[tech] Status of snack machine

[Harry]

Grahame Bowland wrote:

> One question that occurs to me; how do we know that when we put the
> chip in it won't be immediately cooked again because the fault is
> actually somewhere else? Do we have enough information to verify that
> the chip is the only problem - it seems kind of unlikely it'd
> spontaenously fail without receiving some sort of nasty jolt.

It's a valid concern and we had a list of tests in a previous email.

On reflection the tests can be improved upon ..

First step is to fit the chip and test without the motor matrix cables
plugged in. The chip only drives out on those cables so I don't expect
failure when the board is powered up this way.

Next, use a CRO to test that the clocking and data signals are flowing
through the high side (failed) part and low side part (they are
daisychained so serial data has to flow through the pair correctly).

Next is to measure the cables going to the motor matrix to confirm there
are no shorted diodes in the matrix or in the cam switch matrix too. We
started that with my TX3 meter using the diode tester setting but we
didn't confirm that Nick's meter will do that as well; it has a diode
test position but it needs checking. Otherwise finding a meter that
tests the diodes for correct forward voltage (around 0.5volts we
measured on some) rather than a short or open circuit.

Another task is to inspect the receptacles that each of the trays plug
into to confirm that neither the tray plug or socket on the back panel
that receives them is damaged and may be shorting.

Along the way, a task is to clean all of the melted Redskin gunk in the
trays and visually inspect all of the trays for any obstruction that
will stall the screws. Nick did a short test of operating one of the
motors (using clippy leads) directly from the current limited side of
the 24volt motor power supply. That test could be conducted on all
motors to confirm there are no frozen tray screws.

Only after all of those tests should the motor matrix be plugged in and
the driver chip tested under load.

That is the way of diagnosis and repair. You don't just change the
failed "safety shutdown computer power supply line fuse" at the nuclear
power plant and throw the big 2GW circuit breaker to ONLINE ;-) Snack
machine requires similar care.

Harry