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

From: Vintage Computer Festival <>
Date: Mon Feb 16 10:47:41 2004

On Mon, 16 Feb 2004, Robert Maxwell wrote:

> 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...?).

Perfectly normal (actually).
> 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?

It has to do with age. Tantalum capacitors are very prone to blowing just
as you describe on machines that are as old as what you're dealing with
here. In fact, blowing a tantalum cap on an S-100 machine is a rite of

Frank McConnell has a terrific photo of a tantalum blowing on a Sol-20
that I decided to just turn on and let fate deal with. The cap blew with
lots of noise and light and fanfare, but otherwise the system powered up
normally eventually.

> 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 wouldn't worry about any of that really. It's possible that they
might've been damaged, but its more possible that things are just fine and
you should replace the caps eventually.

Sellam Ismail                                        Vintage Computer Festival
International Man of Intrigue and Danger      
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Received on Mon Feb 16 2004 - 10:47:41 GMT

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