Checking X/Lisa Voltages

From: James MacPhail <gg__at_email.domain.hidden>
Date: Tue, 25 Apr 2006 14:36:32 -0700

>Did you check the voltages on the power supply? i have no idea to check
>those under load though... the power supply is difficult to reach

I agree that checking the power supply voltages is an important first step for most troubleshooting.

You can access almost all of the voltages via the I/O board, which is relatively safe to do (no need to be near the high voltage in the power supply or video circuitry).

In particular, on the I/O board you can find:

 +12 at the positive side of capacitor C21

  -5 at pin 8 of U10E (26LS30)

 -12 at the negative side of capacitor C9

  +5 at the maximum pin number of most TTL ICs or the adjacent bypass capacitor

  +5-Standby ( ~5.7 V ) at the anode of diode D9 (present if Lisa is plugged in, even if turned off)

>From the I/O board, you cannot access the +33 volts used by the analog
video card, or the high voltages generated by it, but you can usually tell if the video card is working by the presence of something on the screen (even if it is random).

If you have an internal hard drive that is not working properly, checking the voltages at the hard drive would be appropriate as it may see lower voltages compared to those appearing at the I/O board. (Due to the higher current drawn by the hard drive, and the different wiring path.)

HTH, James

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