Macintosh Plus Horizontal Sweep Repair

A Macintosh Plus with the rear housing off. The analog board stands on the right; the word INTERNATIONAL printed along its top edge marks the 120/240 V version. Photo: htomari, CC BY-SA 2.0, via Wikimedia Commons.

The commonest display fault on the Macintosh Plus analog board is loss of the horizontal sweep: the picture collapses to a single vertical line, or shrinks, wobbles and comes back when the case is slapped. Thomas Lee calls cracked solder joints on the yoke connector J1 "without question" the most common fault on the board. Larry Pina traces the same symptom to cracked joints at J1, the horizontal deflection capacitor C1 and the width coil L2, to C1 itself failing, and to J1 and L2 burning after C1 fails.

The same analog board family is used in the Macintosh 128K, Macintosh 512K and Macintosh 512Ke, in a U.S. (120 V) and an International (120/240 V) version. The horizontal circuit and its designators (C1, J1, L2, T1) are the same on both versions; the horizontal output transistor is Q3 on the U.S. board and Q2 on the International board. Lee describes the Plus board as the same as the earlier models' apart from minor variations.

Symptoms

Which fault fits the picture

What you see	Likely cause	Source
Picture collapses to a bright vertical line after a while; tapping the left side of the cabinet brings it back or makes it flash	Cracked joints at J1 pin 4 (horizontal yoke) and the width coil L2
Intermittent, wobbly, jittery, narrow or missing raster, or a single vertical line down the centre; slapping the case changes it	Cracked joints on the yoke connector J1
Poor linearity: items on the left of the screen wider than those on the right	C1 failing
A few minutes after start-up, bright diagonal lines; the picture darkens and the machine starts to smoke	C1 failed; check J1 for burning
Start-up sound, then a bright vertical line, then smoke	C1 failed; check J1 for burning
Start-up sound, then a dim vertical line, then smoke	R1 (220 Ω ¼ W) burned open

Faults that look similar but are elsewhere:

• A bright dot in the centre of the screen means both sweeps are missing. After service it is usually the yoke cable left off J1.
• A horizontal line is a vertical sweep fault, a different circuit.
• A whine, ozone smell, wavy lines or a darkening, distorted picture point to the flyback transformer and the horizontal output transistor; see Macintosh Plus Flyback Transformer Replacement.
• A dark screen with the start-up sound present may only be the front brightness control turned down, or a cracked joint at J4 pin 1; see Macintosh Plus Troubleshooting.

Safety

Warning: the analog board carries mains voltage and drives a CRT whose anode runs at about 13 kV. Unplug the power cord before each step below; switching off at the rear switch is not enough, because everything between the inlet J5 and the switch stays live while the cord is connected, including the area under the middle pad of the vinyl jacket. Apple's procedure discharges the CRT to the ground lug and removes the anode cap before the analog board is touched, and says never to wear a grounding wrist strap until the CRT has been discharged. See CRT Discharge Procedure. Pina's in-circuit test of C1 below is made with the machine running. Pina and Lee both work on a live set with one hand only, so that a shock cannot pass from hand to hand across the chest. Pina's general safety rules are in chapter 1 of his book.

Apple's drawing for discharging the CRT before removing the power/sweep board: the discharge tool runs from the ground lug to the anode cap.

Tools and parts

• A Torx T15 driver long enough to reach the two screws in the handle recess (Pina uses an 8-inch shaft), and a case-cracking tool or a 1-inch spring clamp.
• A soldering iron, 60/40 rosin-core solder and a vacuum desoldering pump; Pina uses a 15 W grounded iron.
• A lamp to shine through the board from the component side, so that each solder joint can be matched to its part.
• A multimeter. For Pina's in-circuit test of C1, a 10 MΩ resistor in series with the red probe of a 10 MΩ input meter.
• A replacement C1 (below). Lee and Pina both say not to refit the original type.
• If J1 pin 4 or L2 is burned: a new J1 connector and a width coil wound with Litz wire.
• Double-sided foam tape, 1 inch wide, to refix the vinyl jacket.
• Optional: a hot-glue gun, to damp the bases of C1, J1 and L2.

