Chapter 11 Main Logic Board and Component Replacement 11-23
Caution – Voltage measurements will be inaccurate if the power supply wiring
harness is not connected to the main logic board. Without load, the power supply
will not regulate.
The colored wires of the power harness supply the following signals and voltages:
■ Red: +5V
■ Blue: +12V
■ Brown: -12V
■ Gray: POR (Power on reset)
■ Black: Ground
Note – The POR signal is a Transistor-Transistor Logic (TTL) open collector and pull
up is on the main logic board.
The limits for the voltages are:
■ +5V: 4.90 to 5.10
■ +12V: 11.16 to 12.84
■ -12V: -11.6 to -13.2 with a hard drive
■ -12V: -10.2 to -13.8 without a hard drive
If the voltages are correct, you can rule out a defective power supply.
1. Reconnect the cables and SBus cards one at a time and measure the voltages each
time. In this way, you can determine whether one of the system components is
holding down a power supply voltage.
2. Close the system unit using the procedure in Section 7.4 “Closing the System
Unit” on page 7-4.
3. Enter the Forth Toolkit.
a. Turn on the system and the terminal. The system will start to boot.
b. Interrupt the boot sequence by pressing the Break key on the terminal
keyboard.
c. If you see the ok prompt, go to step 3-e. If you see the > prompt, continue with
step 3-d.
d. At the > prompt, type
n and press Return. When the ok prompt appears, you
have entered the Forth Toolkit.
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