dmmartindale@watcgl.UUCP (Dave Martindale) (08/13/85)
In article <4134@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes: > >For a fairly nominal charge, Sigtronics will modify their intercom >so that it will interact nicely with a yoke-mounted PTT switch. If >it is not so modified, anyone who is talking when the switch is down >will transmit. If it IS modified, pressing the PTT switch will >connect the pilot's mike to the transmitter and shut off everyone >else's mike. A toggle switch decides whether the pilot is the person >occupying the left or the right seat. A second toggle switch allows you >to select whether you have a yoke-mounted PTT switch or a portable one. Yes, I can even guess how this is done - just taking the mike keying signal from the intercom's mike output cord and routing it to either pilot or copilot PTT jack inside the intercom. But this lets you have only *one* pilot capable of transmitting. I sometimes fly with two pilots and have both pilots using the radio at different times, so I considered this unacceptable. Perhaps I should have spelled that out. All of the intercoms I discussed can handle two pilots. Unless I'm mistaken, the Telex and Sigtronics cannot be used with two PTT switches unless both are portable ones plugged into the intercom - there is, after all, only one mike cord. The other three intercoms have the ability to make use of *two* yoke-mounted permanently-installed PTT switches, one for each pilot, if the aircraft is so equipped. Or you can use a permanent PTT on the pilot's yoke and a portable PTT on the copilot's. The Sigtronics cannot do this. Only you can decide if the increase in flexibility of the two-mike-cable intercoms is worth the increased wiring mess in the cockpit. >> Designed so that pilot's >> headset is connected directly to the radio when power switch is >> turned off - provides some failsafe protection, but pilot must >> remember to turn the switch off if intercom power fails. > >You do not need to turn the switch off if intercom power fails. >There's no relay or anything like that: the headsets are always >coupled directly to the radio for receiving, period. You use the >radio's volume control to determine how loudly the radio plays in >your headsets. You use the intercom's volume control to determine >how loudly the copilot's and passengers' voices are transmitted. Yes, the headset is passively connected all the time, but when intercom power fails you lose your mike (or so the Sigtronics brochure seems to imply). So you can hear the radio but can't transmit - hardly "safe". Sigtronics arranges that when you turn the power switch off, the pilot's mike and PTT switch are directly connected to the mike output cord allowing trasmission, but you do have to remember to do this. The Telex performs the same switching automatically on loss of intercom power, requiring no action on the part of the pilot. The Oregon Avionics intercom provides the same sort of manual failsafe provision. The RST and SoftComm intercom have no such provisions, but they can be powered by an internal battery. In the case of the RST, and possibly the other, the battery will take over unnoticeably if power via the lighter socket fails. Thus by always having a (fresh) battery in the intercom, you are protected against an intercom power failure, but not electronic malfunction of the intercom. Dave Martindale