Curlytips

Was doing my IMC renewal and suddenly horrendous crackling on the radio. So bad we went 7600. Tried turning off things but no help, eventually found we could stop it completely by reducing power under 2050 rpm. Traded speed for silence and completed flight. On subsequent return flight every time we crept past 2050 if started again. Any ideas what's causing it? Haven't flown her since we got back - but expect same problem if not fixed!

Notes: Couldn't isolate alternator as no split master, and circuit breaker not the type you can pop. A/c is old Cessna but with newish O-360. Noise was staccato crackling, not a whine. Earlier when starting, had had occasional flashes from low volt indicator - had thought caused by old and lazy beacon - when off, no flashes. There are no strobes on a/c. Suggestions include alternator suppressor failing - but I thought that was anti-whine rather than crackling. Regulator?

Anyone had similar experiences and/or is clever enough to know what I need to fix?

ambidextrous

Magnetos perhaps?

Curlytips

Don't think so. Only 250 hours on brand new mags and engine running beautifully - just electrical noise.

Sallyann1234

If it's ONLY the radio showing symptoms and nothing else then I would look closer to the radio itself. In particular check the power supply wiring and connectors. Also the aerial base. There could be a poor connection that is shaking loose when a cable resonates in sympathy with the engine rpm.
Try pulling the radio's cables about on the ground with/without the engine running.

Rod1

If it is ok below 2050rpm it could be the voltage regulator.

Rod1

IO540

I think a voltage reg is a good start, but it is easy enough to put an oscilloscope on the 12/24V bus and see if it is clean, and you can buy a cheap scope for the price of a voltage reg

But you need to eliminate any magneto issue, because of the capacitors (a.k.a. "condensers") are gone then the contacts will wear quickly and in an extreme case you could get an engine stoppage. Don't ever mess with mags...

Sallyann1234's suggestions are good too. Avionics wiring is mostly done by people who have only the barest understanding of electronics (they are nearly all just wiremen) and a lot of it is bodged.

wigglyamp

IO-540 puts the boot in again to all avionics people whether competent or not! Thanks.

dan_vector

Sounds like an EMC issue to me. Poor grounding of the electrical equipment on the aircraft causing potential differences between them. It could also be something simple like a poor ground plane between the ae and airframe. It could also be a bad transmission line (coax cable). Actually the causes could be extensive but I would start with the bonding between electrical components on the aircraft to be honest and work from there. A competent aircraft electrician should be able to help. I doubt the actual radio transrec is unserviceable.

Just my 2c worth (I am an ex-military helicopter avionics/radio tech).

IO540

wigglyamp - I am sure you know that I know you are competent I am also sure you know that not everybody else is. You have seen much more than I have but I have seen enough.

To me, the most suprising thing is what aircraft owners are willing to fly with. If the same person had a barely usable radio in his car, he would get it fixed.

Curlytips

IO540 - Please don't think I'm prepared to fly like this. The fault occured suddenly, and the aircraft has been grounded since the incident and will be fixed before it is used again! My query to the forum was in hope that much more experienced types could maybe save me some grief re repair costs - in that I could make "suggestions" to the repairer rather than accept replacement of everything in sight (with myself having no real knowledge of what might be needed).

Turns out that forum members have pretty wide opinions of what it might be - I'd been hoping that someone would have had experience of just this problem and could have said "Easy - it's the 652 discumnockerator that's developed Diggins disease" or some such guidance.

Never mind - it's going into the shop. Thanks all for you efforts (you still have a chance if you do recognise it!). Curlytips.

airpolice

Curly, I had exactly this issue twice and in one aircraft it was resolved by replacing the Dilithium Crystals, and in the other case I woke up before it got fixed.

Hope that helps.

AP

cockney steve

Could it possibly be intermittent arcing-over (tracking) of the HT cables?

Engines will still appear to run smoothly, but i've seen some real "St. Elmo's fire" on damp evenings.

I make the assumption that the leads are not integral with the magsand were possibly not replaced at the same time.

Captain Smithy

Suspiciously like a classic grounding/shielding issue. Some of the cabling in older types can be crummy at best and all it takes is a badly munched connector or a split coax to introduce all manner of bizarre problems that will have pilots and engineers alike scratching their heads furiously.

Bad earths and non-existant or crap shielding are common and cause all sorts of weird noises on radios. The number of folks who fly around in aircraft who's avionics exhibit all sorts of bonkers noises and yet think nothing of it is quite astounding.

Smithy

sunday driver

Worn brushes?

IO540

That's why I suggested checking the alternator (slip rings not brushes BTW) and its voltage regulator with an oscilloscope.

Without a 'scope, much electronic debugging is just poking around in the dark, changing things until the problem goes away, and then who gets billed for the stuff you changed which didn't make any difference?

If you can simulate it on the ground, it should be easy to find.

neckache

Better check your flux-gate-capacitor! That'll be the bugger

Captain Smithy

Nobody's yet suggested adjusting the wobbulator or replacing the hingymajigg.

Curlytips

Again, thanks to everyone for their input. I managed to get the fault to show up on the ground, so hopefully the shop can now hear it, understand it, and correct. Probably the biggest adjustment necessary, will be my bank balance