-4)

Junction	Good: red/black	Good: black/red	Bad: red/black	Bad: black/red
Anode to gate	Infinity	Infinity	Infinity	Infinity
Anode to cathode	Infinity	Infinity	Infinity	Infinity
Cathode to gate	50 Ω	50 Ω	475 kΩ	Infinity

3. Replace Q8 if it fails. Like CR20 and CR21, it is mounted on a heat sink.

5. Q11 and the switching transistors

1. Test Q11 out of circuit in the same way as the horizontal output transistor; Pina refers to the same tables (see Macintosh Plus Flyback Transformer Replacement#Testing and replacing Q3).
2. Check Q9 (2N3906) and Q10 (the small SCR) whenever R46-R51 have burned.

6. Filter capacitors

1. Look for capacitors that have visibly failed. When none shows anything, Pina measures leakage with the set running: the meter's black lead on the chassis and the red probe on each capacitor's negative joint. New capacitors read about −20 mV; a reading beyond −100 mV marks one close to failure, and he then replaces every capacitor on the list.
2. Replace the capacitors as described in Macintosh Plus Capacitor Replacement Guide.

7. If the analog board is good

1. Swap in a known-good logic board. Pina notes that 128K, 512K, 512Ke and Plus logic boards are interchangeable for this test. If the flupping continues, the fault is on the analog board; if it stops, it is on the logic board.
2. Pina's suspects on the logic board are the TSM chip at D1 and, less often, the 7905CT −5 V regulator Q1 at A8. He tests the 7905CT out of circuit on a digital meter's kΩ range; a new one read 10.6 kΩ both ways between output and common.

Reassembly

1. Refit the vinyl jacket and the rear housing.

Testing

• Switch on. The start-up sound should come with no flup, followed by a normal picture.
• Set +5 V to 5.00 V and realign the video as described in Macintosh Plus Video Adjustment. Pina says to reset the voltage and realign the video after any supply work.
• Run the machine for an extended period, and again from cold, to make sure it does not flup on a cold start.

Related pages

• Macintosh Plus
• Macintosh 128K, Macintosh 512K and Macintosh 512Ke
• Macintosh Plus Low Voltage Repair
• Macintosh Plus Flyback Transformer Replacement
• Macintosh Plus Capacitor Replacement Guide
• Macintosh Plus Troubleshooting
• Macintosh Repair & Upgrade Secrets