Replacements for C1

Part	Source
Original: 3.9 µF non-polar high-frequency aluminium electrolytic, 25 V 85 °C (128K to early Plus) or 35 V 85 °C (late Plus to SE)	Pina
Preferred: 3.9 µF 100 V 85 °C high-frequency non-polar aluminium radial (Nichicon)	Pina, "in every case (128K to Mac SE)"
Also listed: 4.0 µF 25 V HF NP axial (Nichicon); 3.9 µF 250 V metallised polypropylene (Nichicon); 3.9 µF 250 V metallised polyester (Nichicon); 3.9 µF 100 V metallised polyester (Panasonic)	Pina, Table 5-3
A non-polar, low-ESR capacitor rated at 50 V or more, or four or five 1.0 µF ceramic disc capacitors of 50 V or more in parallel, built up as a module and joined to the board with bus wire	Lee

Do not use a small bipolar capacitor of the kind sold for loudspeaker crossovers. Pina warns that it cannot handle the 22.25 kHz sweep current, overheats and can burst; a capacitor much smaller than the original is unsuitable even when its ratings look right. Lee finds 3.9 µF or 4.7 µF fitted at C1 depending on model and date, and notes that the value is not critical. Capacitor values for the rest of the board are on the Macintosh Plus Capacitor Replacement Guide.

Procedure

Opening the case

1. Switch off, unplug the power cord and disconnect every peripheral cable.
2. Remove the battery compartment cover and, if one is fitted, the reset/interrupt (programmer's) switch from the left side.
3. Remove the five T15 case screws: two inside the handle recess, two beside the I/O connectors and one inside the battery compartment.
4. Separate the rear housing with the case-cracking tool, working along the seam in small bites and keeping the tool moving, then lift the housing straight off.
5. Note where the L-shaped RFI shield over the I/O connectors and the small grounding spider on the plastic fin near the power switch opening sit, so they go back in the same place.
6. Discharge the CRT to the ground lug as Apple shows, before your hands go near the top of the analog board.
7. Lay the machine on its right side (seen from the front) on a towel, so that the vinyl jacket on the analog board faces up.

Removing the vinyl jacket

The three sticky pads that hold the vinyl jacket to the 128K to Plus analog board (Pina, Figure 4-5). The area under the middle pad is live whenever the cord is plugged in.

1. Check that the power cord is out of the wall socket.
2. Peel the bottom sticky pad first, slowly, starting it with a fingernail.
3. Peel the pad above the power switch, then the top pad. The top pad sits over J1 and is usually baked on; scrape the residue off the board with a fingernail, without solvents or tools.
4. Clean the old pads off the jacket with a putty knife and set the jacket aside.
5. Remove every trace of adhesive from the joints you are going to work on. Pina warns that soldering through it contaminates the joint and leaves a high-resistance connection.

Finding and resoldering cracked joints

Solder side of an early-Mac analog board with a cracked joint in the centre of the photo (Pina, Figure 5-4).

1. Set the lamp behind the component side so that its light shows through the board, and match the joints of J1, C1 and L2 on the solder side to the parts.
2. Inspect each joint. Pina describes sound joints as "shiny gumdrops" and cracked ones as looking like "tiny nipples", often with a ring around the lead. Lee warns that hairline cracks are not always visible and resolders J1 on every board as a matter of course.
3. Resolder the four J1 pins with fresh solder and enough heat for it to flow to a smooth, shiny joint. Leave the two halves of J1 plugged together while you do it; Lee warns that the heat otherwise distorts the plastic and the halves will not go back together.
4. Resolder the joints of C1 and L2 in the same way, and any others in the area that show a crack. Lee also freshens the flyback transformer's joints while the board is open.
5. If the picture also blinked out or showed flashing lines, resolder pin 1 of J4; if it collapsed to a horizontal line or rolled, resolder pin 5 of J4, pin 1 of J1 and C5 as well.
6. Once the joints have cooled, squeeze the release tab of J1 and pull the top half off. Inspect the four pairs of contacts for oxidation or burning.

Two burned J1 yoke connectors. Pina notes that the damage is always on pin 4, the horizontal sweep connection in series with C1 (Figure 5-18).

