Macintosh Plus Flyback Transformer Replacement

Three generations of compact Macintosh flyback transformer: a black type from an original 128K (left), a grey and white type from an early Plus (centre) and the solid grey bleeder type later used in the SE (right). Pina, Figure 5-10.

The flyback transformer T1 on the Macintosh Plus analog board turns about 12 V into the 13 kV that drives the CRT. Thomas Lee calls it "one famously weak spot" of the compact Macs: undersized to begin with and cooked by the lack of a fan, it usually fails by developing shorted turns. When it shorts it draws heavy current through the horizontal output transistor (Q3 on the U.S. board, Q2 on the International board), so the two are diagnosed and often replaced together.

The earlier Macintosh 128K, Macintosh 512K and Macintosh 512Ke use the same circuit, and Pina's flyback table lists parts from all four. The SE family uses a different board layout and the designators T2 and Q2.

Symptoms

Flyback and horizontal output faults (The Dead Mac Scrolls)

Symptom	Pina's diagnosis	Page
Herringbone pattern on the display at start-up; otherwise usable	Replace T1 (157-0026-B) with the later 157-0042-C	12
Ozone smell, or an occasional high-pitched whine; otherwise usable	Replace T1 with the later type	13, 15
Normal start-up sound, dark or badly distorted display, high-pitched whine, then a burning smell	Replace T1 and check Q3 (BU406), which may be burned; a heavy-duty BU406D is preferred	16-17
No start-up sound, dark display, AC hum from the speaker, the internal drive vibrating; or a loud screech; or a groan	Measure across CR5 with the power off: 44-45 Ω instead of 3.6 kΩ and open means T1 has failed	18-20
Completely dead, no noises	Measure across CR5: 11-12 Ω instead of 3.6 kΩ and open means T1 has failed; a normal reading points to the power supply	21-23
After a flyback replacement, the right side of the picture wiggles like a worm	Q3 was left in place with signs of overheating; replace it	30

Pina treats jitter, wavy lines ("worms"), ringing and an ozone smell as signs that the flyback is about to fail, and says to shut down at once: when it shorts, the high current can damage Q3, C1, J1 and L2. A short whistle at switch-on that then stops is the width coil L2, not the flyback. A picture that collapses to a vertical line is usually C1, J1 or L2; see Macintosh Plus Horizontal Sweep Repair. A machine that is dead or "flups" with a normal CR5 reading has a power supply fault; see Macintosh Plus Power Supply Repair.

Signs on the part itself

Flyback transformers with deformed cases, a sign of advanced failure (Pina, Figure 5-13).

• Bubbled paper labels look alarming but are normal.
• With a ringing noise, the epoxy seals at the top and bottom of the transformer may be broken. Pina does not recommend regluing them.
• In advanced failure the case shows bubbles or stripes that should not be there.
• Once the part is out, burning at pin 4 is common; with an ozone smell the resin under the transformer may have bubbled.
• A burned board around Q3, or a heat sink turned reddish blue, means Q3 has been overheated by a shorted flyback.

Safety

Warning: this job means removing the CRT anode lead. Unplug the power cord, then discharge the CRT before touching the anode cap; see CRT Discharge Procedure. Pina discharges through a 10 MΩ resistor built into an insulated probe and warns that a direct discharge with the cord connected can give a large spark that destroys the LAG chip on the logic board and the 74LS38 (U2) on the analog board. Never wear a grounding wrist strap until the CRT has been discharged.

Tools and parts

• T15 Torx driver with a long shaft, case-cracking tool, CRT discharge tool.
• Soldering iron, solder and a vacuum desoldering pump.
• A multimeter. Pina's transistor tests use an analog meter on its R×100 range.
• Heat-sink compound, a 6-32 × ¼ in machine screw and nut, and a drill with a ⅛ in bit if Q3 is riveted to its heat sink.
• A replacement flyback (below) and, if needed, a BU406 for Q3.

Flyback transformers fitted at T1 (Pina, Table 5-4 and Appendices C and D)

Maker	Marking	Fitted to	Bleeder resistor
ZFAT	AC-058	128K or early 512K
Taiwan Totoku Electric Co.	157-0026-B	128K or early 512K
Tai-Ho, Taiwan	TH-1565	Late 512K, 512Ke or early Plus
Monitron, Taiwan	157-0026-B	Mid-production Plus
Monitron, Taiwan	157-0026-C	Late Plus	Yes
Tai-Ho, Taiwan	TH-1565C	SE	Yes
Lifon, Taiwan	157-0042-B	SE, or a repaired board	Yes

Pina recommends the later SE-style bleeder type as the replacement: it is UL-approved, physically larger, and its built-in bleeder resistor discharges the CRT at power-down. The Dead Mac Scrolls names 157-0042-C as the replacement for 157-0026-B. New old stock is scarce. In a 2025 TinkerDifferent thread, a 157-026C offered by one eBay seller cost about US$150, and Kai Robinson reported that a flyback he had bought from the same seller looked new but had its windings melted and shorted together, which he found by checking the windings before fitting it. Paolo B reported that in his experience any flyback made for the monochrome compact Macs will work.

