Smoke on the Horizon... oh s**t!

From: Robert Maxwell <RMaxwell_at_atlantissi.com>
Date: Mon Feb 16 10:31:48 2004

  5-6 years ago someone decided I could provide a good home for (most of) a
retired NorthStar Horizon... It came without drives, software and the HSIO-4
serial port card for which I did receive a manual.
  I didn't have a pressing need to have it working, so I've taken my time
assembling the parts necessary to revive the beast - I finally had enough
supporting materials to plug it in last weekend.
  The first thing I saw was a bright flash and "pop!", which led to a quick
unplugging. I couldn't locate anything that appeared to have blown, so I
_hoped_ that it was a stray piece of conductive junk that got blown away by
the initial power.
  I then became methodical, and disconnected the power supply from the
motherboard, and used an external supply to try charging each of the great
honking electrolytic capacitors - they were all OK: internal leakage is
really good, considering their age and period of inactivity. With the PSU
running unloaded, the caps charged to 20 volts on the 16V rails (positive
and negative), and about 11V on the 8V line - perfectly reasonable for an
unloaded supply.
  Next I reconnected the boards and repowered the beast - the unregulated
supplies were all holding up without complaint, so I tried booting a disk.
The floppy drive was spinning, and it stopped when I hit Reset, but there
was no drive select or seeking.
  Then something (it used to be a tantalum cap) caught fire. Unplug again.
  The former capacitor was the input filter on the 7812 12-volt regulator on
the HRAM6 64K DRAM card. There was also a dead short on the 7812 output,
caused by its output filter tantalum. There's also a wrecked tantalum on
the input side of the 78L12 regulator on the CPU card: it didn't burn, it
just blew half its coating off (it may have been the original flash...?).

  Now, the questions: what the hell happened? Failures of tantalums (I've
witnessed) are usually due to reversed polarity: not only are these
particular parts long-term residents of the boards, but the unregulated +12V
supply is the highest DC voltage in the machine - how did the - terminal (DC
Ground) become more positive than the +? How did that happen on two S-100
cards when the main supply, motherboard and floppy drive show no problem?
Has any other Horizon (or S-100) keeper experienced anything like this?
  Next, the problems: I've got replacement regulators and capacitors, but
what if the damage spread to the DRAMS (16Kx1, qty. 36) and the boot EPROM
(2708, labelled HGT E000)? If I need them, does anybody have spare
memories, or a copy of the EPROM image? I can even rig a 2716 (one
supply... yay!) as long as I know what belongs inside.

  I really want to know what went wrong before I try plugging this thing in
again.

Regards,
Bob
Received on Mon Feb 16 2004 - 10:31:48 GMT