Cessna PTT switch normal?

We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.

Not recognizing what the problem was, after the first complaint from
ATC, I pressed harder on the switch - making the problem occur 100%
of the time. To work around this, I used the built-in hand mike for
the rest of the flight.

It was only after I got back on the ground and began diagnosing the
problem did I remember a notice from the club admin to all members
that this is just how these switches work, and that you must not
press on the switch too hard. Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Is this really how "these switches" work, or is the switch defective?

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.
...

Mark,

I have never seen a yoke-mounted PTT switch which breaks the MIC line.
Usually,
the yoke mounted switch is only a single pole normally-open switch,
which is wired to connect the PTT line to airframe ground while the
switch is depressed.

If pushing harder effects the Mic audio, this may be happening
indirectly depending on what type of intercom/audio panel is in the
aircraft. Frequently, there is logic inside the intercom which switches
the headset mic from the intercom function to the selected transmitter
when the PTT switch is depressed. It is possibe that pushing hard
causes a high-resistance in the yoke-mounted PTT switch, such that the
resistance is low enough to key the transmitter, but not low enough to
cause the mic to be switched from the intercom bus to the transmitter.

The solution is to replace the yoke-mounted PTT switch.

On Cessna factory installed push to transmitt (PTT) is a three point
normaly open switch in the yoke. When you push the switch all three
points are shorted together. One of the points has a ground wire, one
of the other points is a MIC KEY line going to the audio panel to key
thr transmitter, the last point is a wire providing a ground for the
pilots mic audio. This way the pilot mic audio is not hot when the
switsh is not pushed. This is Cessnas cheep way of providing mic
isolation. On other audio panels the mic audio isolation is provided by
the audio panel. That is a OEM switch big bucks from Cessan!

Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.

Not recognizing what the problem was, after the first complaint from
ATC, I pressed harder on the switch - making the problem occur 100%
of the time. To work around this, I used the built-in hand mike for
the rest of the flight.

It was only after I got back on the ground and began diagnosing the
problem did I remember a notice from the club admin to all members
that this is just how these switches work, and that you must not
press on the switch too hard. Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Is this really how "these switches" work, or is the switch defective?

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

On 09/17/06 21:15, KP wrote:
On Cessna factory installed push to transmitt (PTT) is a three point
normaly open switch in the yoke. When you push the switch all three
points are shorted together. One of the points has a ground wire, one
of the other points is a MIC KEY line going to the audio panel to key
thr transmitter, the last point is a wire providing a ground for the
pilots mic audio. This way the pilot mic audio is not hot when the
switsh is not pushed. This is Cessnas cheep way of providing mic
isolation. On other audio panels the mic audio isolation is provided by
the audio panel. That is a OEM switch big bucks from Cessan!


Thanks. We're being told that what I described is just how the switch
works, not that it's broken and too expensive to replace.

Do you think the switch as I described it is defective - not counting
the design flaw?

It sure seems like a safety issue.


Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.

Not recognizing what the problem was, after the first complaint from
ATC, I pressed harder on the switch - making the problem occur 100%
of the time. To work around this, I used the built-in hand mike for
the rest of the flight.

It was only after I got back on the ground and began diagnosing the
problem did I remember a notice from the club admin to all members
that this is just how these switches work, and that you must not
press on the switch too hard. Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Is this really how "these switches" work, or is the switch defective?

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:

Thanks. We're being told that what I described is just how the switch
works, not that it's broken and too expensive to replace.

Do you think the switch as I described it is defective - not counting
the design flaw?

I don't need knowledge about the internals of the switch to be able to
say that "that's how it works" is not an acceptable answer.

Switches hate the environment they are in... outdoor
enviornment with corrosion, etc.

I've had some luck putting radio shaft contact cleaner/lubricant
in them. On one miniature rotary--that was intermittent--I drilled a
very tiny hole with
a dremel tool (and suction to keep chips out) to let cleaner in.

It would have been a heck of a job to replace this switch.

You might see if you can work some cleaner into it.

