XCOM 760 Radio

[chasers03]

My Lambada XCOM 760 Radio is giving me fits as it refuses to transmit but receives well. I have had it repaired twice over the years and this time I can't make it transmit at all. My antennae is working fine with my handheld connected and I have had it checked for proper VSWR.

Anyone got an XCOM 760 Radio? Steve Chase 864 903 5493

[chasers03]

My Lambada XCOM 760 Radio is giving me fits as it refuses to transmit but receives well. I have had it repaired twice over the years and this time I can't make it transmit at all. My antennae is working fine with my handheld connected and I have had it checked for proper VSWR.

Anyone got an XCOM 760 Radio? Steve Chase 864 903 5493

[Thermalseeker]

High SWR won't stop it from transmitting, the signal will just be attenuated. If you're sure it's not transmitting when you key the radio I would look for a busted solder joint or a broken wire on the transmitter key pin. IOW, something is preventing the transmitter from keying. This problem could be at the PPT switch solder joint(s), too, if you have a PPT switch on the stick(s). There might be a Molex plug (white plastic, 2 pins, quick disconnect) in the wires coming to the PPT switch, usually found at the base of the stick. These plugs are sometimes installed so the stick can be removed without having to remove the switch, too. If it has a Molex plug in line, check for cold or broken solder joints, and/or broken wires there as well. It could also be the switch itself. A quick buzz with an ohm/continuity meter between the transmit pin on the back of the radio and the PPT switch will tell you if you have a broken wire or bad solder joint. One side of the switch will be wired to the transmitter key pin on the radio and the other side should be ground. You should check continuity to both the radio, and from the switch to a known avionics ground (chassis of the radio is a good spot) You should also test the function of the switch. Most PPT switches will be normally open. When you mash the button you are closing the switch which, in turn, grounds the transmit key pin on the radio. The manual that came with the radio should have a schematic that will show the pin functions where all the wires mate to the radio. It's possible the radio could be at fault. Xcom doesn't have a very good reputation in the Experimental realm from what I've read and heard. Sounds more like a wiring or switch problem from your description, though. Certainly worth checking and verifying the wiring and switch before you go to the trouble of pulling the radio and sending it in for repair.

Edited August 26, 2014 by Thermalseeker

[Thermalseeker]

High SWR won't stop it from transmitting, the signal will just be attenuated. If you're sure it's not transmitting when you key the radio I would look for a busted solder joint or a broken wire on the transmitter key pin. IOW, something is preventing the transmitter from keying. This problem could be at the PPT switch solder joint(s), too, if you have a PPT switch on the stick(s). There might be a Molex plug (white plastic, 2 pins, quick disconnect) in the wires coming to the PPT switch, usually found at the base of the stick. These plugs are sometimes installed so the stick can be removed without having to remove the switch, too. If it has a Molex plug in line, check for cold or broken solder joints, and/or broken wires there as well. It could also be the switch itself. A quick buzz with an ohm/continuity meter between the transmit pin on the back of the radio and the PPT switch will tell you if you have a broken wire or bad solder joint. One side of the switch will be wired to the transmitter key pin on the radio and the other side should be ground. You should check continuity to both the radio, and from the switch to a known avionics ground (chassis of the radio is a good spot) You should also test the function of the switch. Most PPT switches will be normally open. When you mash the button you are closing the switch which, in turn, grounds the transmit key pin on the radio. The manual that came with the radio should have a schematic that will show the pin functions where all the wires mate to the radio. It's possible the radio could be at fault. Xcom doesn't have a very good reputation in the Experimental realm from what I've read and heard. Sounds more like a wiring or switch problem from your description, though. Certainly worth checking and verifying the wiring and switch before you go to the trouble of pulling the radio and sending it in for repair.

Edited August 26, 2014 by Thermalseeker

[Eric Greenwell]

My Lambada XCOM 760 Radio is giving me fits as it refuses to transmit but receives well. I have had it repaired twice over the years and this time I can't make it transmit at all. My antennae is working fine with my handheld connected and I have had it checked for proper VSWR.

Your radio must have a "transmitting" indicator. Is that displayed when your press the mike button? If so, the button and wiring are fine, and next is being sure it's not transmitting: are you certain the receiver you are using to listen with is working? If you are certain, then it's almost certainly a problem with the XCOM.

Edited August 31, 2014 by Eric Greenwell

[Eric Greenwell]

My Lambada XCOM 760 Radio is giving me fits as it refuses to transmit but receives well. I have had it repaired twice over the years and this time I can't make it transmit at all. My antennae is working fine with my handheld connected and I have had it checked for proper VSWR.

Your radio must have a "transmitting" indicator. Is that displayed when your press the mike button? If so, the button and wiring are fine, and next is being sure it's not transmitting: are you certain the receiver you are using to listen with is working? If you are certain, then it's almost certainly a problem with the XCOM.

Edited August 31, 2014 by Eric Greenwell

[Thermalseeker]

Which is why I prefaced my commentary with "If you're sure it's not transmitting when you key the radio"..........