Lee gives approximate DC resistances between the flyback windings, for a rough check only. Pins are counted anticlockwise viewed from the solder side, starting at pin 1, which is marked on the board.

T1 winding resistances (Lee, Figure 10)

Between pins	Approximate resistance
1 and 2	about 0 Ω
1 and 3	0.15 Ω
3 and 4	0.1 Ω
7 and 8	3.3 Ω
8 and 5	1.3 Ω

Lee advises suspecting the part only if a reading is very different from these, and checking that windings which should not be connected show no continuity. The high-voltage rectifier inside the flyback is a stack of diodes in series that an ordinary ohmmeter cannot test.

Procedure

Opening the case and removing the anode lead

1. Switch off and unplug the power cord. Remove the battery compartment cover, any programmer's switch and the five T15 case screws, and lift off the rear housing.
2. Discharge the CRT.
3. Remove the anode cap from the CRT. Under the suction cup are two metal prongs shaped like a winged V. Push the centre of the V with the tip of the discharge probe to unhook the first wing; the second then slips out. Lee pinches the cap between thumb and forefinger, pushes towards the forefinger and tilts it up to release one clip, then slides the cap the other way to release the second.
4. Remove the vinyl jacket from the solder side of the analog board; see Macintosh Plus Horizontal Sweep Repair#Removing the vinyl jacket.

The anode connector under the suction cup: two metal prongs like a winged V, hooked into the anode well of the CRT (Pina, Figure 5-16).

Removing the old flyback

1. Lay the machine on its side with the solder side of the analog board facing up.
2. Desolder all nine flyback pins. They are not bent over, so the desoldering pump can sit straight over each one.
3. Before you free the last pin, support the transformer from underneath; otherwise it can drop out and hit the CRT.
4. Inspect the old part and the board under it for burning at pin 4 and bubbled resin.

Testing and replacing Q3

The horizontal output transistor on its heat sink, normal and overheated, with the base, collector and emitter leads marked B, C, E (Pina, Figure 5-17).

Pina's rule: if the picture was badly distorted, wavy or darkening, the flyback has shorted and Q3 must be replaced with it even if it tests good. If there was only jitter, ringing or an ozone smell, Q3 is probably sound. It must still be tested before the new flyback is powered, because a shorted Q3 can ruin it.

1. Desolder the three leads of Q3 (Q2 on the International board), at the top of the board two parts to the right of C1 and one down.
2. Desolder the heat-sink tabs. They have a 45° twist; straighten each with pliers once the solder is clear, so that the tabs slide out of the board without tearing the traces.
3. Test the transistor out of circuit with an analog meter on R×100. Clip the red lead to the first terminal named in each row and the black lead to the second, then reverse them.

BU406 out-of-circuit readings (Pina, Tables 5-5 and 5-6)

Junction	Good: red/black	Good: black/red	Typical bad: red/black	Typical bad: black/red
Emitter to collector	Infinity	Infinity	650 Ω	Infinity
Base to emitter	650 Ω	Infinity	150 Ω	150 Ω
Base to collector	650 Ω	Infinity	650 Ω	Infinity

4. Skip the test and replace Q3 if the board under the heat sink is burned or the heat sink has turned reddish blue.
5. Remove Q3 from the heat sink. If it is bolted, use pliers and a small screwdriver; if it is pop-riveted, clamp the heat sink in a vice and drill the rivet out with a ⅛ in bit. Keep the heat sink: replacements are hard to find.
6. Polish any discolouration off the heat sink, apply fresh heat-sink compound, and fit the new transistor with a 6-32 × ¼ in machine screw and nut. Tighten only until a little compound squeezes out; more bends the tab.
7. Use an original-type BU406; Pina prefers the heavy-duty BU406D. His parts lists warn that the ECG 379 substitute causes severe horizontal foldover at the left edge, and give the RCA SK9085 as an acceptable substitute.
8. Refit the heat sink and transistor to the board and solder the heat-sink tabs and the three leads.

Fitting the new flyback

1. Check the new transformer's windings with an ohmmeter before fitting it.
2. Fit it in place of the old one and solder all nine pins.
3. On a 128K or 512K board, or a machine that was upgraded to a Plus but kept its original analog board, Pina says to change C1 and CR21 at the same time as the flyback.
4. Reconnect the high-voltage lead from the flyback to the anode well of the CRT.
5. Refit the vinyl jacket and the rear housing.

Testing

• Before the first switch-on, check that Q3 has been tested or replaced.
• Switch on. The start-up sound and a steady raster should appear with no whine, herringbone pattern or ozone smell.
• If the right side of the picture wiggles or moves in and out, replace Q3 (and the barrel rectifier CR5 if the movement follows disk activity).
• Recheck the +5 V setting, size, centring and focus; a new flyback changes them. See Macintosh Plus Video Adjustment.

Related pages

• Macintosh Plus
• Macintosh 128K, Macintosh 512K and Macintosh 512Ke
• Macintosh Plus Troubleshooting
• Macintosh Plus Horizontal Sweep Repair
• Macintosh Plus Power Supply Repair
• Macintosh Plus Video Adjustment
• Macintosh Repair & Upgrade Secrets
• CRT Discharge Procedure