Paul Rogers wrote in a message to Matt Mc_Carthy:

 LE> SP-64
 LE> Serial-To-Parallel Converter
 LE> and 64K Buffer
 LE> Product No.
 LE> 8149-1

 LE> It's got a coax power jack, and no indication of what voltage
 LE> polarity it wants. There's also a 8 position DIP switch inside
 LE> (probably for setting serial parameters).

 PR> Got any 74xx TTL inside?  The +5/G were typically 14/7 or 16/8.
 PR> Stick an ohm meter on it and see which is straight through to the
 PR> jack.

That's why I suggested looking for the regulator,  first.    Those
do like their power supply voltage fairly tight.

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