[Thermalseeker]

Which is why I prefaced my commentary with "If you're sure it's not transmitting when you key the radio"..........

[chasers03]

Thanks guys for your good input. I had sent the radio to XCOM's recommended repair man in Kokomo IN, Bob Japundza and for $150 he replaced the EFT which I believe is the modulator part of the radio? (BTW, This same repair was done to the radio 4 years earlier) He and the two XCOM factory (Michael & Ken) folks said that it was likely my antennae causing unusually high VSWR and blowing out the EFT. So before replacing the repaired radio I had the VSWR checked out using my ICOM handheld and then the XCOM. That repair person said the ratio was very low meaning there was low power coming back into the radio and my antennae in the tail was declared OK. (I have been using it with my ICOM handheld very successfully)

I then reinstalled the repaired XCOM and it did not transmit as before. Bob. The repairman said he was sure he sent me a working radio but to return it to him anyway. After he received it he strangely stopped responding to my inquiries about when I could get it back after he had it for two weeks. With pressure from XCOM folks and me he sent it back again and as before with no paperwork or indication in the box of what he did to it. I tried it and after three good transmissions it again failed to transmit as before. As Thermal Seeker recommended, I then had a local avionics fellow fabricate a complete new wiring harness (PTT and all) for bench testing including another antennae. This resulted in no change to the XCOM's failure to transmit. Meaning that my Lambada wiring is likely not the culprit.

So I have now received my new TRIG TY91 radio (which mates well with my TRIG Transponder) and the new harness is being fabricated as I write. Will report here if it works as expected or not. Steve N24PV

[chasers03]

Thanks guys for your good input. I had sent the radio to XCOM's recommended repair man in Kokomo IN, Bob Japundza and for $150 he replaced the EFT which I believe is the modulator part of the radio? (BTW, This same repair was done to the radio 4 years earlier) He and the two XCOM factory (Michael & Ken) folks said that it was likely my antennae causing unusually high VSWR and blowing out the EFT. So before replacing the repaired radio I had the VSWR checked out using my ICOM handheld and then the XCOM. That repair person said the ratio was very low meaning there was low power coming back into the radio and my antennae in the tail was declared OK. (I have been using it with my ICOM handheld very successfully)

I then reinstalled the repaired XCOM and it did not transmit as before. Bob. The repairman said he was sure he sent me a working radio but to return it to him anyway. After he received it he strangely stopped responding to my inquiries about when I could get it back after he had it for two weeks. With pressure from XCOM folks and me he sent it back again and as before with no paperwork or indication in the box of what he did to it. I tried it and after three good transmissions it again failed to transmit as before. As Thermal Seeker recommended, I then had a local avionics fellow fabricate a complete new wiring harness (PTT and all) for bench testing including another antennae. This resulted in no change to the XCOM's failure to transmit. Meaning that my Lambada wiring is likely not the culprit.

So I have now received my new TRIG TY91 radio (which mates well with my TRIG Transponder) and the new harness is being fabricated as I write. Will report here if it works as expected or not. Steve N24PV

[Thermalseeker]

That seems to be a common problem with the XCom's from what I read. I think you did the right thing switching radios. FWIW, the "Microair" radios aren't any better. We had 5 of them on the field here back when I was towing gliders including one in my glider. Every one of them made at least one trip to the factory and most had gone back twice or three times.

I'm an RF Engineer by trade. The general rule of thumb for SWR is 1.5 to 1 or less. I use what is known as a "Time Domain Reflectometer" or "TDR" as we call it when testing for return loss. This device "sweeps" the feed line from the transmit end to the antenna or to a precision load installed at the antenna end, over the frequency band being used. The TDR goes one better than a typical SWR tester as it tells me in feet and inches the distance to each reflection on the line. So, if you have a kink, or a bad connector or water in the line, or if there is a problem with the tuning on the antenna the TDR tells you right were to look. It graphically displays the amplitude of each reflection, too. Very handy when you're trying to sort this type thing out.

[Thermalseeker]

That seems to be a common problem with the XCom's from what I read. I think you did the right thing switching radios. FWIW, the "Microair" radios aren't any better. We had 5 of them on the field here back when I was towing gliders including one in my glider. Every one of them made at least one trip to the factory and most had gone back twice or three times.

I'm an RF Engineer by trade. The general rule of thumb for SWR is 1.5 to 1 or less. I use what is known as a "Time Domain Reflectometer" or "TDR" as we call it when testing for return loss. This device "sweeps" the feed line from the transmit end to the antenna or to a precision load installed at the antenna end, over the frequency band being used. The TDR goes one better than a typical SWR tester as it tells me in feet and inches the distance to each reflection on the line. So, if you have a kink, or a bad connector or water in the line, or if there is a problem with the tuning on the antenna the TDR tells you right were to look. It graphically displays the amplitude of each reflection, too. Very handy when you're trying to sort this type thing out.

