On Mar 2, 2:02*pm, Andy wrote:
On Mar 2, 12:05*pm, Andy wrote:



On Mar 2, 11:25*am, seventripleone wrote:

On Mar 1, 2:35*am, Charlie Papa wrote:

I have a Discus 2cT with about 650 hours on it. *Schempp-Hirth did the
instrument installation, and a more professional looking job you will
not find. *But there was one problem that confounded even the avionics
guy: when the engine was running, pressing the push-to-talk button
would kill the engine until the button was released.

The engine's ignition is a very simple magneto (Solo 2350), and the
radio a Dittel FSG2T radio, - yes, the one with a recall. *How to
explain the ability of the radio to ground the magneto?? *No one even
had a theory. *But when I reinstalled the radio after the recall
service (which was very efficiently and courteously handled by Dittel,
BTW), the problem was gone. *I tried to induce the problem several
different days, but it exists no more.

So what flaw in this radio can cause such an effect? *A black curse?

No, the engine electronics will shut the ignition down when max. revs
for the prop are exceeded. The revs are present in the instrument as a
proportional voltage signal (1V = 1000 RPM) and i'd bet that somehow
the electronics got tricked into thinking that it was the case. A
voltage drop in the power supply from the battery during radio
transmission or an induced spike from the radio's RF power could have
been the reason.

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.

Anyone know the function of the problem radio capacitor?

Andy

I know only that the capacitor was installed incorrectly and that it
affected transmission output. Here's the link:

http://www.eaa.se/dokument/tekniska/...tel_SB_FSG2T-1...

9B

Our company makes a very good living tracking down electromagnetic
compatibility issues like this. Radio frequencies impinging on
unshielded control circuits can end up being rectified and appearing
as a spurious dc signal. We've seen this a lot in electrical control
and industrial process control equipment (a few years ago I watched a
robot throwing gearboxes across a car assembly plant instead of
handing them gently to the next robot in line!). I would strongly
suspect something similar is happening here - 120 MHz is a great
frequency for the purpose.

Mike