Anybody have experience along these lines?
I had used KX-170B, KX-155 and KMA-24 audio panel installed in my '54 model to replace unuseable Edo-Aire's. Everything works great until the engine is started. I hear a whine that goes away when I turn off the Alternator. I understand that. But the squelch is frequently broken when receiving and there is definite engine noise in the background. I have been told that my transmissions are 5 x 5.
A mechanic told me that it could be from the engine itself not being grounded, otherwise, it is a magneto/wiring harness problem. Any advice on the most (cost)effective way to troubleshoot this?
Also, any opinions about the best shoulder harness vendor out there - B.A.S. or some other?
Bruce Hain
4234N
26263
Radio background noise/harnesses
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
There should be Gnds at the engine mount isolators to GNd the engine to the mount and if you're using rubber engine mount to firewall isolators then you definitely need grounds from the mount to the firewall. There's also a large GND strap from the firewall on the R/H side that's bolted to the engine, ensure that its not corroded at the connections.
I don't think that you can have too many Gnds. I've also added an filter (TSO'd) on my alternator output. I think you also have to ensure that the audio jacks are not grounded to the airframe (those pesky little plastic washers with the shoulders are correctly installed) to ensure a good audio signal.
I don't think that you can have too many Gnds. I've also added an filter (TSO'd) on my alternator output. I think you also have to ensure that the audio jacks are not grounded to the airframe (those pesky little plastic washers with the shoulders are correctly installed) to ensure a good audio signal.
while the engine is running and radio noise is present - turn off things one at a time to isolate where noise is coming from. alternator, each mag one at a time, strobes, transponder, one comm at a time, whatever else you have. use breakers if no switch. if suddenly the noise goes away, yippee - now you can hunt it down.
if mags, likely a harness issue - check all the ignition wires for breaks in the shielding, especially where they rub against stuff and where they make tight bends, like at the plugs. have a wire replaced if bad, don't try elec tape. make sure the wire shielding is grounded well inside the mag harness cap - there is a tight-fitting "ferrule" in there that holds the wire to the cap and grounds the shield at the same time. look at the ends of the springs on the ign wire ends where they fit into the plugs - corroded or burned? clean them with a file until shiny and see if that helps, if too worn have them replaced. also there should be a ground wire or a similar press fit deal for your shielding on the p-lead wires. sometimes there is a capacitor in line there, for noise suppression. maybe it wore out? how old are your plugs - the little noise resistors in one or more of them could have just given up.
alternator - shielded wires again, good connections at the alternator and breaker/regulator/switch? worn brushes internally? could be arcing in there - have the alternator checked if so, slip rings probably are burned/pitted/out of round. bad diode in the rectifier? often makes a whine, also reduces output.
anything else, including other avionics - clean connections, check shielding, make sure transponder antenna has good ground, etc.
engine grounding - should be a strap around one of your engine mounts to pass ground from engine to mount, another to pass from mount to firewall - make sure they are in good shape and clean. sometimes braided, sometimes just a strip of aluminum.
battery connections - clean these too, and the master contactor connections.
good luck - a lot of times newer radios are more susceptible to noise sources than older stuff because they are more sensitive, which might be why the problem just showed itself.
Hans
if mags, likely a harness issue - check all the ignition wires for breaks in the shielding, especially where they rub against stuff and where they make tight bends, like at the plugs. have a wire replaced if bad, don't try elec tape. make sure the wire shielding is grounded well inside the mag harness cap - there is a tight-fitting "ferrule" in there that holds the wire to the cap and grounds the shield at the same time. look at the ends of the springs on the ign wire ends where they fit into the plugs - corroded or burned? clean them with a file until shiny and see if that helps, if too worn have them replaced. also there should be a ground wire or a similar press fit deal for your shielding on the p-lead wires. sometimes there is a capacitor in line there, for noise suppression. maybe it wore out? how old are your plugs - the little noise resistors in one or more of them could have just given up.
alternator - shielded wires again, good connections at the alternator and breaker/regulator/switch? worn brushes internally? could be arcing in there - have the alternator checked if so, slip rings probably are burned/pitted/out of round. bad diode in the rectifier? often makes a whine, also reduces output.
anything else, including other avionics - clean connections, check shielding, make sure transponder antenna has good ground, etc.
engine grounding - should be a strap around one of your engine mounts to pass ground from engine to mount, another to pass from mount to firewall - make sure they are in good shape and clean. sometimes braided, sometimes just a strip of aluminum.
battery connections - clean these too, and the master contactor connections.
good luck - a lot of times newer radios are more susceptible to noise sources than older stuff because they are more sensitive, which might be why the problem just showed itself.
Hans
'56 "C170 and change"
'52 Packard 200
'68 Arctic Cat P12 Panther
"He's a menace to everything in the air. Yes, birds too." - Airplane
'52 Packard 200
'68 Arctic Cat P12 Panther
"He's a menace to everything in the air. Yes, birds too." - Airplane
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cessna170bdriver
- Posts: 4063
- Joined: Mon Apr 22, 2002 5:13 pm
I installed the mount on my overhauled engine today and installed the ground straps between the engine and mount. I used strips of sheet aluminum about 3 inches long and an inch or so wide with a 7/16 hole in one end for the engine mount bolt and a 1/4 hole in the other end for one of the clamp bolts in the engine bracket. I use similar straps for the firewall isolators, only they have 3/8 holes for the bolt in one end and 5/8 or so for the underside of the isolator. I thought about using braided ground straps, but have had trouble finding connector lugs for the 7/16 bolts. I've been using this setup since 1990 and have never had an electrical noise problem.c170b53 wrote:There should be Gnds at the engine mount isolators to GNd the engine to the mount and if you're using rubber engine mount to firewall isolators then you definitely need grounds from the mount to the firewall.
