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Perhaps an evil spell??
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 |
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