Rheostat Audio Interference

[Jonathan Goodish]

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

[Juan Jimenez]

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

[RST Engineering]

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.

[Jonathan Goodish]

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

[RST Engineering]

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

[Jonathan Goodish]

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

[Tauno Voipio]

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

[[email protected]]

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

[RST Engineering]

> 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

[[email protected]]

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

Remove .spam.sux to reply.

[Juan Jimenez]

"Jonathan Goodish" > wrote in message
...
> 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.

Jonathan,

Remember Occam's Razor? It applies just as much to electricity as it does to
science fiction movies.

Juan

[Jonathan Goodish]

In article >,
"Juan Jimenez" > wrote:
> Remember Occam's Razor? It applies just as much to electricity as it does to
> science fiction movies.


Maybe, and if I'm going to bet on something, it's going to be on a bad
transistor based on the experience of some local mechanics. In any
case, I hope it's a simple fix one way or the other, because I don't
think I could ever work under the panel for a living.


JKG

[[email protected]]

Might want to read up.

If emitter followers are driving a load with a capacitive component, it
is reflected
to the input as a negative resistance. You can prove this with only
the simplest
hybrid pi model. It's a parasitic oscillation, not a loop oscillation
which couldn't
happen with gain < 1 as you point out. You add enough series base
resistance
to swamp out the negative resistance, making it stable. There is
enough capacitance
in the wiring to excite the phenomena. This is why you see resistors
in the base
circuits of emitter followers all the time.

Darlingtons are much worse. You can show that with the hybird pi model
as well.
The oscillation will be near fT. That's a few mhz for 3055 type
devices.

Bulbs usually burn out open, true. What about some other short? The
circuit has NO short current protection other than the beta of the
transisitor. The base current becomes 1/beta of whatever short circuit
current flows. If the pot is set very near the max end of it's range,
dissapation will destroy the upper part of the resulting divider stick.

I have replaced enough of the panel mount edge adjust pots in Bonanzas
which have
this exact setup to know. Know what those cost?

The P-fet works great because you can get the output clear to the rail,
not the rail - 1 diode drop
as you are limited to with the 3055 approach or 2 drops in the
darlington approach.
It's inherently current limited by IDss as well. Draw it out: The
source goes to +14,
the drain to the lamps to ground. The control pot goes with the hot
end to +14, the
wiper to the gate, and the other end to the bulbs. So it also has a
small amount
of loop gain-- makes the adjustment very smooth. The huge gate-drain
capacitance
of the v-fet structure miller multiplied by the gain of the FET ensures
stability under
all conditions.

Maybe you should write it up for kit planes.

I won't be applying to RST anytime soon, but I did think you were
better than this.

Bill Hale

[RST Engineering]

> wrote in message
oups.com...


> Might want to read up.

Have. Lots.

>
> If emitter followers are driving a load with a capacitive component, it
> is reflected
> to the input as a negative resistance. This is why you see resistors
> in the base
> circuits of emitter followers all the time.

I really don't want to get into the whys and wherefores of parasitic
oscillation in this ng. Hell, the guy only wants his dimmer fixed. And
yes, a 100 ohm base decoupling resistor has been standard for me for the
last 40 years, mostly because I really can't control how well the collector
is bypassed for DC, audio, and RF simultaneously. It is difficult (not
impossible) to get parasitic oscillation where the collector is bypassed
from DC to daylight (or at least out to Ft).

The only place I *won't* use a base decoupling resistor is in a VHF power
amplifier where the input impedance is an order of magnitude lower than the
decoupling resistor.


>
> Bulbs usually burn out open, true. What about some other short? The
> circuit has NO short current protection other than the beta of the
> transisitor. The base current becomes 1/beta of whatever short circuit
> current flows. If the pot is set very near the max end of it's range,
> dissapation will destroy the upper part of the resulting divider stick.

How do we know that there is no short circuit current protection? Did you
take it apart or do you have a schematic of this particular dimmer? We sure
could have saved a lot of wild ass guessing about the problem. He MIGHT
have it overloaded, but without knowing the particulars of this
installation, we are doing rectorandom guesses at the problem.


>
> I have replaced enough of the panel mount edge adjust pots in Bonanzas
> which have
> this exact setup to know. Know what those cost?

No, and I have a hard time believing that Beech, the overdesigner of the
industry, put something out without short circuit protection of some sort.
However, if you have replaced them, then you are one up on me. No, I don't
know what they cost, but a simple current shutdown (with or without
foldback) is less than half a buck's worth of parts at the front end.

