Macintosh Plus Focus Repair

Pina's photo of the focus suspects on a U.S. analog board: R16 is in line with the pointer and R9 two resistors above it, just below the flyback transformer (Figure 4-17).

When the picture on a Macintosh Plus stays out of focus whatever the focus control is set to, or goes in and out of focus as the brightness is turned up, the cause is usually one of three things: burned-open resistors in the focus circuit, a loose or corroded CRT socket cable, or tarnished pins on the neck of the CRT. Pina links the burned resistors to heat in sets run without a fan.

The Macintosh 128K, Macintosh 512K and Macintosh 512Ke use the same circuit. On the U.S. analog board the resistors are R9 and R16; on the International board they are R12 and R20, in the same places.

Symptoms

• The focus control R54 has no effect and the picture is out of focus: R9 or R16 open.
• Focus works but is intermittent, or fails as the brightness is turned up: the CRT socket cable has slipped or its contacts have corroded.
• Pushing the socket cable home does not help: tarnished CRT pins.
• Out of focus straight after a board swap or service: the new board only needs its focus control adjusting; see Macintosh Plus Video Adjustment#7. Focus.
• An occasional bright flash across the screen is a cracked joint at pin 1 of J2, not a focus fault; resolder it.

The focus and cut-off feed to the CRT: cut-off control R53, focus control R54, R9, R16, connector J2, the CRT socket cable and the CRT socket pins (Pina, Figure 4-16).

Safety

Warning: the analog board carries mains voltage and the CRT anode about 13 kV; the focus voltage on the CRT socket cable is several hundred volts. Unplug the power cord before touching the socket cable or the CRT neck, use one hand only, and wear goggles: the neck of the CRT is thin and fragile. Discharge the CRT before unplugging the socket cable; see CRT Discharge Procedure.

Tools and parts

• T15 Torx driver, case-cracking tool, a multimeter with a megohm range, an inspection lamp, soldering iron and desoldering pump.
• Replacement resistors, ½ W carbon film even where the original was ¼ W: R9 1 MΩ and R16 100 kΩ on the U.S. board.
• Metal cleaner and cotton swabs for the CRT pins. Do not put contact stabilant on the CRT socket: Pina found it stopped the cut-off circuit working, leaving the screen overly bright for some hours.

Procedure

Opening up

1. Switch off, unplug the power cord and remove the rear housing.
2. Discharge the CRT.

1. Testing R9 and R16

1. Check again that the cord is unplugged. Set the meter to its megohm range and set up the lamp to shine through the board from the component side.
2. Find R16 and R9 on the component side (R9 two resistors above R16, just below the flyback), then their joints on the solder side. They often look undamaged.
3. Measure across each in circuit, either probe either way round, and compare with Pina's readings:

In-circuit readings (Pina, Table 4-4)

U.S.	International	Value	Good	Bad
R9	R12	1 MΩ	824 kΩ	Over-range, or 3.5 MΩ
R16	R20	100 kΩ	101 kΩ	Over-range, or infinity

4. Replace any open resistor with a ½ W part of the same value, using the general desoldering procedure in Macintosh Plus Low Voltage Repair#Replacing a zener.
5. Check the repair: start up with fine text on the screen and turn the focus control (R54 on the U.S. board) gently from one end to the other through the vinyl jacket. The screen should go in and out of focus.

2. Seating the CRT socket cable

The CRT socket cable runs from J2 on the analog board to the socket on the neck of the CRT (Pina, Figure 4-18).

Heat cycling, transport or an accidental brush during upgrade work can work the socket loose from the CRT by as little as 1/32 in, after which its uncovered contacts corrode.

1. Unplug the power cord and put on goggles.
2. With one hand, push the socket gently all the way home on the neck of the CRT. A small push is enough.

CRT socket cable (Pina, Table 4-3)

J2 pin	Wire	Function	CRT pin
1	Red	Cut-off	6
2	None	Key	None
3	Blue	Focus	7
4	Green	Brightness	1
5	Black and white	Ground	None
6	Brown	Ground	4
7	Black	Heater	3
8	Yellow	Cathode	2

Lee's connector drawing numbers J2 differently, with nine positions and pin 4 absent, so match wires by colour and function.

3. Cleaning the CRT pins

Unplugging the socket from the neck of the CRT to inspect the pins (Pina, Figure 4-19).

1. Unplug the power cord and discharge the CRT.
2. Pull the socket straight off the neck of the CRT. Apple's take-apart slides the CRT socket cable straight off the neck and warns never to grasp the neck itself.
3. Look at the pins. Clean copper pins are bright; pins streaked dark blue are tarnished.
4. Clean them with cotton swabs dipped in metal cleaner and dry them carefully.
5. Push the socket back on, straight and fully home.

Testing

• With fine text on the screen, the focus control should move the picture in and out of focus, and focus should hold as the front brightness knob is turned up.
• Set the focus as described in Macintosh Plus Video Adjustment#7. Focus, then refit the vinyl jacket and the rear housing.

Related pages

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