The parts to fix the spot welder showed up this afternoon. I was thrilled.

After putting in a new fet driver, I put my DVM in series with the power supply
to the fet board. The fet board draws only 28ma ac. So it’s basically in fine
shape, so far.

The first time I took the probes and pushed them up against a piece of nickel
strip to test the spot welder, there was a large flash and pop! The probes had
voltage across them!! Almost like the foot pedal was being pressed ahead of
time. Yet it wasn’t. I put the scope across the output of the control board that
drives the fet driver. I couldn’t get the scope to trigger right. So I put the
probe on the output of the fet driver. The scope was triggering on something
kind of random. I adjusted the scope and there is a pulse on the fet driver
output that is 18.6v p-p. But only 500 nanoseconds long. Something around
2.5mhz. Wow. The normal pulses are between .1-35 milliseconds. But now I finally
got the scope to sinc up nicely to this odd pulse. So I adjust the other scope
channel to the signal that triggers the fet driver on the control board, and
it’s the same frequency, but only a 4v p-p pulse that I have not identified the
source. So the fet driver is actually being triggered to fire the 4 farad cap by
this micro tiny pulse. When I hit the foot pedal I get the correct pair of
pulses. So now I have found out why my spot welder probes are hot before I even
hit the foot pedal. This happened yesterday too! Some how this must have smoked
the fet driver. I whipped out my EMI probe and am now sniffing over the system
for a source that fits this pulse. The processor is 20mhz. A 4:1 harmonic
perhaps? I disconnected the fan and the lcd hoping they were involved, but no
such luck.

Oh brother, is this how I am spending my Friday night?

Party on Garth!

Here is the phantom pulse. Ch 1 is the output of the fet driver. Channel 2 is the input of the fet driver.