Classic Mac Tech Docs, v1.1: No warranties expressed or implied. Use at your own risk!
Classic Mac Tech Info ©2000 Thomas H. Lee, rev. April 17, 2002; All rights reserved Page 6 of 18
The bottom inverter activates Q4, effectively increasing the negative rail for the output
buffers to –12V from ground for a brief interval.
The vertical deflection yoke measures about 8 ohms or thereabouts at DC, and should
show no continuity to the horizontal deflection windings. As with other such measure-
ments, only gross departures from the nominal DC values should be considered cause for
suspicion.
5.0 Power Supply Circuit
Another Achilles heel of the classic Macs is the switching power supply, specified to pro-
vide a maximum output of something like 60-65 watts. Unfortunately, the original
designer underestimated the special demands that switchers place on components (such as
filter capacitors and rectifiers), especially when there is no fan to keep things cool. As a
result, the unit is underdesigned and prone to failure.
The power supply may be decomposed into two main modules. One simply takes in
120VAC and produces a raw, +170V unregulated source. The second converts this high
voltage into three regulated DC outputs: +5, +12 and –12V. Only the +12V output is actu-
ally regulated directly; the rest are slaved to the +12V output through transformer turns
ratios. A linear regulator IC on the motherboard produces a –5V output from the –12V
produced by the main power supply.
The first module takes the signals from all three terminals of the AC power line, and filters
them through line filter LF2, which is trifilar wound on a toroid. A fuse and spike-sup-
pressing capacitor C38 protect the unit from gross faults. One more line filter, LF1, with
R61, C33 and C37 complete the functions of protection and EMI suppression ahead of the
on/off switch. Thermistor R39 limits surge currents through the bridge at powerup. Capac-
itors C35 and C36 filter the output, and bleed resistor R41 improves safety by discharging
the deadly filter capacitors when the computer is turned off. This part of the circuit is very
reliable, and has no known pathological failure modes.
FIGURE 4. Raw 170VDC power supply
2.5A
250V
C38
0.1µF
LF2
LF1
R61
470k
C37
4.7nF
C33
4.7nF
SW1
On/Off
R39
C35
100µF
R41
100k
100µF
200V
C36
200V
120VAC
Grn
wire
ground
Chassis
ground
Primary
ground
+170VDC
Vac (to startup circuit)
CR23-26
R39 is a thermistor. CR23-26 are 1N4007.
++
1
2
1
32
4
3
4
LF2 pins are counted counterclockwise (foil side).
Pin 1 is marked on foil side of PCB.
LF1 pins are marked on component side of board.
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Kommentare zu diesen Handbüchern