Jörgen,
Your Mark-V is probably similar to my FT-990 in that there is one 0.01 uF/2
kV capacitor connected from each of the "hot" power line wires to chassis
ground where 230 V enters your power supply. The protective ground pin,
instead, goes direct to chassis ground. The capacitors act as a voltage
divider and if you don't have any ground connection, the chassis potential
will be at half of the nominal AC voltage, i.e. 115 V. You can't get much
current out of the capacitive voltage divider, but you can feel that the
voltage is there and it is the voltage which kills the COM-chips.
The capacitors are on the power network side of the switch so it is enough
that the power cable is plugged into the wall outlet. Also, it does not help
to turn around the plug, because the capacitors will always make sure that
chassis ground stays at half the nominal voltage.
Since I think that the capacitors are necessary to keep RF from the
transceiver from reaching the power network, make sure that common chassis
ground is established before the radio is connected to the AC power, as you
have learnt the hard way.
You are not alone. I lost a couple of COM-chips for my Commodore 64 when I
was writing early CAT control software for FT-757GX and FT-767GX around
1985...
73,
Kjell
PB3SM - SM6CPI
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