Well ... it is most likely the connector on the power supply.
You already have it apart, if you have all of those disconnected, so you
can look easily.
Pull the wide single-row connector (orange plastic) from the row
of pins on the power supply. Look along its side. If any of the
plastic over the pins has turned slightly brown, it has developed high
resistance and is heating, and the voltage is spiking and falling.
If it is *very* brown, or black, it has probably heated things
up badly enough so the solder has melted from around one of the pins on
the underside the power supply's circuit board. (You'll have to pull
the power supply out of the system to verify this, and to work on it.)
Since it is a 7300, not a 3B1, I suspect that the power for the
hard disk drive is fed through the system board, so it burns the
connector a bit sooner. (Later 3B1 power supplies have a cable soldered
directly to the board to carry power to the hard disk drive, and thus
reduce the current through the rest, though it also typically has a full
2MB of RAM on the system board, so the current load there increases.
If the connector is dark brown from overheating, it really
should be replaced. It is possible to pull the back cover off it, then
ease the indivdual wires from the stiff ribbon cable out of the
individual slots, and install them in a replacement connector of the
same type by pressing down on either side of the metal of each pin, with
perhaps the tips of some needle-nose pliers.
IIRC, the connector has either 18 pins or 38 pins -- I forget
But, in any case, spray the pins with a good contact cleaner,
and slide the connector on and off the pins several times to clean off
any oxidation. What I use to really like was something called
"Cramolin", but it got discontinued because of something toxic or bad
for the ozone layer, and it was replaced with something called De-Oxit.
Either of them come in two colors, intended to encourage you to
spray with one, wipe it clean, and then spray again prior to
Of course, it could be something else, but this is what I would
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