Radio WTF

[bones]

Chook heres one to do your head in cause it has sent me to the drink, again,Microair 760/ uhf combo was fitted to last gyro i had, and was working perfectly as far as i know, removed the radios and wiring harness and gave the gyro away.decided to fit it all in to the ol faithfull so i was legal to fly into airports with a vhf, so i fits it all up, quick ground test showed all good, got a radio check from a plane over head said a 4 sitting on the ground near the hangar, not bad.decided to go for a fly to air test it all, after about 6 secs of trans the radio shuts off WTF, turn it off and back on again with same result so i heads back to get my multimeter(MM) thinking i got a bad voltage drop which is what shut off the radio.You got a drink yet?Now with motor OFF, MM reads 12.6v on the feeds to the radios, press the PPT on VHF and it shoots up to 24-27v, WTF, so i tested the UHF 12.5v press PPT it starts to slowly drop, so it all good, so the problem is in the VHF (or so i thought).So i come home to ring Microair, i had to tell him about 3 times as he couldnt believe it, no idea, but to try earths ect.Today i goes back out full of confidence with a different type of aerial to try also.Now today the voltage is only going up to 17v but now both the VHF and UHF are doing it, double WTFSo i first changed the aerials, no difference WTFi run a new wire for both + and - directly from the gyro battery instead of taking off where it was, still going to about 17v,, WTFSo i then went back to original aerial, and now the VHF has drop in V ONCE then jumping again and the UHF is jumping to 17V, WTFchanged back to the new aerial again, same as above VHF , UHF jumping, so it not the aerial Basically frigged around for about another hr trying other stuff to no good result, so i goes over to the LAME next door to ask him if he seen it before, the younger fella comes to look(not believing me) he left after 5 mins saying something to the effect of that sh*t shouldnt happen, as i was now worse jumping to over 30v.So the final straw was a test again and now the voltage is jumpimg to 33-35v on both radios, obviously i not holding the PPT on very long cause i dont want to toast couple of grand of radios.I promptly turned off everything and closed the hangar and drove to the pub got beer, so much for OcsoberChook if you have followed all that and you wish to comment i is listening(well reading)

[Brian]

I know absolutely nothing about radios Bones but I"m guessing the internals of one of the radios has gone berserk & is feeding various amounts of voltage indiscriminately back into the system.Pull the radios out, give them to a very good radio person to sort out. Actually, once you get them out connect up a temporary battery & give one radio a bit of a shake while you have the multimeter hooked up & see what happens.Remembering years ago I took off & all of a sudden there was an awful noise on the UHF, just about to go crook & I sees the high voltage thing flash up & it disconnected at 20 odd volts & the GPS had also quit so I landed & found the alternator had gone stupid. Put a new alternator on & all went back & to normal.Nothing to do with this but I hear a new noise from the UHF the other day & I turned the engine off....stilll there, pulled the positive wire on the leckiy & noise stopped !! Lecky is still working but I"m guessing he"s not well.

[chook]

You guys really need a radio tech over there don"t you?OK there are only 2 ways that you can be getting a higher voltage than what your battery is supplying.One is what Chopper is suggesting and that will be from the alternator. What sort of alternator are you running?The second is that the earth on your radio is resistive and the output from the antenna is somehow feeding it in. Remember that the rear chassis of your radio should have a screw connection for a chassis earth. This is radio frequency specific and should ideally be a braid connection back to your gyro earth point. The braid I use is the same as the outer conductor on the coaxial cable that would be feeding your antenna. It is made up of many wires and forms a "knitted sock" around the middle conductor.You mention that

[bones]

Chook,glad you caught this thread.Now i have fitted this exacte type of set up on at least 8 gyros and never a problem and this systen was working prior to removing so it got me buggered.Yes motor OFF so it nothing to do with voltage supply.Microair assure me there is nothing within the radios(if shagged) could be doing this.The problem is not consistant it keeps changing, yesterday it was only 24v this morning it 17v and this arvo it was 30+v. Also the UHF was not doing it then it was, then the VHF wasnt doing it now it is again, now they are both doing it everytime.the case of the VHF is definately earthed checked that.the aerial i dont think could be doing it even the dude at microair said this, no voltage goes to the aerialMM are digital, i even borrowed the LAME MM that was still in wrapping from being calibrated.Tomorrow i am going to pull it out of the gyro and bench test it, if it still plays up it goes back to Microair to see if they can work it out, but i near guanantee it wont do anything wrong once it gets there

[chook]

Bones exactly where are you measuring this voltage?So you turn the master on, radio on and then what do you do?Is there anything else operating? Fuel pump? strobe? Thinking about it both of those items can produce back EMF but again there would have to be a faulty earth or resistive power feed for that to happen. All voltages larger than about 13v should be absorbed by the battery.Regards.........Chook.

[mad max]

I"ve just skipped through this conversation and could be talking crap but through experience I"ve had transmitting RF jump into my multimeter and give an inaccurate false high reading when the meter is close to the RF -- I"ve even had RF interfere with an ECU and stop the engine in flight while transmitting in a sparowhawk.

[chook]

No Max you are not talking crap and that was why I asked what type of meter he is using. The digital meters are particularly sensitive to RF if they are not shielded correctly. People really don"t appreciate the power coming out of our little antennas!As you know its difficult trying to diagnose remotely. I will keep asking questions to try to reduce the search area then it will probably be "piccie" time.Regards......Chook.

[Disco]

The last two posts say what"s important IMO. The battery would absorb any spurious emf so if the voltage was really high, there would probably be enough current into the battery to blow the fuse and the new invention of a power generating radio is born.

[aussie_paul]

We had a 503 in a single seat once that when the PTT was used the engine rpm gauge dropped 400 rpm. RF can have an effect on various electronics.Aussie Paul.

[mad max]

I"ve always believed that it"s the very high frequency rather than high energy that upsets the multi-meter. The meter gets brain damage because it"s not designed to read such high frequencies. The meter becomes a "signal strength meter"

[birdy]

Bones, look closely at the MM wen you test it and be sure there aint a small MV after the readout.Iv had plenty of WTF moments too, only to see I was readn MV, not V.

[MilesW]

I"ve just skipped through this conversation and could be talking crap but through experience I"ve had transmitting RF jump into my multimeter and give an inaccurate false high reading when the meter is close to the RF -- I"ve even had RF interfere with an ECU and stop the engine in flight while transmitting in a sparowhawk.Another vote for RFI.I have a digital altimeter that sometimes has me "climbing" at about 6000 feet a minute when I am using UHF. Even on the ground parked up???Doesn"t appear to have damaged the altimeter. Miles