>
> The P-fet works great because you can get the output clear to the rail,
> not the rail - 1 diode drop
> as you are limited to with the 3055 approach
> Maybe you should write it up for kit planes.

You probably want to look at January, April, May, June, July 2001 Kitplanes
where I used N-channel, P-channel, NPN, and PNP transistors as the output
devices, explaining exactly what the tradeoffs were between each of the
devices. All in all, about ten designs.


>
> I won't be applying to RST anytime soon,

And I thank you kindly.


but I did think you were
> better than this.

When I'm in sci.electronics.design, I'm really quite careful about the
nuances of design. When I'm trying to get some poor guy's lamp dimmer to
work on RAH, RAO, or RAP, I'm a little less careful about being precisely
technically correct. For a dimmer that apparently worked correctly once
upon a time, poor design is about the last place I try and look. I'm not
above reengineering a crappy design, but if it really does have a parasitic
oscillation at the top end of the range, you'd have thought that in my last
45 years in this biz I'd have come across one, no?

Now, let's get back to fixing this sucker with what we DO know. Did the
fact that he can't key his radio transmitter or hear his radio receiver when
the unit was acting up mean anything to you? I doubt parasitic oscillation
keeps the transmitter key line from kicking the transmitter on. Use ALL the
clues, not just the one you are most comfortable with.

Jim

[RST Engineering]

OK, let me reiterate what you have told me. Please take your time to
respond so that we don't go chasing down blind alleys. THere are some
things you have said that are contradictory.

1. THe nav lights (you are talking about the ones on the wingtips -- red,
green, white in the back) are working just fine. What would this have to do
with the problem?

2. The internal instrument lights are working just fine. I'm presuming
what you are telling me is that if you connect a wire directly from the +12
supply to the instrument lights then they illuminate at the proper level.
You DID take the dimmer out of the circuit for this test, didn't you?

3. WHen you turn up the dimmer potentiometer so that the "instrument
lights" (do you mean the radio backlights or do you have some sort of
lighting system on the gyros?) then you get a certain amount of noise,
mainly in the headsets but some in the speaker as well.

4. You also said somewhere along the line that you cannot transmit when
this noise occurs, but that you CAN transmit using a handheld mic. However,
when you key up the transmitter, the radio lights dim.

5. You have a fixation on a "bad" transistor or a "bad" potentiometer.
Please let's not guess at solutions until we can prove something.

6. You said that you can't hear radio transmissions, yet in a subsequent
post you said that you CAN hear radio transmissions. Which is it?

7. I guess I'm a little unclear about the difference between "nav" and
"instrument" lighting. Can you elaborate?

8. I don't suppose there is a chance in hell that you have a schematic of
the dimmer?

9. What sort of test equipment can we presume as we toddle down the fixit
trail? A handheld AM broadcast band receiver is a hell of a good buzz
detector.


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. 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

It sounds like there are two likely possibilities: bad potentiometer or bad
power transistor. When I crank the pot up so that the internal radio lights
illuminate, I get an escalating buzz that quickly squelches the panel mount
intercom. Although I can hear the buzz in the headset (but not as easily in
the aircraft speaker), I can also hear radio broadcasts, but I can't
transmit and am unable the communicate via the intercom. When I turn the
lights back down, the problem goes away.

However, when I key the hand mic with the radio lights illuminated, the
radio lights dim significantly. I thought this was odd considering I don't
ever remember that happening in the past, and it certainly didn't happen
previously when the mic was keyed via the intercom.

A similar circuit that controls the panel lights produces no such effects.



Any ideas? Does this sound like a bad potentiometer, transistor, or

something else?

[David Lesher]

"RST Engineering" > writes:



>Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out open.

I wish.... I had some hard experience with shorted ?317's I think
they were -- the 14v version of 327's. And lots of owners of
early-generation X10 modules discovered they would fail into full-on
when their 120vac lamp failed into full-off.

The clear solution for the OP was what the NASA LeRC 10x10 Supersonic
Wind Tunnel used for ""dimmers"" for the seven 40KHP drive motors
-- large glass tanks of salt water. The electrodes were cranked in
deeper to speed things up....

Oh, if you go that route, be sure and stick to positive G maneuvers
or you'll have a mess....
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[RST Engineering]

Describe the failure mode that lets a bulb short out.

