jasona900 Posted March 16, 2018 Report Posted March 16, 2018 So I have been trying to resolve this for a week with no luck.. Time to call in the masses for help. I picked up my '78 J from annual a week ago from today. Flew up with a buddy in his plane, and then flew home that afternoon about 10 miles in trail. While talking to him using my GNS-480, I noticed every time I transmitted, the values displayed on my Electronics International UBG-16 engine monitor would fluctuate. All of the values displayed would go down. The graphs would all go down. As soon as I released the push to talk, the values would return to normal. This happened regardless of which cylinder EHT/CHT value I had displayed. If I had OAT, oil temp, voltage...any value displayed would decline when I hit the PTT. I was already well beyond the halfway point by the time I noticed this, or I would have turned around and gone back to the shop. After some troubleshooting on the ground, here is what I know: - It does NOT do this when I transmit on my COMM 2 (SL-30) - It DOES do this even with the engine off... so I have determined it is not interference coming off the alternator or mags - I looked under the panel and could not find any loose ground wires - I checked both comm antennas and the cables are securely fastened to the antennas - I do not know what or if anything was done behind the panel during annual - I have called and emailed the shop that did the annual, and they still have not gotten back to me - There IS foil wrapped around the cables going into the UBG-16. I intend to add extra foil around the cables this afternoon, but the existing foil does not look like it was disturbed during annual. Adding extra foil may just hide an actual issue.
N201MKTurbo Posted March 16, 2018 Report Posted March 16, 2018 I would suspect it is more of a grounding or power problem than an RF issue. I could be wrong. I would start by putting a meter on the points where the UBG-16 is grounded and where it is powered. Key the mike and see if the voltage changes. Make sure the UBG-16 is solidly grounded. If it is not to difficult, swap the comm antennas between the two radios and see what happens. 1
jasona900 Posted March 16, 2018 Author Report Posted March 16, 2018 1 hour ago, N201MKTurbo said: I would suspect it is more of a grounding or power problem than an RF issue. I could be wrong. I would start by putting a meter on the points where the UBG-16 is grounded and where it is powered. Key the mike and see if the voltage changes. Make sure the UBG-16 is solidly grounded. If it is not to difficult, swap the comm antennas between the two radios and see what happens. My initial thought was a grounding issue as well. That could still be the case.. but I also figured that if it was in fact a grounding issue, then my COMM #2 would likely affect it as well when the mic is keyed. I don't have the differences in power output between the two radios in front of me.. but my guess is that I would see even some sort fluctuation with the 2nd comm keyed. I plan on spending tomorrow morning on continued troubleshooting.
FBCK Posted March 16, 2018 Report Posted March 16, 2018 (edited) My UBG-16 does the same thing and its been doing it for years. I had the UBG-16 sent back a number of years ago for a new screen, which I understand they basic gut the thing and send it back to you, didn't change anything with regard to the on and off when transmitting. Edited March 16, 2018 by FBCK
jasona900 Posted March 16, 2018 Author Report Posted March 16, 2018 4 minutes ago, FBCK said: My UBG-16 does the same thing and its been doing it for years. I had the UBG-16 sent back a number of years ago for a new screen, which I understand they basic gut the thing and send it back to you, didn't change anything with regard to the on and off when transmitting. That is interesting.. did it do it with both radios, or just with one?
FBCK Posted March 16, 2018 Report Posted March 16, 2018 I cant remember if its on both radios or not. I will be out there next week, Ill make a point of looking at that though. Ive also had most of my avionics done since then, a new IFD540, new audio panel with no change. I also remember the avionics guy telling me he cleaned up the wires on the UBG at that time, no change.
jasona900 Posted March 16, 2018 Author Report Posted March 16, 2018 I talked to an avionics guy the other day, he mentioned that since the wires going to the UBG are not shielded, a potential fix is to wrap the bundle in foil. Mine is wrapped, but not completely. May be worth sticking your head under your panel to check. I am going to add more foil tomorrow. A pretty cheesy fix..but if it works, I'll take it.
Piloto Posted March 16, 2018 Report Posted March 16, 2018 (edited) I installed a UBG-16 myself on my M20J and never had a problem. And my #1 COM is 20 Watts (GNS 530AW) and the HF is 100 Watts. The interference problem with the COM could be related to the antenna location of each COM. If COM 1 uses the forward antenna then it is likely RF is leaking into the UBG-16 through the plastic cowling into the CHT/EGT wiring or through the plastic glareshield. Try contact cleaner on the UBG-16 connector. José Edited March 16, 2018 by Piloto
jasona900 Posted March 17, 2018 Author Report Posted March 17, 2018 Thanks Jose. I’m going to re-check the connections in the morning.. I don’t think it is the antenna location, as I didn’t have this issue before the annual. They did remove the interior to conduct the corrosion inspection on the steel cage. So I guess there is a chance the antenna cable got nicked.. but I think those chances are low, but possible. I’ll fly it back to the shop before I tear out the interior to find out.
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