Rheostat Audio Interference

This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before. The nav
lights and internal instrument lights (I have KX170Bs, KMA20 audio
panel, etc.) work just fine. The problem is that when I turn the
rheostat up so that the internal instrument lights come on, there is an
escalation in electrical noise in the intercom to the point that the
intercom cuts out (i.e. I can't communicate with any other person in the
airplane, can't hear radio transmissions, etc.) When I turn the
rheostat down to the point where the instrument lights are out, but the
navs are still on, there is no problem. I have no problem with the
panel light rheostat.

This problem wasn't always present, and I was caught off guard by it
tonight. I did swap the original non-shielded intercom wiring with
shielding wiring a couple months ago, and brought everything back to a
single-point ground, but I can't image that this has anything to do with
the rheostat issue.

Does anyone have any ideas?



Thanks,
JKG

You could always eliminate the possibility of the rheostat being dirty or
corroded by throwing some contact cleaner at it and seeing if the problem
persists.

"Jonathan Goodish" wrote in message
...
This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before. The nav
lights and internal instrument lights (I have KX170Bs, KMA20 audio
panel, etc.) work just fine. The problem is that when I turn the
rheostat up so that the internal instrument lights come on, there is an
escalation in electrical noise in the intercom to the point that the
intercom cuts out (i.e. I can't communicate with any other person in the
airplane, can't hear radio transmissions, etc.) When I turn the
rheostat down to the point where the instrument lights are out, but the
navs are still on, there is no problem. I have no problem with the
panel light rheostat.

This problem wasn't always present, and I was caught off guard by it
tonight. I did swap the original non-shielded intercom wiring with
shielding wiring a couple months ago, and brought everything back to a
single-point ground, but I can't image that this has anything to do with
the rheostat issue.

Does anyone have any ideas?



Thanks,
JKG

The problem with true rheostats (as opposed to the variable resistor
controlling a solid state device such as a transistor) is that just as you
approach the "bright" end there is a large amount of current and a
significant resistance, with the result being a maximum of power dissipated.
At "full on" there is maximum current but zero resistance, hence zero power
dissipated in the rheostat. At "full off" there is a maximum of resistance
but zero current, hence zero power dissipated in the rheostat. Somewhere in
between is the max power dissipated in the rheostat point.

Having said all that good stuff, maximum power means maximum heat means
maximum oxidation of the rheostat wire. Either a GOOD cleaning with a
toothbrush and contact cleaner or a little judicious application of very
fine (400-600 grit) wetordry sandpaper followed by a good cleaning to get
the sandpaper and oxide particles out of the rheostat will probably solve
the problem.

Jim


"Jonathan Goodish" wrote in message
...
This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before.

In article ,
"RST Engineering" wrote:
Having said all that good stuff, maximum power means maximum heat means
maximum oxidation of the rheostat wire. Either a GOOD cleaning with a
toothbrush and contact cleaner or a little judicious application of very
fine (400-600 grit) wetordry sandpaper followed by a good cleaning to get
the sandpaper and oxide particles out of the rheostat will probably solve
the problem.


Jim,

Thanks for the input. I believe that I have potentiometer/transistor
pairs that perform the dimming function, rather than a true rheostat
(even though Piper still calls it a rheostat in the maintenance manual).
Based on what I've researched, I suspect that I may have a faulty
transistor. I will swap the old one with a new one and see if that
solves the problem.



JKG

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO.

"Faulty" transistors (whatever the hell THAT means) will not create RFI.
Somewhere something is an arcin' and a sparkin'. Transistors aren't
"faulty"; they either am or they ain't.

Jim



"Jonathan Goodish" wrote in message
...
In article ,
"RST Engineering" wrote:
Having said all that good stuff, maximum power means maximum heat means
maximum oxidation of the rheostat wire. Either a GOOD cleaning with a
toothbrush and contact cleaner or a little judicious application of very
fine (400-600 grit) wetordry sandpaper followed by a good cleaning to get
the sandpaper and oxide particles out of the rheostat will probably solve
the problem.


