Jim:
I have the Cessna RT-359A service manual in front of me. There is NO
disabling of the transmitter during self test. In fact several months
ago I watched the voltage on the cathode of the transmitter tube with
a scope and it pulses when the test switch is depressed. There are
only 4 wires and a coax cable to the TX tube. These are Cathode,
filament, anode , ground and RF out. The anode is +1400 volts DC,
the filament is +6.3 volts and ground is of course at ground. The
cathode is pulled to ground by an NPN transistor to transmit.

On page 5-9 paragraph 5-10 step 2 the manual tells you to press the
test switch to measure the TX peak power and frequency of the
transmitter.

King and Narco may or may not disable the transmitter for self test.
I do not have schematics for these transponders so I can not say one
way or the other but I see no reason to do this since it would add
extra circuits and extra cost to do this. They may do this on the
more modern units since the logic may be in an EPLD and no extra
production cost would be incurred to add this small piece of VHDL
code.

I used to have my email address as part of the post but a few months
ago I started to getting 20 to 60 bounced email messages per day that
I had not sent from people I had never heard of. Apparently the
spammers were not only using my email address to spam me at the rate
of 30 to 40 per day they were also spoofing my address on the spam
they were sending. Due to this I now no longer show even a human
readable address but if you insist here it is: frerichsATroDOTcom.
If you want my address and phone number look up airport identifier
AL10. If you email me your phone number I will give you a call and we
can talk about this.
This all started over an EMC question and I am the EMI "expert" where
I work. (The definition of expert is: A drip under pressure.)
John

On Wed, 10 Mar 2004 19:52:48 -0800, Jim Weir wrote:

(John)
shared these priceless pearls of wisdom:

-Lets see if I got this correct. A micro power oscillator injects RF
-pulses into the receiver which then thinks this is a radar
-interrogation pulse and fires out a normal TX pulse out of the
-transponder.

Nope. The transmitter's local oscillator (or a microfragment thereof) is
injected into the receiver's front end. The receiver tests whether or not the
microtransmitter was received.

The transmitter is inhibited by a digital signal during test.

You got it wrong. Sorry.

If you want to carry on this conversation, post your real name and real email
addy.

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