Radio "Hash"

I've been here before with a similar question, but maybe I can be a
little more specific now. I have a vintage 1972 aircraft with a Lyc
O-360 (McCulloch J-2). When I bought it, she had the original Bendix
radio in it. I replaced it with an ICOM A-200 and a brand new Comant
antenna, or rather a radio guy did. He can't figure out why there is a
constant hash noise. I took her in for my annual, and the A&P said
that the old Bendix mags do not have shielded wiring, and that's the
cause of the noise. I didn't go any further with it, as I now fly out
of a little private strip and can turn the radio down after I leave the
pattern. But I did have to talk to Charlotte approach the other day,
so this thing has to be resolved.

Is it an easy task to replace the unshielded wiring on the old Bendix
mags? I hate to replace the mags, as they're working great. They have
510 hours on them. I used to get the hash on primarily one mag, but
now it's equal on each. Am I correct in assuming that the hash gets
into the radio from the pos. input, or can the ground contribute?
Would wiring the radio power directly to the pos anode of the battery
with a circuit breaker inline, and running a separate ground for the
radio help, or am I just whistling Dixie? I did solve the problem of
an erratic volt-meter in another plane by giving it it's own ground.
Thanks for any help and suggestions.

Kensandyeggo wrote:
...
I took her in for my annual, and the A&P said
that the old Bendix mags do not have shielded wiring, and that's the
cause of the noise...
Am I correct in assuming that the hash gets
into the radio from the pos. input, or can the ground contribute?

The unshielded ignition is radiating radio frequency interference at
the COM frequencies (118+MHz) all the way from the plug wires to the
COM antenna. You have a whole battery of spark gap transmitters running
at the same time. The interference is NOT getting into your radio via
its power wiring.

Would wiring the radio power directly to the pos anode of the battery
with a circuit breaker inline, and running a separate ground for the
radio help, or am I just whistling Dixie?

Dixie! The only way to cure it is to shield the ignition system. Unless
you know for a fact that the "towers" on your existing mags are
internally shielded, you will have to replace the mags too. Don't
forget to install shielded P-leads, too.

Kensandyeggo wrote:
... I have a vintage 1972 aircraft with a Lyc
O-360 (McCulloch J-2).

I'm surprised that any aircraft build in 1972 wouldn't come with
shielded ignition.

Kensandyeggo wrote:
... I replaced it with an ICOM A-200 and a brand new Comant
antenna, or rather a radio guy did. He can't figure out why there is a
constant hash noise.

You need a new radio guy!

Kensandyeggo wrote:
... I replaced it with an ICOM A-200 and a brand new Comant
antenna, or rather a radio guy did. He can't figure out why there is a
constant hash noise.

A couple of questions for Ken:

1. Can you verify that you didn't have the "hash" with the old radio?
2. How do the spark plug wires attach to the plugs? If it's just a simple
clip, then you indeed have an unshielded ignition system. But if you have to
turn a nut on the plug wire to remove it from the plug, you've got at least a
shielded harness and plug.
3. Does the noise vary with engine RPM, and does it literally sound like
popping? "Hash," to me, implies a hissing type noise. Does the noise stop if
the engine isn't running?
4. Does anything else in the aircraft affect it? Several years back, I was
having a bad interference problem on my comm radio. Turned out it was crap from
the transponder, leaking into the radio via a gap in its case. A handful of
aluminum foil fixed the problem.

If the noise seems dependent on engine RPM, you probably have an ignition
shielding problem...especially if you can verify the lack of a shielded plugs
and mags. The good news is that this is fixable...a new set of plugs, mags, and
harnesses will do the trick. The bad news is, it'll cost a bundle.

Here's a link that may help:

http://www.bowersflybaby.com/tech/fe...tm#radio_noise

Ron Wanttaja

Well, it's all moot now. I've had the gyro up for sale for awhile as I
needed the cash when a family partner backed out of a deal AFTER I
signed the lease for a store in downtown......I mean uptown Charlotte.
The guy made me an offer last Thursday evening that I accepted and came
to get the gyro on Friday the 13th. Maybe that should have told him
something. He told me he was signed off by a gyro CFI, and after a few
laps around the pattern after paying me, he decided to cut short the
familiarization and head home to Ohio. There was a slight crosswind
and I told him to lean the cyclic into the wind until the small rudders
became effective. The nose wheel steering is very springy and almost
nonexistant. He spun up and started drifting left toward a small farm
building. He lifted off and immediately made a sharp left turn
downwind instead, climbed about 75 feet, got behind the power curve and
came down like a flying manhole cover. He walked away unscathed, but
the ship was totally destroyed. You can see what this beautiful
classic looked like before he trashed it by going to
www.barnstormers.com and doing a search with "J-2." The pics will be
up for a few more days. I know I couldn't feel as bad as he does, but
I was sick seeing such a beautiful craft destroyed. Thanks for
everyone's help anyway.


Ron Wanttaja wrote:
Kensandyeggo wrote:
... I replaced it with an ICOM A-200 and a brand new Comant
antenna, or rather a radio guy did. He can't figure out why there is a
constant hash noise.

A couple of questions for Ken:

1. Can you verify that you didn't have the "hash" with the old radio?
2. How do the spark plug wires attach to the plugs? If it's just a simple
clip, then you indeed have an unshielded ignition system. But if you have to
turn a nut on the plug wire to remove it from the plug, you've got at least a
shielded harness and plug.
3. Does the noise vary with engine RPM, and does it literally sound like
popping? "Hash," to me, implies a hissing type noise. Does the noise stop if
the engine isn't running?
4. Does anything else in the aircraft affect it? Several years back, I was
having a bad interference problem on my comm radio. Turned out it was crap from
the transponder, leaking into the radio via a gap in its case. A handful of
aluminum foil fixed the problem.

If the noise seems dependent on engine RPM, you probably have an ignition
shielding problem...especially if you can verify the lack of a shielded plugs
and mags. The good news is that this is fixable...a new set of plugs, mags, and
harnesses will do the trick. The bad news is, it'll cost a bundle.

Here's a link that may help:

http://www.bowersflybaby.com/tech/fe...tm#radio_noise

Ron Wanttaja