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#11
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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 |
#12
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Perhaps an evil spell??
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**** :-) |
#13
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Perhaps an evil spell??
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 |
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