Rheostat Audio Interference

"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

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

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

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

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?

"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

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.

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

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

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