For the electronics experts here

Well I had an old student that just got an education in radio shops. He
has an older 172. He took the airplane in for a pitot static check and to
certify a transponder. Spent all day there. First is that they came back
and said he had a blind encoder that was bad, even though he got no
complaints from controllers on the way in.

When he got back in it to fire the airplane up, BOTH radios were also dead.
They quickly looked at it and found a burnt out diode in one on the bench
and the other they kept to check.

All this is HIGHLY unlikely without some help. Then I asked does the
airplane have an external plug for ground power. hmm He said yes. Its an
older 172 with a 12 volt system, but I suspect the shop hooked it up to a 24
volt ground power unit.

Would these symptoms be consistent with a young kid hooking a GPU up at too
high of voltage? Hes learning that many shops you are lucky to get your
airplane back undamage, even if they don't fix what you took it there for.
Thanks in advance.

Mark