26 Aug
2010
26 Aug
'10
1:45 p.m.
Maybe HP's QC isn't quite what we like to
believe it to be. I've
MAybe not... I think I've had the odd reveres small electrolytic too...
mentioned in the past an HP9815 calculator
(1970's) in which a power
supply filter electrolytic capacitor was factory installed with reverse
polarity. It has never presented a problem however, so in that instance
I didn't correct it, just noted it in my log for the machine.
Now that I would _certainly_ replace. The reason is that the 9815 (and
9825 for thatr matteer) do not have corwbar cirucits in the PSU. If the
PSU malfucntions in the 'right' way -- e.g. the chopepr transistors
shorts (on the heatsink on the side of the transforemr box), the 5V line
ca jum pt abotu 30V. Needless to say that's expensive...
So if there was anything remotely suspect about that PSU. I'd fix it.
It's a difficult PSU to test because it's all on the lower PCN that
contains the printer and tape interface circuits. But that's not a big
enough loard for it. What I do is remvoe the kyeboard/procesosr board
assembly, take uout the expansion slot unit and the tape drive, and
disconnect the printer (particualty the DIL header fo the prinhead which
contains driver circuity). Then connect a 6V bulb across the 5V lone on
the lower PCB and power up. If the PSU oges crazy, all it will ruin are a
few common logic parts on that PCB, no custom parts.
-tony