Jim



"David Lesher" > wrote in message
...
> "RST Engineering" > writes:
>
>
>
>>Unmitigated horsepoop. Bulbs don't burn out shorted. Bulbs burn out
>>open.
>
> I wish.... I had some hard experience with shorted ?317's I think
> they were -- the 14v version of 327's. And lots of owners of
> early-generation X10 modules discovered they would fail into full-on
> when their 120vac lamp failed into full-off.

[RST Engineering]

The problem comes from the
> "click" switch at the end of travel of the pot. The entire string of
> post lights & internal radio lights is switched directly with the little
> "click" switch. On my airplane, this is 2.5A tungsten through a little
> switch!

And not to forget the inrush current which is somewhere between 3 and 10
times the quiescent current with an incandescent bulb.


>
> *I* think the OP has a bad alternator diode or 2. Turning on the panel
> lights draws > 2A which starts the whining. Pressing transmit draws > 8A
> which drowns out the transmitter with the whining and pulls the system
> voltage down so that the lights dim. Cherokee 140's have a very marginal
> 40A alternator to begin with.

Hm. His complaint didn't seem to be "whine" but more of a static or
rat-a-tat-tat sort of noise. Let's wait for the OP to get back to us with
exactly what he is hearing.


>
> If the OP checks by turning on pitot heat or the landing light I think
> you can diagnose this right away.

THat's true. If it is alternator diodes. I was just thinking that the
dimming he was seeing was from not having the engine turned on and running
on battery alone. That was one of the next questions as soon as he finishes
answering what I've already asked him.


Jim

[Jonathan Goodish]

In article >,
"RST Engineering" > wrote:
> > If the OP checks by turning on pitot heat or the landing light I think
> > you can diagnose this right away.
>
> THat's true. If it is alternator diodes. I was just thinking that the
> dimming he was seeing was from not having the engine turned on and running
> on battery alone. That was one of the next questions as soon as he finishes
> answering what I've already asked him.


That is correct. The problem exists both with the engine running and
not running. Any other load in the electrical system has no impact on
the problem--it is specific to the intercom and the circuit that
controls the avionics backlighting.


JKG

[RST Engineering]

Did you get the list of questions I sent you yesterday? I'm holding fire on
making suggestions until I get some of the answers back.

Jim


"Jonathan Goodish" > wrote in message
...
>
> That is correct. The problem exists both with the engine running and
> not running. Any other load in the electrical system has no impact on
> the problem--it is specific to the intercom and the circuit that
> controls the avionics backlighting.
>
>
> JKG

[Jonathan Goodish]

In article >,
"RST Engineering" > wrote:
> 1. THe nav lights (you are talking about the ones on the wingtips -- red,
> green, white in the back) are working just fine. What would this have to do
> with the problem?

Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.


>
> 2. The internal instrument lights are working just fine. I'm presuming
> what you are telling me is that if you connect a wire directly from the +12
> supply to the instrument lights then they illuminate at the proper level.
> You DID take the dimmer out of the circuit for this test, didn't you?

No. With the dimmer in, the instrument lights (radio backlights)
illuminate and dim just fine.



>
> 3. WHen you turn up the dimmer potentiometer so that the "instrument
> lights" (do you mean the radio backlights or do you have some sort of
> lighting system on the gyros?) then you get a certain amount of noise,
> mainly in the headsets but some in the speaker as well.

I have two circuits, panel lights and the radio backlights. The circuit
in question is the radio backlights.

You are correct, when I turn the pot so that the radio backlights
illuminate, I get noise that escalates in volume as I turn the lights
up. The lights appear to illuminate normally. When I get more than a
little current in circuit, the intercom squelches the mics on all
positions.



> 4. You also said somewhere along the line that you cannot transmit when
> this noise occurs, but that you CAN transmit using a handheld mic. However,
> when you key up the transmitter, the radio lights dim.

That is correct. The hand mic causes the radio backlights to go from
full bright to dim when it is keyed.



> 5. You have a fixation on a "bad" transistor or a "bad" potentiometer.
> Please let's not guess at solutions until we can prove something.

I don't necessarily have a fixation on anything, otherwise I wouldn't be
asking for opinions. It's pretty obvious that something is wrong, but I
don't know what that "something" is at the moment.


>
> 6. You said that you can't hear radio transmissions, yet in a subsequent
> post you said that you CAN hear radio transmissions. Which is it?

I can hear transmissions at all times, even when the buzzing is present
through the intercom when the radio backlights are at full brightness.



> 8. I don't suppose there is a chance in hell that you have a schematic of
> the dimmer?

Yes, I do.