Jim,

Thanks for the input. I believe that I have potentiometer/transistor
pairs that perform the dimming function, rather than a true rheostat
(even though Piper still calls it a rheostat in the maintenance manual).
Based on what I've researched, I suspect that I may have a faulty
transistor. I will swap the old one with a new one and see if that
solves the problem.



JKG

In article ,
"RST Engineering" wrote:

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO.

"Faulty" transistors (whatever the hell THAT means) will not create RFI.
Somewhere something is an arcin' and a sparkin'. Transistors aren't
"faulty"; they either am or they ain't.


Well, "faulty" means it isn't working.

The bottom line is that something is wrong, and either way I'm going to
have to endure the pain of pulling the switch assembly out of the panel
and take a look.



JKG

Jonathan Goodish wrote:
In article ,
"RST Engineering" wrote:


NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO.

"Faulty" transistors (whatever the hell THAT means) will not create RFI.
Somewhere something is an arcin' and a sparkin'. Transistors aren't
"faulty"; they either am or they ain't.



Well, "faulty" means it isn't working.

The bottom line is that something is wrong, and either way I'm going to
have to endure the pain of pulling the switch assembly out of the panel
and take a look.

Check all the connections in the circuit. The
noise may be from an arcing connection.

--

Tauno Voipio
tauno voipio (at) iki fi

I struggled with a problem like this on a Glassair III some friends
built.

Had a transistor in an Darlington emitter follower configuration. The
dimmer pot
drove the base. The lights were from the emitter to ground.

Trouble was, the thing was oscillating when at mid brightness
positions. Too much
capacitance on the output. A known problem with emitter followers.
One forgets
that they still have gain at a few tens of mhz. When it took off you
could hear it
in several of the radios. Darlington configurations have worse
stability problems.

I solved it by puttting about 100 ohms in the base right at the
transistor.

These circuits are designed by people not very skilled in the art.
They also suffer
the problem that if a bulb burns out shorted or there's an inadvertent
short on the
string of lights, the transistor fails. There is nothing to limit the
current.
That will usually take the pot too, especially if
it's near the high end of its range. The 100 ohm resistor will solve
that, too.

If it's not a darlington, the resistor will have to be smaller.

The cool way around all this is to design it with a P-FET power device
configured like an op-amp.

Bill Hale

wrote in message
ups.com...

Trouble was, the thing was oscillating when at mid brightness
positions. Too much
capacitance on the output. A known problem with emitter followers.

Horsefeathers. Emitter followers have less than unity voltage gain and are
stable as rocks. And how does a resistive LED load become capacitive?
Difficult to imagine.



One forgets
that they still have gain at a few tens of mhz. When it took off you
could hear it
in several of the radios. Darlington configurations have worse
stability problems.

I solved it by puttting about 100 ohms in the base right at the
transistor.

Tens of millihertz? Try again. And the base of the transistor at mid-gain
has more than 100 ohms of resistance in the control pot.




These circuits are designed by people not very skilled in the art.
They also suffer
the problem that if a bulb burns out shorted or there's an inadvertent
short on the
string of lights, the transistor fails. There is nothing to limit the
current.
That will usually take the pot too, especially if
it's near the high end of its range. The 100 ohm resistor will solve
that, too.

Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out open.
If the transistor fails ( a million to one odds), the pot is open-circuited
and will not be damaged. You have absolutely no experience in the matter,
so why waste our time and bandwidth with your ignorance?





If it's not a darlington, the resistor will have to be smaller.

Don't apply for an engineering job at my company.



The cool way around all this is to design it with a P-FET power device
configured like an op-amp.


Why not an N-FET, or an NPN, or a PNP, all of which will solve the problem
elegantly.


Jim

RST Engineering wrote:

wrote in message
ups.com...

Trouble was, the thing was oscillating when at mid brightness
positions. Too much
capacitance on the output. A known problem with emitter followers.

Horsefeathers. Emitter followers have less than unity voltage gain and are
stable as rocks. And how does a resistive LED load become capacitive?
Difficult to imagine.

True for a real emitter follower, however if you have an N-stage
darlington with N=3, it can oscillate.

This was the subject of an IEEE Transactions article in the early 80's
and a source of great embarrassment for an engineer I worked with that
didn't read the article until after putting the design in production.

snip

--
Jim Pennino

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