Macintosh Plus

This page applies to the Macintosh Plus primarily, but information here might also be relevant for the 128 and 512 variants.

Warning: While running, the voltages on the analog board and around the CRT can be lethal.

Wobbly / wavy / shaking screen
The computer might work fine otherwise and boot, or the screen only shows garbage. But whatever is displayed is unstable, with the outer lines of the display swaying back and forth at about 10Hz.

Causes

 * 1) Unstable or too low voltage from the power supply on the analog board.

1. Confirm correct voltage
Turn on the computer and measure the voltage on the 12V rail, which is most easily done by measuring the voltage between pins 7 or 8 on the external floppy connector to pin 1 or the chassis ground. It is specified it to be within 11.9 and 12.75 Volts. Then check the 5V rail via pin 6 on the same connector, which should be between 4.85 to 5.15 Volts.

2. If voltage is too low
The output voltages of the power supply can be controlled by a variable resistor on the analog board. After removing the computer case, it is adjustable using a flat-head screw driver through a hole in the board. Look for the hole marked "Voltage" on the protective sheet on the analog board, just above the speaker holes.

If the voltages are out of spec, adjust the resistor while the computer is running and you're measuring the voltage. First get 12V within spec, as the 5V rail is derived from it.

3. If the voltage is unstable or cannot be adjusted
If adjusting the resistor doesn't make any difference, double check that you're turning the right control. If that's the case, it is possible that the optocoupler used in the feedback circuitry of the power supply is faulty, and thus the variable resistor is without effect. The voltages will also be very low, around 9-10V on the 12V rail and 3V on 5V. It's the small 6-legged DIP chip found just above the brightness control knob, at the bottom of the analog board, marked U3.


 * 1) Using diode mode on a multimeter, measure between its pins 1 (where the dot is) and 2. It should leave some current flow in one direction, but fully block when you flip the test leads. If there's no continuity in both directions, the internal LED is dead. The chip needs replacing. (Note: Maybe there are other failure modes and even if this checks out, it's might be worth replacing just in case.)
 * 2) While it may not look like it, the chip is likely to be socketed. Try gently prying it out of the board. If it doesn't budge, desolder it.
 * 3) A 4N35 optocoupler is a cheap and widely available drop-in replacement.

Once the optocoupler is replaced, turn down the voltage to minimum before trying again. Better to under- than to overvolt the system.

Disassembly Notes
Warning: While running, the voltages on the analog board and around the CRT can be lethal.