>
> 9. What sort of test equipment can we presume as we toddle down the fixit
> trail? A handheld AM broadcast band receiver is a hell of a good buzz
> detector.

I have an AM radio and a digital VOM and that's about it at the moment.




JKG

[RST Engineering]

OK, one last question. How long has this exact radio stack (radio,
intercom, headset, etc.) been in the airplane, and have you ever witnessed
it working correctly? Has anything ELSE electrical in the airplane changed
between the time it worked correctly and the present time?

I'm going to go out on a bodacious limb and say that there is some vital
connection between your radio light dimmer and your PTT line. THere should
be absolutely no connection between the two.

Jim




"Jonathan Goodish" > wrote in message
...
> In article >,
> "RST Engineering" > wrote:

>> 1. THe nav lights (you are talking about the ones on the wingtips --
>> red,
>> green, white in the back) are working just fine. What would this have to
>> do
>> with the problem?
>
> Nothing, other than the fact that they are activated by the same switch.
> in the Cherokee.

The same switch as what? Are you saying that by turning the radio lights
pot from off to just barely on (switch snap) also turns on the nav lights?


>
>
>>
>> 2. The internal instrument lights are working just fine. I'm presuming
>> what you are telling me is that if you connect a wire directly from the
>> +12
>> supply to the instrument lights then they illuminate at the proper level.
>> You DID take the dimmer out of the circuit for this test, didn't you?
>
> No. With the dimmer in, the instrument lights (radio backlights)
> illuminate and dim just fine.

OK, let's agree on some terminology so we don't keep running down the same
rabbit path. Does the SAME dimmer run the instrument (post) lights as the
radio lights? If not, let's have an INSTRUMENT LIGHT dimmer and a RADIO
LIGHT dimmer.

>
>
>
>>
>> 3. WHen you turn up the dimmer potentiometer so that the "instrument
>> lights" (do you mean the radio backlights or do you have some sort of
>> lighting system on the gyros?) then you get a certain amount of noise,
>> mainly in the headsets but some in the speaker as well.
>
> I have two circuits, panel lights and the radio backlights. The circuit
> in question is the radio backlights.
>
> You are correct, when I turn the pot so that the radio backlights
> illuminate, I get noise that escalates in volume as I turn the lights
> up. The lights appear to illuminate normally. When I get more than a
> little current in circuit, the intercom squelches the mics on all
> positions.

Squelching the microphones turn them off. Are you saying that the radio
dimmer turns ALL the microphones off when the lights start to get bright?


>> 6. You said that you can't hear radio transmissions, yet in a subsequent
>> post you said that you CAN hear radio transmissions. Which is it?
>
> I can hear transmissions at all times, even when the buzzing is present
> through the intercom when the radio backlights are at full brightness.
>
>
>
>> 8. I don't suppose there is a chance in hell that you have a schematic
>> of
>> the dimmer?
>
> Yes, I do.
>
>
>>
>> 9. What sort of test equipment can we presume as we toddle down the
>> fixit
>> trail? A handheld AM broadcast band receiver is a hell of a good buzz
>> detector.
>
> I have an AM radio and a digital VOM and that's about it at the moment.

[Jonathan Goodish]

In article >,
"RST Engineering" > wrote:
> > Nothing, other than the fact that they are activated by the same switch.
> > in the Cherokee.
>
> The same switch as what? Are you saying that by turning the radio lights
> pot from off to just barely on (switch snap) also turns on the nav lights?

Yes.


> OK, let's agree on some terminology so we don't keep running down the same
> rabbit path. Does the SAME dimmer run the instrument (post) lights as the
> radio lights? If not, let's have an INSTRUMENT LIGHT dimmer and a RADIO
> LIGHT dimmer.

No. We are talking about the radio light dimmer circuit.



> > You are correct, when I turn the pot so that the radio backlights
> > illuminate, I get noise that escalates in volume as I turn the lights
> > up. The lights appear to illuminate normally. When I get more than a
> > little current in circuit, the intercom squelches the mics on all
> > positions.
>
> Squelching the microphones turn them off. Are you saying that the radio
> dimmer turns ALL the microphones off when the lights start to get bright?

Yes.


> OK, one last question. How long has this exact radio stack (radio,
> intercom, headset, etc.) been in the airplane, and have you ever witnessed
> it working correctly? Has anything ELSE electrical in the airplane changed
> between the time it worked correctly and the present time?

