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Top Document: Atari 8-Bit Computers: Frequently Asked Questions
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13.9) Why should I disconnect the 810/1050 power supply before connecting or disconnecting SIO cables?

Rich Mier writes:

You've been plugging and unplugging the SIO cable with the 1050 power pack
plugged in, right?  That's a no-no.  Most of the time it's Okay, but about 1
in 10, 20 times, it will blow out 'U-1'.  It's a CA/LM 3086 I.C. at the right,
rear of the main board.  A 14 pin DIL chip.  Actually it is an array of 5
transistors.

Unplug the power pack from the 1050, then unplug the SIO cable.  Power can be
ON on the CPU.  The problem has to do with the secondary winding of the Power
Pack.  Remember, the problem only occurs 1 out of 10 - 20 times that you do
it, not all the time.

It doesn't really matter if the 1050 Transformer has power on or off, it
'Might' happen if plugged into the 1050.  It is really bad on 810's.

One thing, if the system has been turned off for, oh say, 5 - 10 minutes it
won't matter.  By then all the capacitors should be bled(sc?) to 0 volts.