Rheostat Audio Interference

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

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.

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

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

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

"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

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

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

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

"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