Hi James,



Thanks for your feedback. I think we are talking about two different

generations of Piper / Mitchell autopilots. Mine is an Altimatic II,

installed in a 1964 Piper Aztec. It obviously is original equipment,

factory installed. All the electronics are composed of germanium

transistors, so that also dates it.



My info all comes directly from the Piper Service Manual entitled

"Autocontrol I & II, Altimatic I & II Service Manual". The Piper part

number is 753 798 with a first publish date of June 1961. It contains

permanent revisions numbered PR710920 and PR721020, dated

September 20, 1971 and October 20, 1972, respectively. There are

also pages with various dates between these dates.



The 10Mhz frequency information is contained in the theory of operation

section and it describes in good detail the construction and operation of
the roll

vane in the AI. It shows it as being composed of an Iron Ferrite piece

on one side and of an Aluminum piece on the other. As the aircraft

banks to the Iron side, it says the inductance of the circuit is increased
and

when it banks to the Aluminum side, the inductance of the circuit decreases.

It also details a similar pickup in the AI for pitch and a variable
capacitor in

the DG, variable capacitors in the each servo's "follow-up" circuit and
variable

capacitors in the amplifier and control console. It goes step by step on
how

a change in bank and a change in pitch and altitude changes each reactive

component and how all of the these reactive components work together to

maintain a given resonant frequency.



Maybe the 10MHz figure is just an example - I have not measured the

frequency. However, the roll and pitch servos in my 1964 Aztec definitely

have the feedback or "follow-up" capacitors. I've seen them with my own
eyes

and checked their timing marks to see if they are lined up properly.



There are RF type variable capacitors in the control console and in the
console

amplifier to adjust the circuit reactance (level the wings and level the
pitch). There

is also an RF type variable cap in the altitude bellows assembly to detect a
change

in pressure altitude. The trim-turn and pitch controls on my control
console are

also variable caps, along with an altitude / pitch offset adjustment.



The amp has two cans which look like IF transformers with paper labels on
them

that state "Do not adjust discriminator". The whole thing is connected
together

with coax cables and the service manual is clear about using the proper
length

and type of cables otherwise the resonant frequency will be shifted. The
service

manual shows a drawing of these cable as being composed of a hollow tube
that

contains a curly wire on the inside and a braided shield on the outside.
The whole

thing is covered with an outer jacket of rubber or plastic material. It
reminds me

of the radio antenna coax used in automobiles in the 1960s.



When I went looking for an instrument shop to overall my AI, I ran into
several that

stated they would not work on "those old RF type gyros". All of this taken
together

makes me believe that the system does in fact use an RF signal as the method

to detect changes in pitch and bank and to detect the position of the
servos.



From the limited information I can find, it appears that the later Mitchell
/ Piper

versions switched to the pickup system that you describe. Maybe this was
the

Autocontrol IIB / Altimatic IIB and later versions? I don't know.



I wish I had a schematic of my console amplifier. My service manual is
lacking

in this area. It treats the amplifier as a black box. It does provide a
schematic

of the radio coupler, but no theory of operation information, or other
useful info

like the excitation frequency it supplies to the DG, voltage levels, etc.
Any

information you'd care to share along those lines would be greatly
appreciated.



Just FYI, the earliest Autocontrol and Altimatic systems used what the
service

manual refers to as a "0" heading directional gyro. This device did not
allow a

specific heading to be selected, but rather just indicated a change in
heading which

was used to keep the wings level. Also, these early models had amplifier
modules

attached to the back of the AI and to the back of the DG. There was no
central

"console" amplifier. When I first got the service manual and saw this, I
knew these

didn't exist in my airplane and thought there were missing components.
However,

as I dug in more, I finally figured out that these were consolidated in a
later version

and that the DG was changed to a resolver type wired to the radio coupler
rather

than the older RF type wired to the autopilot amplifier.



What is the excitation frequency of the radio coupler to the DG in the later
Mitchell /

Piper autopilots? Maybe it is the same in mine.



FYI, my coupler has a 4-pin Amphelnol connector for the radio input and a

9-pin Amphenol connector for connection to the DG and autopilot amplifier.

In my case, the coupler could not be eliminated from the system by simply

moving connectors around.



I'd be happy to learn all you're willing to teach me about these things. If
you have

any schematics or theory of operations on the early Autocontrol / Altimatic
I/II

amplifiers and/or on the early radio couplers, I'd really appreciate getting
a copy.



Ronnie



"jmk" wrote in message
ups.com...
Hmmm... I have to admit, that's quite a bit different than a standard
Autocontrol (aka Century) for a normal Piper system - and I would ask
if you are sure of your information? The standard "wing level"
indication from the AH is indeed off a pickup coil, but it's about 5
KHz, no RF. And there is no frequency shift - the inside of the
Century AH/AI just has two coils and a metal plate behind them (looks
like an iron butterfly). As the thing shifts from one side to the
other you get a shift in amplitude, either in phase or out of phase
(180 degrees) with respect to the ROLL EXCITATION signal. [Or the HDG
EXCITATION signal, if it goes through the nav coupler.] With the AH
centered in roll, the amplitutde is zero. There is no frequency shift
regardless. It would be great if there was some sensor back from the
ailerons, but there isn't - so it has no way of knowing their position.

Having said all the above, your "list of the usual suspects" is the
same one I would come up with. #3 is the one that surprises most
folks, but if you have a nav coupler then it uses HDG EXCITATION to
drive the ROLL coils, so lose of HDG signal affects just operating in
the WINGS LEVEL mode. You can move the ROLL cable that goes from the
computer to the NAV Coupler to go to the AI instead and restore use of
ROLL EXCITATION for a test.