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PTT issue

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dan(at)azshowersolutions.
Guest

Posted: Wed Jul 13, 2011 8:35 am Post subject: PTT issue

As of this morning my PTT Buzz ghost is gone. I haven't done an engine start 

yet, so I'm not going to do back-flips (don't wanna jinx it) but I am a happy 

camper. As per my e-mail below to Bob N. , I started tracing wires and noticed 

the red and white tristed pair which were "salvaged" wires meant for an engine 

instrument. I know these thermocouple wires are dissimilar and now I know this 

is a big no no to use them in a system for power or return. 

Thanks Bob and All for the ideas and comments. 

Dan B

----- Forwarded Message ----

 	  Quote: 	
		   From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>

 To: Dan Billingsley <dan(at)azshowersolutions.com>

 Sent: Tue, July 12, 2011 9:59:17 PM

 Subject: Re: PTT issue

 At 02:56 PM 7/12/2011, you wrote:

 > Bob,  As I was working on the issue at hand with the PTT buzz, I noticed that 

I

 > used a twisted pair of theromocouple wire (originally intended for an engine

 > instrument) as the hot wire from my buss to my on off toggle switch and then

 > back to the hot wire on the radio. I remember taking a second look at using 

this

 > twisted pair while installing it, but dismissed it. I tend to think is is a

 > non-issue as the radio works great while receiving, but I am now at the point 

of

 > troubleshooting I will entertain any plausible problem.

 That needs to come out. TC wire is very high resistance stuff

 and not at all suited to carrying power. In fact, a high resistance

 in the power lead might very well be the cause of your problem.

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 13, 2011 5:33 pm Post subject: PTT issue

At 11:31 AM 7/13/2011, you wrote:

 	  Quote: 	
		  

<dan(at)azshowersolutions.com>



As of this morning my PTT Buzz ghost is gone. I haven't done an engine start

yet, so I'm not going to do back-flips (don't wanna jinx it) but I am a happy

camper. As per my e-mail below to Bob N. , I started tracing wires 

and noticed

the red and white tristed pair which were "salvaged" wires meant for 

an engine

instrument. I know these thermocouple wires are dissimilar and now I 

know this

is a big no no to use them in a system for power or return.



Thanks Bob and All for the ideas and comments.


	


    What Dan was experiencing was a manifestation of

    'negative resistance' . . . a euphemism for the

    effects on closed loop system stability where root cause

    is an un-accounted for increase in resistance somewhere

    in the loop. The resistance causes devices with gain to

    believe they're seeing positive feedback. Positive feedback

    + gain can produce instabilities, i.e. oscillations.



    The classic example discussed here on the list at

    various times over the years was the "galloping ammeter"

    in airplanes where the power path from the bus through

    a breaker, alternator switch, ov protection system and

    perhaps a dozen crimps and mated pins in connectors

    was carrying BOTH field_current AND sense-voltage

    for a simple regulator.



    After years of service, the sum total of resistance

    in this path might rise by several hundred milliohms.

    At some point, this became a 'negative resistance' value

    that caused the voltage regulator to 'chase' a moving

    target for bus voltage . . . voltage the regulator could

    SEE at the end of the wire was waving in the breeze in

    response to changes of field current as the regulator

    attempted to get it's arms around design goals.



    Interestingly enough, many such problems appeared to

    go away after a mechanic replace THE master switch or

    THE circuit breaker. The mechanic would then pronounce

    the replaced device as 'bad' and push the airplane

     out the door . . . only to have it return in perhaps

    a few months with the same problem.



    This is discussed in more detail on page 9 of



http://www.aeroelectric.com/articles/Alternators/Know_Your_Charging_System.pdf



    This is a situation where TOTAL replacement of all

    components in the field-power/voltage-sense path would

    offer a new lease on regulator stability probably for

    tens of years.



    Dan's situation involved a transceiver that didn't

    draw enough current in receive to produce a distasteful

    voltage at the back of the radio. But when he attempted

    to transmit, the voltage fell below some value that

    caused some part of the transmitter to shut down.

    The current would drop, the transmitter would recover,

    the offended circuit would do it's thing again. In this

    case it was a pretty fast rate of shut-down, recovery and

    shutdown that produced a near musical note.



    These situations are pretty rare except where they are

    literally designed in. The engineers that wired alternators

    in Cessnas in 1968 had no idea that aging of the quality

    of joints in that power/sense path over 30+ years would

    cause the system to go unstable.  None-the-less, it happens

    on lots of older airplanes all the time and very few mechanics

    understand why or how to really fix it.



    Dan's particular discovery for this phenomenon turned

    out to be a craftsmanship/materials issue. MUCH easier

    to find and fix than design errors. His radio is going

    to be just fine in 30 years . . . but as you read these

    words, many an ammeter on lots of Piper and Cessna aircraft

    is waving in the breeze.



    Bob . . .

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dan(at)azshowersolutions.
Guest

Posted: Wed Jul 13, 2011 11:22 pm Post subject: PTT issue

Bob,

Thanks for the very well written explanation of negative resistance.  I 

appreciate all of the effort you give to this list...it is invaluable to so many 

of us trying to learn/ understand the nature of these things called electrons. 

The next time I rent an ole Cessna with a waving ammeter, I'll give it a wave 

back for you.

Dan



---

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