Re: Smoke from the (portable) XT

With regards to the "3-legged capacitor", if I had any doubt about C56
being a tantalum before, I have no doubt now. Because the "explosive
nature" of tantalums has been well known for a long time, the "3-legged
variety" were often used to prevent reversing the polarity of the
tantalum. Right at the moment, I can't remember if the middle leg is
the + and the outer legs are the - (tied together), or if it's the
other way around. When you remove the planar from the 5155, you should
be able to verify which way is right easily -- check continuity on any
of the +5V decoupling capacitors, I think that all of the outer legs
will be tied to ground. Sourcing these "3-legged tantalums" might be a
bit tricky, though they were also used on many IBM PS/2 planars (an
8580 planar is littered with them). Once you know the proper polarity,
you can replace it with a standard 2-leg capacitor, though you might
also need to jumper the outside two pins if the designer used the
"safety tantalum" as a trace point.

The planar does not tell the power supply when it has a good voltage,
the power supply tells the planar that the voltages are nominal by
raising the "Power Good" line. Either your power supply is improperly
sensing an overcurrent condition (that is, it's broken), or you don't
have the minimum required load on the +5V and +12V supplies with your
floppy and hard drive. The minimum required load on the +5V supply is
about 3.5 amps. My old "bench specification" called for a 1.5 ohm, 25W
power resistor between pins 1 and 6 on the P9 connector to test
5150/5155/5160 power supplies not connected to a planar.

Rick Ekblaw

.

Relevant Pages

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