On Mar 4, 3:27*am, seventripleone wrote:
On Mar 2, 9:05*pm, Andy wrote:

Anyone that designed a system to transmit a critical parameter as a
fraction of a reference voltage but did not also provide the reference
voltage *to the receiving system would be better employed flipping
burgers. *In other words, a properly designed engine speed monitoring
system would be independent of the battery voltage.

RFI could have strange results though. *Maybe the incorrectly
installed capacitor allowed RF on the radio power line and that
coupled into the engine monitor.

I'd generally fully agree with you on this one, but there is no signal
outside of the electronics box (case mostly metal and made by Ilec
BTW). Except that you can connect an external voltmeter to act as RPM
indicator on older units and on new ones there is perhaps an internal
DVM module to do the same.

The box senses the RPM as ignition pulses via the earthing lines from
the two ignition modules (coils) and does the conversion internally.
Depending on said signal three LEDs are lit to indicate in which PRM
range (green, yellow, red) the engine is turning and the ignition is
switched off (earthed) when a threshold of ~7000 RPM is exceeded.

The above does contain some informal reverse engineering concerning
the internal workings of the box on my side to be honest but i'd
happily buy you all a round if i'm wrong.

(IMHO it's much better to try to contain the RF in the radio and
antenna cable than try to harden everything in the glider against it.
Otherwise i'd end up with tinfoil around my b**** :-)

Thanks for the replies, - at least the reasoned and helpful ones. I
am at Seminole Lake and spent a half hour yesterday with the very well
informed TA discussing the mystery. Additional information, that may
be relevant:
• I don't know if the problem has existed from the beginning, as with
the engine running I can't hear myself think. It was only recently,
while flying with a friend, I got low and advised I was going to light
the engine. He asked me to advise him when it was running, and so
even though I knew I would not hear a reply, when it lit as it very
reliably has, I pressed the PTT and had an Oh Sh*t moment. I released
the button and the engine caught again. I again tried to transmit,
and the engine died. The following day I verified while just off tow
that each push killed the engine.
• I did not look for the effect on the LED's which might have been
showing an 'overspeed' signal
• I tried the PTT when the radio was removed for the recall, and there
was no effect
• I don't have the knowledge to trace ground leads and the like, and
so did not

TA theorized, if I can explain it accurately, that RF interference was
picked up by the wires from the rev sensor and the sensing unit
interpreted these as excess RPM, and grounded the magneto. This is
similar to the speculation from seventripleone. 711 has offered me
his unrepaired radio to try to again induce the problem, and eliminate
the possibility that it arose from frayed wires in the harness.

Me, I like the explanation that I let the smoke out. Thanks all.

Charlie Papa