Thorp Air Command - T18.net

I recently picked up a t18 in Phoenix and have been trying to get the radio working better. It has/had an ignition noise in the radio that was highly aggravating. It is not an alternator whine, nor strobes etc. all of that was checked and shut off during running to check it out. I have put ferrite beads on p-leads, antenna wire, alternator wire. I disconnected the key switch ground going back to the bus. Pleads are now grounded back to the mag housing with the key switch being used to open and close the connection. That seemed to help some, but the ignition noise is still breaking through the squelch. At altitude I switched between mags to try and isolate it. On one mag the noise went away, but a different (higher pitched noise not heard while switch was on both) was heard. When going to the other mag I did hear the ignition noise. I have 3 possibilities. One is bad ignition wires(they appear to be older and are quite limber but I have no way of testing them. 2. Mag itself is going bad. 3 possible antenna /wire or both is causing this.
During each "noise "interim I could see the RX light come on and I noticed when I hit my PTT button it would stop. Sometimes it took several pushes but it consistently would stop the noise and the RX light would go off. I am being heard loud and clear and my reception is good on close transmissions.
Before I just start throwing money at it does anyone have ideas?
Mark

Ask a local A/P to test the plug wires
Cubes

Plug wires then mag capacitors.

_________________
Bob Highley
N711SH
SN 835
KLAL

Tx-Swordguy,

Cubes and Bob both make super good suggestions, I would follow up on as I trust their knowledge and experience.

One other crazy thought for you to consider, but I may have misunderstood you. But risking fear of pride for showing my naivety, I will throw out a possible wild goose chase layman's idea to consider.

Are you saying that a transmit light (RX) is on when you hear the noise? But when you hit the PTT button to transmit, the noise will go away. And if you play with the PTT enough times, you can occasionally get the noise to disappear? If so, then perhaps it is a bad PTT button. My crazy thought is that the PTT is NOT behaving binary like it should. That is; either 100% off, or 100% transmit mode.

"What if..." the PTT button is really say 99% off instead of 100% off, This might be enough to indicate "RX" yet not truly be transmitting. But when you positively engage the PTT it is 100% transmit. The intermittent ability to get the noise to go away would be explained when you randomly push the PTT enough times, it occasionally goes to 100% off. and that is when the noise disappears. But in the more commonly 99% state it causes the noise. But occasionally you will get it to go to 100% off, and the noise goes away.

Is it possible to replace the PTT button or otherwise isolate/disconnect the PTT to see if the "ignition" noise goes away and that the PTT is the culprit?

I was able to borrow an ignition cable tester so will give that a try and report back.
Flyingfool= The radio has both a receive indicator RX, and a transmit indicator TX. When the noise breaks through the RX indicator comes on as if I am getting a signal from another plane. WHen I hit the PTT button the RX indicator goes off during the transmission and the TX indicator comes on. I can consistently get the noise and the RX indicator to go off (it may take 2-3 pushes of the PTT). the noise and RX comes back sometimes after getting a transmission from another plane but mostly on its own. The last flight the noise and RX would be intermittent. When I first bought the plane it was constant so it has improved, but not completely gone away.
thanks for the help

Something else to consider. Does the aircraft you bought have any accessory wiring connections, like a cigarette lighter adaptor to USB charger or other(?)? I have read some people have had some issues and possibly some ghost Rx indications/interference from these accessories.

Something else to check. Maybe unhook everything/pull fuses or breakers etc. and see if the problem changes. We test every new wire installed in our aircraft at work during new installations/STC's even if it is a dormant/provisional wire that is not hooked up on either end due to EMI/EMC related interference.

Worth a shot.

Rob

I tested the ignition cables. All tested ok however one was not giving as strong an arc in the test window and even appeared to "sputter" a bit (the tester never completely failed that wire just sputtered and not as strong and arc). I tested it several times and it consistently did this as opposed to the rest that gave a good strong, steady arc. I forgot to OHM test the right side plugs when I had them out to clean and gap but I tested the Champion plugs on the left and 1=5.4k , 1=9.1k and 2 at 21.1K ohms. The wire in question is also where one of the 21,1K plugs were. I am wondering if the combination of the 2 items are causing the voltage to exit and get picked up by the radio. I did start noticing some very slight missing last time I flew intermittently. It starts, runs strong (until the 2 times I felt a miss).
Mark

I bought all new tempest plugs. That is the cheaper option at this point . All plugs ohmed greater than 5k so will see if that helps. Hopefully that will keep the voltage where it should be going and hopefully reduce the ignition noise.

Are the wires to the headset and mic jacks shielded with the shield connected only at the radio/intercom end?

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA