The International Cessna^® 170 Association

On two previous occasions over the 19 yrs I have owned my "170-C" I have experienced a high pitched noise when I would push my ptt button. On both occasions it turned out to be a diode in my alternator that needed to be replaced and upon doing so the problem went away. Several months ago the same problem popped up again and I took my alternator to a well respected shop and was told that other than cleaning up the unit, etc. that everything was OK. No damage to the diode's. Unfortunately the problem didn't go away. The technician gave me a copy of an article that had been in one of the aviation magazines regarding radio noise and the main thing it referred to was being sure the engine was grounded to the airframe. I have checked that as best I could and tried using a wire jumper from the airframe to the engine to see if that might be the culprit. NADA! The high pitched sound varies with engine rpm, lower at idle to higher with more rpm's, but at cruise rpm there is no noticable high pitched noise. Hangar talk with several fellow pilots all seem to think it is not likely that the noise is coming from the mag's and that I probably should get one of the alternator filters (mine does not have one) from a source such as Spruce and that it will likely eliminate the problem. Anyone care to comment or add suggestions?

The high pitch whine generally comes from the alternator. I experienced this with my 1965 Cherokee 180C. The diodes were checked and found OK. I put on a filter which reduced the whine but did not eliminate it. Later, the alternator quit. I exchanged it for a rebuilt, and, no whine! Apparently, they can cause noise even though they check Ok. If you can try another alternator maybe you could tell.
Some day I'll talk about strobe noise - that causes a whine that pulses with the strobes. Very annoying!

Although your shop may have checked the diodes, they may not have checked the brushes which can "bounce" due to a worn/uneven commutator or "slipper"-ring. Check brush length, brush spring tensions, and check for a dead loop in the field. (or simply exchange that alternator.)

I guess I will give the alternator noise filter a try and see if it might eliminate the problem. I notice that filter at Spruce has increased in price from $37.00 to $55.00 in the past couple of years. Sure don't think the cost to mfg & distribute them has gone up that much, but guess we don't have a lot of options.

George, you suggested to "just simply replace" the alternator. Are you saying you have access to one that would make such an undertaking "simple"? Last time I checked those 60 amp alternators were going for about $300.00 less the core. Do you know something I don't?

170C wrote:I guess I will give the alternator noise filter a try and see if it might eliminate the problem. I notice that filter at Spruce has increased in price from $37.00 to $55.00 in the past couple of years. Sure don't think the cost to mfg & distribute them has gone up that much, but guess we don't have a lot of options.

George, you suggested to "just simply replace" the alternator. Are you saying you have access to one that would make such an undertaking "simple"? Last time I checked those 60 amp alternators were going for about $300.00 less the core. Do you know something I don't?

I just figgered that somebody with pockets as deep as yours .... (you've already got about $900 invested in that alternator).... wouldn't have any problem spending another $300 !

(After all, you're probably willing to send $55 to Spruce for a capacitor that fellow member Wendell Wyborny could probably give you the specs on and you could obtain at the local electronics shop for about $4.)

Well George, you are probably right. Someone probably could have told me how or where to get a lower cost noise filter, but nobody spoke up when I needed that information Seriously I do appreciate everyone's input on my inquiry. That is one of the benefits of belonging to such a super organization And besides George, the one I should have gotten that you suggested wouldn't be FAA-PMA approved so the airplane would probably fall out of the sky Howsomever, I'll report the results of my "noise" after receipt and installation of the filter. Now, i'm wondering if I can install that or maybe I need to hire an IA to do so

Ole Pokey and George

I think I'll fly to Duluth next year just to meet you two,just to see what your really like.
W.

W J, I hope you will fly up to the convention next year. Maybe you can figure out George Now me, I'm a real straight arrow type guy, but George-------well I will leave it up to you to figure him out. You might as well throw Ole Gar into that mix too

Ole Pokey

Every time I open the the pic of Ol Gar I laugh...........that easily could be me on any given day. I do hope to get to the convention if all works out ok.
W

At the risk of embarrassment I want to update my results with the alternator whine in my radio when I transmit. After being sure it was a diode in the alternator & having the shop check it out (it wasn't), purchasing an alternator noise filter (no help), putting on a temporary ground wire between the engine mount & the airframe (wrong again) I was about ready to pull the alternator again & have a different shop check it for diode problems. I had a middle of the night . I reached under the instrument panel and wiggled the radio tray a bit and then started the plane, taxied to the fuel pumps and there was no whine . I then flew to a flyin, but going there & returning I had the same ole whine So I pulled the radio out and remembered that the tray was a really tight fit and knowing at the time ( a yr or 2 back) that I eventually wanted to move that radio to the other side of the instrument panel, I did not secure the tray to the angles on either side because without assistance, I couldn't manage to get a nut onto the small bolts due to the close quarters, etc. The radio wasn't going anywhere and it worked just fine (or so I thought). Well after pulling the radio & transponder out and properly securing the tray to the mounting angles on either side, it now appears that the problem has been solved However, had I secured the tray as I should have initially or checked that when the noise began, I probably would have cured the problem without all the expense and anguish Maybe my screw up will help others to avoid the mistake I made. Again, I appreciate all the good suggestions, etc. you folks offered

I was offline for most of September and didn't see this post when it came up...

Yup, generally a filter is just a cover-up that masks the problem (IF it does anything at all). A properly installed and properly functioning system shouldn't need a filter. Sounds like your radio tray just wasn't grounded properly.

Miles

It's true that the metal cases our radios are mounted within act as RF shields against stray signal. I'm told that's one reason (among others) that portable GPS units will never be approved for navigation....they are mounted in plastic (not properly shielded against external RF interference.)

Guess that means we shouldn't/can't leave our cell phones on while flying for fear of it interferring with some of our navigation equipment. I have read conflicting data on this regarding airliners. They must have a way to prevent the problem if they are considering allowing passengers to use their cells at some point. Personally I hope they don't ever permit it. I would hate to sit within hearing distance of someone gabbing, whether on business or pleasure, for the duration of a flight

Yea Miles-------that radio tray wasn't properly grounded and that appears (hopefully) to have been the problem even though it was wedged in there tightly, but the screws seemed to have made the difference and should have been in there from the get-go

Everyone have a nice Thanksgiving