Avionic question for Mr. Jim Weir

Hello !

I have some trouble with a King KX 175B Nav/Comm Radio:
I have connected a 14 V Unit to 28 V Power.
(The aircraft has two voltage buses)
Interesting, it does recieve very good and the lamps wont burn out, but once
you set it to transmit, thats it.
It will transmit once and then never again.
It still does recieve.

Did somebody out here made the same mistake ?
How can I fix it ? Is there a fuse inside ?
Any help is greatly appriciated.

Henning DE

First law of electronics:

"The $300.00 picture tube will blow first to protect the $0.25 fuse."

Although you may be lucky and find there is a fuse in this radio, the first
law will normally operate. I'd take an educated guess and say the
transmitter output stage is burnt out. If it transmitted for a few seconds
at this higher voltage, the heat generated in the output transistor will
probably have caused it to go open circuit.

As embarrassed as you may feel, it will help the technician that repairs it
to know about the cause! This could be reflected in a lower price to repair
than to just chase the problem around without knowing the cause. Most radio
gear will handle a partial overvoltage of 16 to 18 volts as part of the
normal operation expected with an engine driven alternator, the 28 or so
volts that has been applied would be well over any design expected event.

Hope this helps,
Peter

"Henning DE" wrote in message
...
Hello !

I have some trouble with a King KX 175B Nav/Comm Radio:
I have connected a 14 V Unit to 28 V Power.
(The aircraft has two voltage buses)
Interesting, it does recieve very good and the lamps wont burn out, but
once
you set it to transmit, thats it.
It will transmit once and then never again.
It still does recieve.

Did somebody out here made the same mistake ?
How can I fix it ? Is there a fuse inside ?
Any help is greatly appriciated.

Henning DE

Most likely you fried the RF power output transistor and possibly the
RF driver transistor. The good news is 95% of the radio still works.

You cannot replace these transistors your self as this requires a FCC
general class radio telephone license to work on the transmitter and
some test equipment you obviously don't have, (or you would not have
asked the original question in the first place), to retune and check
for spurious emissions.

On Fri, 17 Sep 2004 20:51:43 +0200, "Henning DE"
wrote:

Hello !

I have some trouble with a King KX 175B Nav/Comm Radio:
I have connected a 14 V Unit to 28 V Power.
(The aircraft has two voltage buses)
Interesting, it does recieve very good and the lamps wont burn out, but once
you set it to transmit, thats it.
It will transmit once and then never again.
It still does recieve.

Did somebody out here made the same mistake ?
How can I fix it ? Is there a fuse inside ?
Any help is greatly appriciated.

Henning DE

I have some questions rather than jumping to conclusions...


-I have some trouble with a King KX 175B Nav/Comm Radio:
-I have connected a 14 V Unit to 28 V Power.
-(The aircraft has two voltage buses)

Do you mean that you connected the 14 volt radio to the 28 volt bus and then
powered it up? If so, what do you mean by saying that the aircraft has two
voltage busses? That there is a center-tapped 14 volt bus and you connected it
to the 28 volt bus by mistake?


-Interesting, it does recieve very good and the lamps wont burn out, but once
-you set it to transmit, thats it.
-It will transmit once and then never again.

Do you mean that every time you turn the radio off and then back on that it will
transmit once and then not again? Or that it transmitted once when it was
powered up to 28 volts and that it now will not transmit at all?

Jim



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

"Jim Weir" schrieb im Newsbeitrag
...
I have some questions rather than jumping to conclusions...


-I have some trouble with a King KX 175B Nav/Comm Radio:
-I have connected a 14 V Unit to 28 V Power.
-(The aircraft has two voltage buses)

Do you mean that you connected the 14 volt radio to the 28 volt bus and
then
powered it up? If so, what do you mean by saying that the aircraft has
two
voltage busses? That there is a center-tapped 14 volt bus and you
connected it
to the 28 volt bus by mistake?

Well, Jim, it is kind of a mistery to me. The Aircraft is a old six
cylinder C 172 (around `67 or so). On one of the avionics fuses has 28
Volt while the rest of the electrical system is 14 Volt. I did connect the
14V radio to 28 Volts.

-Interesting, it does recieve very good and the lamps wont burn out, but
once
-you set it to transmit, thats it.
-It will transmit once and then never again.

Do you mean that every time you turn the radio off and then back on that
it will
transmit once and then not again? Or that it transmitted once when it was
powered up to 28 volts and that it now will not transmit at all?

When I first powered up the radio everthing looked fine. I had a wattmeter
in the coax line to the comm antenna. When I pressed the mike key the first
time there was a power output ( about 5 watts) to the comm antenna. When I
pressed the mike key again, there was nothing and it stayed that way. The
radio does not transmit anymore.

Henning

Jim



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com