It's all been in there for years. We used to fly quite a bit at night,
but haven't so much in recent years. We did the other night, and that's
when I noticed this problem. I don't remember it having been there
before. BUT ...

I didn't have much time tonight, but I did collect some more data. With
the master on, the avionics master off, and the radio light dimmer on,
the electrical noise is still in the headset even though the intercom is
not powered since the avionics master is off. The noise escalates as
the Century 1 autopilot gyro spins up... that's where the whine is
coming from. When I pull the connector out of the Century 1 to power it
down, the whine is instantly gone, but I can still hear scratches
through the headset when I move the radio light dimmer.

There is a terminal block under the panel to which it appears some of
the avionics is grounded, including my intercom. When I pulled the
intercom ground off of this terminal block and grounded it to the
airframe, even near to the block, the problem disappeared and I regained
normal intercom operation with the radio lights on full bright and no
noise in the intercom. I now wonder if the terminal block is grounded
to the airframe, or what is going on there. Would it be typical for a
terminal block to be anything OTHER than a ground? I didn't have
anything with me to measure voltage on this block or check for
continuity with ground.



JKG

[Jonathan Goodish]

In article >,
Jonathan Goodish > wrote:
> There is a terminal block under the panel to which it appears some of
> the avionics is grounded, including my intercom. When I pulled the
> intercom ground off of this terminal block and grounded it to the
> airframe, even near to the block, the problem disappeared and I regained
> normal intercom operation with the radio lights on full bright and no
> noise in the intercom. I now wonder if the terminal block is grounded
> to the airframe, or what is going on there. Would it be typical for a
> terminal block to be anything OTHER than a ground? I didn't have
> anything with me to measure voltage on this block or check for
> continuity with ground.


I am wondering if this terminal block could be the radio light bus
instead of ground. It seems like an odd location, but I have no
expertise with avionics installations so I don't know what is common and
what is not. It would seem logical based on the symptoms and
resolution. I will have to check it with a meter tomorrow.



JKG

[RST Engineering]

You jumped two steps ahead of me in the troubleshooting, but you FOUND IT.

Terminal blocks can be ANYTHING that the manufacturer wants to put in as a
break in the system. Some of the terminals can be ground, some can be light
voltage, and some of them (check me if I'm wrong) can be PTT terminals. My
best guess is that somehow (and a loose ground is a perfect way to do it) is
that your stick mounted PTT switch was getting lamp dimmer voltage, which
would have held it up above transmit level (3 volts) no matter WHAT your PTT
switch commanded.

The bitch about terminal block grounds is that once they loosen just a
LITTLE bit, they heat up (I^2R losses) which makes them corrode just a bit
more, which heats them up more, which makes them corrode a bit more ...)

But you FOUND IT.

Good for you.

Jim



"Jonathan Goodish" > wrote in message
...
> In article >,
> Jonathan Goodish > wrote:
>> There is a terminal block under the panel to which it appears some of
>> the avionics is grounded, including my intercom. When I pulled the
>> intercom ground off of this terminal block and grounded it to the
>> airframe, even near to the block, the problem disappeared and I regained
>> normal intercom operation with the radio lights on full bright and no
>> noise in the intercom. I now wonder if the terminal block is grounded
>> to the airframe, or what is going on there. Would it be typical for a
>> terminal block to be anything OTHER than a ground? I didn't have
>> anything with me to measure voltage on this block or check for
>> continuity with ground.
>
>
> I am wondering if this terminal block could be the radio light bus
> instead of ground. It seems like an odd location, but I have no
> expertise with avionics installations so I don't know what is common and
> what is not. It would seem logical based on the symptoms and
> resolution. I will have to check it with a meter tomorrow.
>
>
>
> JKG

[David Lesher]

"RST Engineering" > writes:

>Describe the failure mode that lets a bulb short out.


I'm not an expert on same; just a victim. But from conversations
with someone at GE Nela Park decades ago; the filament breaks, and
can fall down from both gravity and err "sprong"ing when it lets go...

If the shortened filament end touches the OTHER post, it will draw
lots more current since it is shorter. It very soon burns out, but
in the meantime....

Or a length comes loose at both ends; it falls across the posts at the
bottom and.....


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[RST Engineering]

We must not have been talking to the same people at GE, because one of the
things I did in my former life was do reliability studies on some of the
stuff we threw up into orbit -- like annunciator light bulbs (WAY, WAY
before LEDs could remotely be considered reliable enough for space flight).

