IBM 5155 Power supply schematics or info

Alberto Rubinelli - A2 Sistemi alberto at
Wed Nov 9 17:41:43 CST 2005

> I think what I might do next is get a spare comparactor chip of the same
> type (LM339)  and connect the power and ground pins to the power and
> ground pins of the chip on the board (I am not _sure_ they both have the
> same power and ground connections, check this!). Then link the inputs --
> right way round -- to each of the 4 comparators in the undervoltage
> circuit in turn. Put a suitable pull-up resistor (to the comparator power
> connection on the output).

I have found the voltage under threshold isolating one by one the output of
the four comparator (desoldering the pin) and there is two voltage out of
range : +12V is 11V and -12V is 10V. In fact the two comparator of the two
voltage, if connected, shut down the power supply.

I have test the power supply without the two comparator output connected,
and it work , with load ... but I have tested it for a short time, no more
than 20-30 sec, I'm afraid of dangerous effect :)

Tomorrow I try to undestand how these voltages are low.


Alberto Rubinelli          Mail : alberto at
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