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_at_p850ug1.demon.co.uk>
To: <classiccmp_at_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
>
>
Received on Sun Aug 13 2000 - 12:31:52 BST