help needed with power supply

[Thad P Floryan]

Re: checking and adjusting the power supply in the UNIXpc ...

Start by opening the system up (removing the top cover) and identifying
the subsystems illustrated below:

          ---------------------------------------------+
           +----------------------+  +---------------+ |
           |[oooo]                |  |               | |
           |                      |  |               | |
           |                      |  |           (C) | |
           |                      |  |               | |
           |       HARD           |  |    POWER      | |
           |       DISK           |  |   SUPPLY      | |
           |                      |  |           (B) | |
           |                      |  |               | |
           |                      |  |               | |
           |                      |  |               | |
           |                      |  |               | |
           |                      |  |           (A) | |
           |                      |  |               | |
           +----------------------+  +---------------+ |
                                                       |

[oooo] is the power connector on the hard disk
(A) is the +12VDC adjustment potentiometer (silkscreen label on the board)
(B) is the  +5VDC adjustment potentiometer (silkscreen label on the board)
(C) is the -12VDC adjustment potentiometer (unlabelled)

Acquire a disk-drive power "Y" cable which looks something like:

                           +-+
                ___________|o|
               /___________|o|  JACK
              //___________|o|
             ///___________|o|
   +-+      ////           +-+
   |o|_____////
   |o|_____///
   |o|_____//
   |o|_____/ \
   +-+     \\\\
  PLUG      \\\\           +-+
             \\\\__________|o|
              \\\__________|o|  JACK
               \\__________|o|
                \__________|o|
                           +-+

The "Y" cable is essentially a mini-extension cable whose wires are commoned
from the one plug to the two jacks.  The two middle wires are for ground (and
are typically BLACK), and the two outer wires are for +5 (typically RED) and
for +12 (typically YELLOW) [ but DON'T rely on the color coding! ]   These "Y"
cables are usually available at electronic parts stores or flea markets for
between $1 to $3.

The following steps assume you possess some common sense and know enough to
unplug the system from AC power before opening it up and "playing" with the
parts.  It's always wise to have a buddy in the room when you're working
on the opened system (if for no other reason than to provide him the
opportunity to laugh his ass off when you screw up and get thrown across the
room when you touch a live wire :-)

The steps to check or adjust the power supply voltages to the disk drive are:

1) disconnect the power cable from the disk drive and connect it to the PLUG
   on the "Y" cable.  Connect one of the JACKS of the "Y" cable to the disk
   drive; you'll be using the sockets on the other JACK as test points for
   measuring the voltages.

2) power up the system (with the cover still removed).

3) measure the voltages on the "free" JACK; they should be within 5% of the
   ratings per:

       NOMINAL      LOWER     UPPER

        +5          +4.75     +5.25

       +12          +11.4     +12.6

4) if the voltages are NOT between the LOWER:UPPER range for each voltage,
   use a plastic CRT adjusting tool on the respective power supply ajustment
   potentiometer and S-L-O-W-L-Y turn the potentiometer until the voltage is
   within the range.  A clockwise adjustment will raise the voltage and a
   counter-clockwise adjustment will lower the voltage.

   I usually adjust each voltage "slightly" high; e. 5.10 and 12.25 are the
   levels I typically set.

5) After checking/setting the voltages:  power down, remove the AC power cord,
   remove the Y cable, re-connect the power cable to the disk drive, and close
   the case.

Of all the systems I've checked, I've only found one whose +12 was low, so
assuming you haven't been experiencing a rash of HDERR entries in
/usr/adm/unix.log and that the HD itself is OK, you should be back on the air!

If you have been seeing a plethora of HDERR entries in /usr/adm/unix.log,
then you'll be interested in a treatise "Sparing Out Bad Blocks" that I'll
be posting shortly.


Thad Floryan [ thad at cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]