14 Nov
2002
14 Nov
'02
4:46 a.m.
On Thu, 14 Nov 2002, Mark Tapley wrote:
I think there is no such line. On the mainboard connector to the PSU,
there are Red (3), Black (4), Orange (1), and Blue (1) wires.
Red are all connected together, and all go to appx. 5 V when the system
is on. Black are all connected together and to ground and are always at
0 V. Orange is one wire, and goes to appx. 12 V when the system is on.
Blue is one wire, and goes to appx. 12 V. when the system is on.
Black is nearly always ground, red is commonly used for +5V. This wouldn't
be the first supply I've heard of that didn't have a PG connection.
Most DEC machines have at least one power-OK line from
the PSU.
I want to figure out which that one is. The red and black cross-connections are true on the
mainboard and on the
PSU. They are not all adjacent on the connector, which makes not much
sense to me, but that's how it is. Red and Black on the mainboard
connect to the corresponding colors on the Disk power connector.
It may have made it easier for the engineers who were laying out the power
busses on the mainboard.
As for voltage: I really need a good DVM. Calibrated
the 12V setting at
work against a good 4-digit fluke DVM, so I think my reading of 12.1V
for orange and -12.0 for blue is close. Did not have time to check at
5V, but afterward measured 3 Ray-o-Vac Renewal D cells in series at 4.7
V, so I may not be way far off. If so, my reading of 5.3 to 5.4 for the
Red group is worrisome. This reading is consistent even at the Disk
power connector. Anyone else have a good VOM, a working VLC, and 3
Renewal cells? Or just the VOM and either of the other two?
It sounds like your DMM is close enough. Since I'm not familiar with these
supplies, I can't say if 5.3-5.4 is too far beyond the norm, but I usually
don't get too worried unless it reaches ~5.5 and beyond for +5V.
I know more about it, now. Turned it on cold, it ran
for a while, then
failed, then started to try to boot again. Blew the hair dryer on it, it
went into hard fail almost immediately and did not try to boot. Blew a
fan on it, it went OK and actually ran far enough to give me a login
prompt. In both cases I was trying to aim at the part of the mainboard
near the power connector, but the system is so small I was probably
affecting most of the mainboard and possibly a lot of the PSU as well.
It certainly sounds like something is overheating. You might want to try
using some cardboard to carefully direct the airflow from your hair dryer.
Do these systems have an internal fan? I haven't been inside one enough to
remember, but if they do, is the one in your system running properly?
Near the power connector are two power-looking
components (as in they
have big leads, and in one case a heat-sink attach point, whereas nearly
everything else is microscopic surface-mount stuff). One is a
power-transistor looking thing with a heat-sink attach point (but no
heat-sink) that has 3 pins and says
LT1086CT
9151
Linear Technology 1.5A adjustable voltage regulator, TO-220 package.
Datasheets for this part can be found here:
http://www.linear.com/prod/datasheet.html?datasheet=233
The right leg is connected to 5.3 V (Red-wires).
The center leg stays around 3.4 V, system running or failed.
The left leg stays around 2.15 V, running or hung (as far as I can tell).
Pins from left to right:
1 ADJ (ground for fixed voltage regulators)
2 Vout (also connected to the mounting tab)
3 Vin
It seems like the board uses the regulator to regulate +5V down to 3.3V.
The other is an 8-pin DIP that says
M9124
LM393N
QST
Low Power Low Offset Voltage Dual Comparator
Datasheets can be found here:
http://www.national.com/pf/LM/LM393.html
Are these two part of a crowbar circuit, or reset
circuit, or some such?
Should I be able to see some clue about why the system hangs by looking
at their pins?
I expect to find a crowbar would be inside the PSU, and not on the
mainboard. I don't think these components would be used for a crowbar
anyway. It really does sound like your system has a thermal problem. I'd
suggest some careful directing of the air from your hair dryer to help
pinpoint the problem area.
-Toth