At 08:04 AM 3/30/99 +0530, you wrote:
>The SMPS is AT type. Also there is leakage current in the case
>enough to give a good shock. The leakage occurs only when load
>is connected (ie motherboard).

I recently had a problem after I replaced an AT power supply.
When the motherboard was in the case, all I could get at boot
was constant beeping...like what happens when there is a memory
problem (bad RAM or none at all). The system booted fine when
the board was removed from the case and sitting on the table top.

After a lot of troubleshooting, I concluded that the on/off switch
was apparently inducing fluctuations in the ground voltage of the
case. There is a ground wire that connects the switch to the
computer case and it was apparently attached correctly. But I
noticed that this wire twisted once around the wire that carried
the AC current. (This twist occured just after the different wires
came out of the single thick insulation sheaf that carries the
different wires from the power supply to the on/off switch...
a few inches before the switch.)

I straightened the ground wire, wrapped a ton of electrical tape
around the switch and and rescrewed it to the case. The problem
was solved. I don't believe that the switch was touching the case
improperly before I insulated it; the metal base of the switch was
screwed to the case the same way before and after, the ground wire
was attached the same before and after, and the motherboard was
screwed into exactly the same posts before and after. But the
problem was solved.

Maybe you should examine the computer's front on/off switch for
correct insulation, shielding, and grounding. Check the route
that the wire connecting the power supply and front on/off switch
takes as well. Unlike with an ATX power supply and motherboard
(where only a small and momentary DC current is involved), the
front switch for an AT power supply goes directly to the power
supply and (I think) carries a large alternating current whenever
the computer is powered up. A changing current (an AC current for
example) will induce currents in nearby conductors or wires.

Regards,
Bill

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