Bill Hale

Mark Hansen wrote:
On 09/17/06 21:15, KP wrote:
On Cessna factory installed push to transmitt (PTT) is a three point
normaly open switch in the yoke. When you push the switch all three
points are shorted together. One of the points has a ground wire, one
of the other points is a MIC KEY line going to the audio panel to key
thr transmitter, the last point is a wire providing a ground for the
pilots mic audio. This way the pilot mic audio is not hot when the
switsh is not pushed. This is Cessnas cheep way of providing mic
isolation. On other audio panels the mic audio isolation is provided by
the audio panel. That is a OEM switch big bucks from Cessan!


Thanks. We're being told that what I described is just how the switch
works, not that it's broken and too expensive to replace.

Do you think the switch as I described it is defective - not counting
the design flaw?

It sure seems like a safety issue.


Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.

Not recognizing what the problem was, after the first complaint from
ATC, I pressed harder on the switch - making the problem occur 100%
of the time. To work around this, I used the built-in hand mike for
the rest of the flight.

It was only after I got back on the ground and began diagnosing the
problem did I remember a notice from the club admin to all members
that this is just how these switches work, and that you must not
press on the switch too hard. Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Is this really how "these switches" work, or is the switch defective?

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.

Not recognizing what the problem was, after the first complaint from
ATC, I pressed harder on the switch - making the problem occur 100%
of the time. To work around this, I used the built-in hand mike for
the rest of the flight.

It was only after I got back on the ground and began diagnosing the
problem did I remember a notice from the club admin to all members
that this is just how these switches work, and that you must not
press on the switch too hard. Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Is this really how "these switches" work, or is the switch defective?

Mark,
Sounds like you have a short to ground in the mic line. Have the switch
pull and see if it is contacting anything underneath.

Michelle

On 09/18/06 07:36, Dave Butler wrote:
Mark Hansen wrote:

Thanks. We're being told that what I described is just how the switch
works, not that it's broken and too expensive to replace.

Do you think the switch as I described it is defective - not counting
the design flaw?

I don't need knowledge about the internals of the switch to be able to
say that "that's how it works" is not an acceptable answer.

I agree, Dave. What I'm wondering is if there might be a poor design
involved here, and that this failure is "typical" for the factory switch.

I realize, of course, that a switch shouldn't act this way - but I
wonder if a replacement Cessna switch will likely act the same way.

Thanks,

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:
On 09/18/06 07:36, Dave Butler wrote:
Mark Hansen wrote:

Thanks. We're being told that what I described is just how the switch
works, not that it's broken and too expensive to replace.

Do you think the switch as I described it is defective - not counting
the design flaw?

I don't need knowledge about the internals of the switch to be able to
say that "that's how it works" is not an acceptable answer.

I agree, Dave. What I'm wondering is if there might be a poor design
involved here, and that this failure is "typical" for the factory switch.

I realize, of course, that a switch shouldn't act this way - but I
wonder if a replacement Cessna switch will likely act the same way.

You have an old, worn-out switch and it shouldn't act like it
is. We have a 1978 182RG that has that 3-wire switch, and I had to
replace it two weeks ago. Cost $100 (Canadian) from Cessna, which of
course is too much, but it worked like a charm. Had tried to run
cleaner through the old one, but nothing helped. It's a tiny, precision
snap-action switch, not a "poor design," and those contacts oxidize
and/or burn out after 28 years. Things don't last forever. There's
nothing you can do besides replace it if you want it to work properly.
It's only going to get worse.

Dan

Mark Hansen wrote:

Of course, that's just not Human
Nature ;-) If the switch doesn't work when you press it, you press
it harder.

Sort of like typing harder on the keyboard when the computer acts up :-)


Is this really how "these switches" work, or is the switch defective?

I vote for defective (and that the person telling you this from the club
doesn't know what they're talking about).

I hate PTT problems. Ours acted up a few months back in the Arrow and
we wound up having the wiring replaced. It was a mess...wrong type of
wire used, spliced, just plain ugly. It's a wonder that it worked well
for the first year we had the plane.

--
Jack Allison
PP-ASEL-Instrument Airplane

"To become a Jedi knight, you must master a single force. To become
a private pilot you must strive to master four of them"
- Rod Machado

(Remove the obvious from address to reply via e-mail)