If you look closely at an incandescent light bulb (especially the aviation
versions like the 327-28 volt and 330-14 volt) you will see that the
internals of the bulb start with a little dot of ceramic called the "bead".
The wires that come out of this bead are called "spreaders" or "stringers";
there is an optional electrically inert third wire called a "support" that
we can talk about later.

The spreaders are angled out at a fairly precise angle to keep a broken
filament from coming into contact with the other spreader and causing
exactly the failure mode you describe. It is geometrically impossible for a
dangling filament to come in contact with the other spreader. THe support
does exactly the same thing for a long-filament bulb -- holds up the
filament while it is still a lamp and keeps the broken filament from
touching the other spreader when the lamp burns out.

The only possible failure mode would be for the filament to break at both
ends simultaneously and drop down onto the bead in such a manner that the
slightest vibration would not cause the filament to drop harmlessly into the
bottom of the lamp base. While there is a mathematical probability that
this could happen, there is also a mathematical probability that the lamp
could disassemble itself and reassemble itself in a far corner of the
universe. I'm not sure which one is more probable.

Anyway, it seems the OP has found the "ground wire that ain't a ground wire"
and solved the problem, which is a good thing.

Jim



"David Lesher" > wrote in message
...
> "RST Engineering" > writes:
>
>>Describe the failure mode that lets a bulb short out.
>
>
> I'm not an expert on same; just a victim. But from conversations
> with someone at GE Nela Park decades ago; the filament breaks, and
> can fall down from both gravity and err "sprong"ing when it lets go...
>
> If the shortened filament end touches the OTHER post, it will draw
> lots more current since it is shorter. It very soon burns out, but
> in the meantime....
>
> Or a length comes loose at both ends; it falls across the posts at the
> bottom and.....
>
>
> --
> A host is a host from coast to
> & no one will talk to a host that's close........[v].(301) 56-LINUX
> Unless the host (that isn't close).........................pob 1433
> is busy, hung or dead....................................20915-1433

[Jonathan Goodish]

In article >,
"RST Engineering" > wrote:
> The bitch about terminal block grounds is that once they loosen just a
> LITTLE bit, they heat up (I^2R losses) which makes them corrode just a bit
> more, which heats them up more, which makes them corrode a bit more ...)
>
> But you FOUND IT.
>
> Good for you.


Thanks to Jim and others for your input. These types of things can be
like trying to find a leaking pipe.


JKG

[RST Engineering]

> Thanks to Jim and others for your input. These types of things can be
> like trying to find a leaking pipe.


That's why we are here. Good on 'ya for finding it for yourself. That's
what this is all about.

Jim

[David Lesher]

"RST Engineering" > writes:

>If you look closely at an incandescent light bulb (especially the aviation
>versions like the 327-28 volt and 330-14 volt) you will see that the
>internals of the bulb start with a little dot of ceramic called the "bead".

Well, I was not aware we were talking exclusively about aviation/space
rated lamps, but as it turns out while working for LeRC; I almost
emptied Building 142 with those unshortable lamps. I was driving
them with NE555's which turned out to be effective, if somewhat
smoky, fuses. The technician and I just looked at each other while
the cloud rose toward the smoke detector....the one going to the
sprinkler system and Evac alarms.

The NE555 fuse has an audible annunciator as well; at least when
the charred pieces fall down into the vent fans that spit them out
with a clatter...

In one of those "I'm not making this up.." aspects, my then-boss is
now a participant in this newsgroup, and I'm sure he remembers the
design/assembly in question. {But I ..cough... don't think I ever
bothered him with this particular FUBAR at the time...}

As for the X10/120v lamp aspects, also circa 1980, I'd looked into
it when my BiL wanted to know why his kept failing. I suspect
triac/et.al designs have progressed since then and maybe it's no
long an issue.



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[RST Engineering]

My original comment holds...

horsefeathers.


Jim




"David Lesher" > wrote in message
...
> "RST Engineering" > writes:
>
>>If you look closely at an incandescent light bulb (especially the aviation
>>versions like the 327-28 volt and 330-14 volt) you will see that the
>>internals of the bulb start with a little dot of ceramic called the
>>"bead".
>
> Well, I was not aware we were talking exclusively about aviation/space
> rated lamps, but as it turns out while working for LeRC; I almost
> emptied Building 142 with those unshortable lamps. I was driving
> them with NE555's which turned out to be effective, if somewhat
> smoky, fuses. The technician and I just looked at each other while
> the cloud rose toward the smoke detector....the one going to the
> sprinkler system and Evac alarms.