13 Aug
2000
13 Aug
'00
5:31 a.m.
I'll certainly second the statement about the Tantalum cap's. I once had a
fire in my AMPRO system because of a shorted Tantalum, and, having seen the
smoke emanating from the rear of the PSU (I hd used a PC-style case)
immediately assumed the fault was in the often considered flaky power supply
(this was in the mid-80's).
In the process of prying the grommet with all the various power leads out of
the supply case, I stuck a 1/2"-wide screwdriver I was using as a prybar,
about 1" into my hand. (think about where you can stick a screwdriver 1"
into your hand and not go out the other side ... ) Half a day and over $1k
in medical expenses later, I looked at the Little Board and saw the burned
spot where the cap had been. If Only I'd looked before I lept . . . <sigh>
About a year ago I once again was powering up an old Ampro Little Board,
and, sure enough! ... the -12 was pulled down by a shorted tantalum bead.
Apparently they do not age well.
BTW, AUI uses -12 and +12 from the supply on most cards that support it.
The coax gets an isolated negative voltage generated locally on the NIC,
generally with a little switcher module. These are handy to hang onto when
you're faced with a broken NIC. They generally make a nice -9-volt source
for boxes that have no other negative supply, and that's satisfactory for
RS-232.
Dick
----- Original Message -----
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Sunday, August 13, 2000 10:35 AM
Subject: Re: [OT] Power and NIC questions
>
> I need to double check a diagnostic I'm making. I have a board that
fried
> the a 1/2W ceramic resitor (in the -12V section
of the PS). It also took
> out a tant cap and the fuses for the +/- 12v (between the MB and the
PS).
Are you sure it wasn't just that the tants shorted and took out the
resistor and fuses? I've had plenty of tantalum bead capacitors do that
over the years.
I suspect the RS232 driver chips but I need to
know if the ethernet nic
(aui & bnc) use -12v. Anyone?
You're going to love this : Probably not, but it might do!
The thing is that the AUI port, and the internal 10base2 transceiver need
a 12V supply. But that supply can float with respect to the data lines
(which are transformer-isolated anyway) and with respect to local ground.
Now most manufacturers use the +12V and ground lines from the PSU for
transceiver power. But at least one used ground and -12V. The only way to
be sure is to check where the power pins on the DA15 connector go, and
where the input to the DC-DC converter for the built in transceiver goes.
-tony