16 Feb
2004
16 Feb
'04
2:36 p.m.
On Mon, Feb 16, 2004 at 11:31:48AM -0500, Robert Maxwell wrote:
[...]
Now, the questions: what the hell happened?
Failures of tantalums (I've
witnessed) are usually due to reversed polarity: not only are these
Not only reversed polarity, but also aging.
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.
The three-voltage circuits usually become defective when the lowest voltage
is missing (-5V) while the others are applied (+5V, +12V). The preferred
power-up sequence is: -5V, +5V, +12V. Since the defect has blown just the
+12V supply, chances are good that nothing bad happened, and the DRAMs as well
as the 2708s are still okay. But you should be aware that likely the same
that happened to the 12V tantals might also happen to the +-5V regulator
circuits (although less likely due to lower voltage). It's a good idea to
replace the various tantal and electrolyte caps with new ones. Capacitors
older than 10 years will likely lose their capacity or voltage tolerance due
to aging of the dielectrical insulation.
Holger