So far I've been able to get away without a large strap from the firewall directly to the engine, although it's not a bad idea. My Jasco alternator does have a dedicated ground post that I wire directly to the firewall at the battery ground strap with about #16 (not sure of the size) wire.c170b53 wrote:There's also a large GND strap from the firewall on the R/H side that's bolted to the engine, ensure that its not corroded at the connections.
c170b53 wrote:I think you also have to ensure that the audio jacks are not grounded to the airframe (those pesky little plastic washers with the shoulders are correctly installed) to ensure a good audio signal.
Bruce, if the problem wasn't there before this installation, then check your audio jack installation per c170b53's post. Any time you have low level signals, it's best to run a separate ground wire back to the source (intercom, radio, etc.) and not rely on airframe ground.pilotlaw wrote:I had used KX-170B, KX-155 and KMA-24 audio panel installed in my '54 model to replace unuseable Edo-Aire's. Everything works great until the engine is started. I hear a whine that goes away when I turn off the Alternator..
Miles
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
Bruce,
When I brought my plane home last october, we had some very scratchy sounding radios, a slight whine as some switches were activated and an additional intercom scratching when going over bumps. All very indicative of grounding issues.
When we tore into it - this is what we found:
You could grasp the lower edge of the panel, while everything was running and duplicate the intercom noises. Looking under the panel, we found grounding lugs (up to 7 , yup 7 on one lug and others through partially painted pieces of aluminum. We took all of the lugs off, installed a block to attach the grounds and that problem was reduced dramaticly but not eliminated.
Next we started checking intercom grounds and found that not all the jacks were wired back to a common ground. It's a good idea to do so and most avionics shops will do it when installed. We corrected that and the intercom was clear.
Now for the radios, checked the rack and associated grounds, pins etc. All seemed in order. Long story short, we pulled the headliner down, tested the coax with a multi meter and then used some aligator clips to ground from where the coax connected to the antenna and bingo - the noise cleared up. Seemed as though the radios were installed with the old coax cable which turned out to be bad.
If you are familiar with them, grab a multi meter and use it to check between connections and use a few leads with aligator clips to test what is suspect.
I will at some point rip ALL the wiring out of my bird and update it. Things tend to get patched, jury rigged and made-do with more often than is good for us with our older planes - especially with what you can't see.
Some of the other members have provided some great tips on the grounding straps etc. (cool paint on the engine Miles - Powder Blue?)
Good luck
When I brought my plane home last october, we had some very scratchy sounding radios, a slight whine as some switches were activated and an additional intercom scratching when going over bumps. All very indicative of grounding issues.
When we tore into it - this is what we found:
You could grasp the lower edge of the panel, while everything was running and duplicate the intercom noises. Looking under the panel, we found grounding lugs (up to 7 , yup 7 on one lug and others through partially painted pieces of aluminum. We took all of the lugs off, installed a block to attach the grounds and that problem was reduced dramaticly but not eliminated.
Next we started checking intercom grounds and found that not all the jacks were wired back to a common ground. It's a good idea to do so and most avionics shops will do it when installed. We corrected that and the intercom was clear.
Now for the radios, checked the rack and associated grounds, pins etc. All seemed in order. Long story short, we pulled the headliner down, tested the coax with a multi meter and then used some aligator clips to ground from where the coax connected to the antenna and bingo - the noise cleared up. Seemed as though the radios were installed with the old coax cable which turned out to be bad.
If you are familiar with them, grab a multi meter and use it to check between connections and use a few leads with aligator clips to test what is suspect.
I will at some point rip ALL the wiring out of my bird and update it. Things tend to get patched, jury rigged and made-do with more often than is good for us with our older planes - especially with what you can't see.
Some of the other members have provided some great tips on the grounding straps etc. (cool paint on the engine Miles - Powder Blue?)
Good luck
Kyle Takakjian
Truro, MA
52 C-170B, N8087A
Truro, MA
52 C-170B, N8087A
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cessna170bdriver
- Posts: 4063
- Joined: Mon Apr 22, 2002 5:13 pm
I hadn't noticed how blue it looks in the photos. It must be either the flourescent lights or the camera flash because it's actually a light to medium gray. Weather permitting, I'll be bringing it up from Mojave next weekend, and I'll take some pictures out in the daylight. There are more pictures at http://www.geocities.com/cessna170bdriver , but I just noticed a lot of them have a blue cast also.Kyle wrote:Some of the other members have provided some great tips on the grounding straps etc. (cool paint on the engine Miles - Powder Blue?)
Miles
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
Thanks fellas:
All good advice! Thanks for the detailed pictures, Miles. I would have written sooner, but have been out-of-the-country for a bit. Nice to be back, but the logjam on everything else is interferring with my getting anything done on 4234N (I know, a case of poor prioritizing!). Inspiring responses from all you guys, though, thanks. I am printing them all out and taking them to the airport next trip out.
Bruce Hain
'54 N4234N
26263
All good advice! Thanks for the detailed pictures, Miles. I would have written sooner, but have been out-of-the-country for a bit. Nice to be back, but the logjam on everything else is interferring with my getting anything done on 4234N (I know, a case of poor prioritizing!). Inspiring responses from all you guys, though, thanks. I am printing them all out and taking them to the airport next trip out.
Bruce Hain
'54 N4234N
26263