J1 yoke connector

J1 pin	Wire colour (Pina)	Function (Pina)	Function (Lee)
4	Green	Horizontal yoke	Horizontal yoke
3	Yellow	Ground	Ground
2	Red	+5.00 V	Vertical yoke low
1	Blue	Vertical yoke	Vertical yoke high

Lee explains that the vertical yoke on these boards is referenced to the +5 V supply, which is why Pina lists pin 2 as +5 V.

Burned J1 or L2

1. Replace J1 if pin 4 shows any burning. Pina found that once the plating has burned off pin 4 it never makes good contact again.
2. If no replacement connector is to hand, Lee's alternative is to desolder J1 from the board, remove its mate from the yoke cable and solder the four yoke wires straight to the board. The board then has to be desoldered from the yoke if it is ever removed.
3. A scorched L2 can stay if its slug still turns; free a stuck slug with a metal hex tool only with the power cord out, then return it to the middle of its travel. Replace L2 if the slug cannot be freed.

Testing and replacing C1

A 100 V Nichicon fitted at C1 at the top of the analog board (Pina, Figure 5-6).

1. Look at C1. Lee's rule is that a visibly bulging top, or a bulge at the base where it meets the board, means the capacitor is dead.
2. To test it in circuit, Pina adds a 10 MΩ resistor in series with the red probe of a 10 MΩ input meter, which doubles every DC reading. Clip the black probe to the chassis, switch the machine on (it need not boot) and touch each C1 joint on the solder side. A good C1 reads 18-21 V at the top joint and between −14 and −100 mV at the bottom joint, after doubling. Anything beyond −100 mV at the bottom joint means failure is close.
3. Switch off and unplug the cord before going on.
4. Desolder C1 and remove it.
5. Fit the replacement. A 100 V radial part goes in like the original. An axial part needs one lead bent back and sleeved with insulating tubing. A metallised-film part must be bent over so that its top is no higher than the top edge of the board, or the rear cover will not go back on.
6. Pina's advice is to replace an original 25 V C1 whenever the set is open, faulty or not.

Dim vertical line: R1

1. Check R1 (220 Ω ¼ W) for an open circuit and replace it.
2. Pina also desolders the linearity coil beside it and resolders it with all-new solder, which he says stops the fault returning.

Reassembly

Pina's new pad positions: the top pad moves left, off the J1 hot spot (Figure 4-7).

1. Optionally, run hot glue around the bases of C1, J1 and L2 so that they are fixed to the component side of the board. Pina uses it to damp the high-frequency vibration that cracks these joints.
2. Cut three pieces of 1-inch double-sided foam tape about 1 inch square and stick them to the jacket. Move the top pad to the left of its original place, which was directly over J1.
3. Refit the vinyl jacket. Pina points out that it must go back: the inside of the rear housing is coated with conductive paint.
4. Plug J1 back together if it was separated, refit the RFI shield and grounding spider, and refit the rear housing and its five screws.

Testing

• Switch on. The start-up sound and a full raster should appear. Tap the left side of the cabinet: the picture should stay steady.
• Check linearity: a grid of squares should be the same width on the left as on the right.
• Check the picture size and focus, and the +5 V setting if any parts were changed. Apple's raster size is 7 in (177.8 mm) wide by 4.7 in (119.4 mm) high; Pina works to 7.11 × 4.75 in. See Macintosh Plus Video Adjustment and Macintosh Plus General Maintenance#Voltage check.

How the horizontal output transistor (Q3 on the U.S. board), the flyback transformer T1, the width coil L2, C1 and the yoke connector J1 are connected (Pina, Figure 5-1). The horizontal yoke current runs through L2 and C1 to J1 pin 4.

Related pages

• Macintosh Plus
• Macintosh 128K, Macintosh 512K and Macintosh 512Ke, which use the same analog board family
• Macintosh Plus Troubleshooting
• Macintosh Plus General Maintenance
• Macintosh Plus Capacitor Replacement Guide
• Macintosh Plus Flyback Transformer Replacement
• Macintosh Plus Video Adjustment
• Macintosh Repair & Upgrade Secrets
• The Dead Mac Scrolls
• CRT Discharge Procedure