[chasers03]

This is an update on my XCom radio replacement with a Trig TY91. While the new radio was being shipped we made up a complete new wiring harness including mike jacks, push to talk switches and used my existing antennae only to find that the XCom still would not transmitt. The new Trig was then connected to the XCom plug and existing harness with an additional wiring harness and the installation went without a hitch. I have made local flights and one round trip (87 nm one way) to Atlanta's PDK using flight following all without a communication problem. So as Thermalseeker say's "Xcom doesn't have a very good reputation in the Experimental realm from what I've read and heard". I am further disappointed that XCom's Indiana repair man Bob Japundza demonstrated such low integrity as to charge me $150 for a repair he did not do.

Thermalseeker maybe someday I will pay you a visit over the mountains and learn how you use a Time Domain Reflectometer. Thanks you guys for your responses. I wish we had more sharing of whats happening with our flying motorgliders on this forum.

BTW I am rethinking my ownership of a motor glider vs a faster more comfortable LSA. The Bristill and the Phoenix are top contenders at the moment.

[Michael-Coates]

Steve, I repaired your radio twice replacing the PD2505 FET. Doing so takes three hours of labor. Six hours of my time on your radio. It had a Narco service tag on the radio, so that is the third time that part was replaced as there was evidence in the radio of that FET being replaced. The second time I repaired your radio I left it on overnight and transmitted continuously for 10 second intervals at least ten times to check the thermal endurance of the FET.

The ONLY way the PD2505 will fail is high SWR, ie bad antenna.

How many hours of my time should I spend with someone who won't listen? It left here FINE AND IN WORKING ORDER, TWICE. So what do you do? Accuse me of not repairing it, and CCing the world. That's a great way to get someone who was trying to help you in walking away. Good luck to you sir, in the mean time I will gladly help others that need XCOM repairs as this is the only example of hundreds where the customer wasn't happy.

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Michael Coates here, the owner of XCOM Avionics and also the Pipistrel USA distributor. The only time we see failures as mentioned above is when people have a bad antenna. Please allow me to explain what happens, the XCOM VHF radio and in fact every VHF radio made is designed to operate with a tuned aerial.

This means an aerial which has a VSWR usually of 1.5:1 or lower which is ideal for the best airborne operations.

The XCOM VHF radio has 'protection' to allow a VSWR of up to 3:1 and still operate without failure. Any aerial with a VSWR higher than this will damage the radio.

It is unlikely to damage it in the first 10 seconds of transmission and is more likely to damage the radio in the first day/week/month of operation although sometimes this can be years based on how much you transmit because you are basically transmitting a signal down a piece of coax cable to be broadcast to the world and a lot of this signal power is getting reflected back into the radio which is a perfect way to blow up the radio.

To put it in simplistic terms it is like having your car engine running and you block the exhaust, the pressure builds up until it will eventually cause some problem with the engine and the engine will stop, It is just the same with any VHF radio.

If you have a bad aerial, you can go and put any other VHF radio on it and go transmitting but as I mentioned above it may transmit for the first day/week/month but it will eventually fail because of the bad aerial, it is normally not an instant failure. So to say 'I have used a handheld and it works or I have put another brand of radio on and it works' is grossly premature.

We know by the failed parts that this particular failure can only occur from a bad aerial (and its the same part that's failed 3 or 4 times - this makes it really obvious), the XCOM VHF radio is no more susceptible to bad aerials than any other VHF radio.

I am glad that in this case the customer has found a good TEMPORARY solution by replacing the radio but expect he will see another failure in the near future.

A bad aerial will damage any VHF radio just like you would not expect your car to operate normally if you welded the exhaust shut.

I hope these short ramblings help clear up a little bit of information and I also am happy to point you to a resource on our website if the forum allows links called the XCOM aerial dummies guide which explains in simplistic terms everything about aircraft aerials. Please let me know if you have any questions whatsoever. Many thanks Michael Coates

http://www.xcom760.com/education/XCOM-Aerial-Dummies-Guide.pdf

[chasers03]

Thanks Michael for your response. Adhering to your concerns, I again checked my antenna's SWR yesterday and it was slightly less than 1.2 to 1, the same as was measured by my local avionics shop two months ago when you alerted me to this possibility as the cause of radio's failure. I guess 'the proof will be in the pudding' as they say and if the Trig doesn't fail "in the near future" I'll know that I made the right decision. I do know for now that my Trig TY91 radio works well and also receives local airport frequencies from my Garmin 496, a feature I did not have before.

[Michael-Coates]

Thanks Michael for your response. Adhering to your concerns, I again checked my antenna's SWR yesterday and it was slightly less than 1.2 to 1, the same as was measured by my local avionics shop two months ago when you alerted me to this possibility as the cause of radio's failure. I guess 'the proof will be in the pudding' as they say and if the Trig doesn't fail "in the near future" I'll know that I made the right decision. I do know for now that my Trig TY91 radio works well and also receives local airport frequencies from my Garmin 496, a feature I did not have before.

The XCOM will also handle your frequencies from the Garmin and has done for about 10 years, you just need to wire it in. Please send through your antenna plot from 118.0 to 136.9 MHz for evaluation and the last calibration report for the VSWR unit so we can see whats going on. Thanks