View Full Version : Autopilot Servo

Paul Sharp
07-08-02, 12:30 PM
re: 1967 T337B, w/Cessna Navomatic 300 single-axis autopilot -

It has always worked fairly well in heading mode - a little looser in trackng under the "Nav" mode, but worked OK.

But on last trip, autopilot started commanding left turn. Nothing I did would change the settings. Worked that way in either the DG or Nav modes, no matter where the heading knob was turned.

Called Autopilot Central in Tulsa. They said it was most likely to be the servo unit, which if I remember right he said was in the right wing, accessible via an inspection plate between the flap and aileron.

If it turns out to be bad then they charge $895 to overhaul the servo.

Any suggetions from others out there?

1 - Is it hard to remove and reinstall the servo?
2 - Would it be cheaper to buy a surplus servo?
3 - Other places to recommend that do good work for less?


07-08-02, 12:39 PM
NOPE ITS not hard to remove.. but, when you put it back in.. you have to re-rig the plane... and can't fly it while its out... sorry.. GMAs

Bob Cook
07-08-02, 01:21 PM
This autopilot is the old ARC type. I have the manuals. What is the servo part number?

1) could be the clutch
2) There are transistors (series pass) that control direction. One of these could be bad.
3) the motor (least likely) is at fault.

Most of these have been pulled out by now and there are a ton of servo motors around. Call STEC, sure they have some and also have seen them on Ebay.

I might find you one for about 400 dollars but that part number is crital. The elevator servo is the same unit.



Paul Sharp
07-08-02, 06:03 PM
Yes, mine is the old ARC unit. I don't know the servo unit number - will have to pull the access panel, at least, to get that. Don't know if it's readable with merely removing the panel or if you have to remove the unit to get it.

GMAS, not being able to fly isn't a nice option. Also, not knowing about the rigging involved, I'm curious why it has to be re-rigged?

Sounds like if the re-rigging is required I'll have to find a competent shop to do that.

Bob Cook
07-08-02, 09:26 PM
The electronics portion can come out without taking the motor out if I remember correctly.
You should be able to get the unit number off the equip list (original).

There are two large transistors that is probably one is the culprit.
One controls one direction and the other, the other direction. I am sure one of these are blown.

If you have an electronics person you can check using a voltmeter in which a control voltage determines direction. You can check this at the plug. There is a p/n tag on the unit giving model, revision number of the unit. From this I can give you the correct pinouts and semi conductor part numbers. You should be able to ground test it by putting engaging autopilot and trying to assist the wheel while turning the roll command pot (console). If your assisting helps and the motor appears to be working then it may be the clutch that is not working properly.

Ground test first and listen for noises and report back. Also check trim servo and if this is normal you know the main computer power supply is working properly. The main computer had +/- pwr supply problems.

You need to eliminate the servo first as it is the simplest to test then work backwards.

It is not that difficult if you can isolate the problem. There are three major components and it is hard for a radio shop to trouble shoot the system out of the aircraft. Many times the problem is in the wiring and not in the electronics portion. In fact it can be dirty connectors at the servo causing the problem.

let me know how you make out.


Paul Sharp
07-09-02, 12:08 AM
OK, Bob. THanks for the input. I'll go down and have a look at it and see what I can do.

Kevin McDole
07-09-02, 01:22 AM

Maybe this is a long shot, but I'll suggest it anyway. Don't those old Cessna autopilots have a knob that allows you to turn the aircraft (overridnig the NAV or HDG mode)? I think it's a black knob on the autopilot that looks like an egg timer knob, and it's surrounded by a 270 degree white collar. If that knob/collar is rotated, then the plane will turn in the direction it's pointing. The knob should point straight up. Make sure that's not your problem.

Paul Sharp
07-09-02, 03:28 PM
Kevin, mine has a 3 knobs - -

1 - Left: Turns A/P off, to Heading, or Omni (VOR/GPS - separate switch on panel).

2 - Right: Sets heading of A/P when in "Heading" setting.

3 - Center: "Pull Turn" or "Push Heading-Omni" and also has a small outer lever at the bottom which you can push left or right by say 30 degrees total (small adjusting for imbalances or whatever).

I've tried the various combinations, and nothing makes a difference. The A/P keeps the plane in a left turn no matter what settings are used (except "Off" of course).

Bob - Here's info from the O.E. List on my A/C:

ITEM NO: 501-A
DESCR.: Cessna Nav-O-Matic 300 (includes new A.R.C. directional gyro, new R.C. Allen horizon gyro, and dual vaccum system).
REF. DRAWING: 1470067
WT. LBS.: 33.5
ARM IN.: 114.0

On the DESCR stuff I'm not sure all is like it was, although it may be exactly. But the stuff about the 2 "new" gyros has me wondering? Does that mean they're somewhere in the aircraft or have they been removed, or what?

I'm planning on going out in the morning, taking the wife with me to man the knobs while I do a ground check to see if I can hear the motor running. I'm assuming I can just turn on the Master and then the A/P and I should hear something, at least if the wheel is being turned or whatever?

Then I'll try to remove the appropriate panel and see if I can get any numbers or info on the servo.

Paul Sharp
07-09-02, 03:34 PM
Oh, forgot to mention that the sticker on the control head itself (the "Controller-Amplifier" says "Type C-594A-1 (28V)" on it, S/N 127. It also has the following numbers on it:

20405-5 (top circuit board)
29610-0028 (top circuit board)

ARC 29407-4 (bottom circuit board)
29611-0028 (bottom circuit board)

(I don't know what's in between the circuit boards.)

I did check the connectors, pins, sockets to make sure things were making contact, etc. for the 3 plugs that plug into the back.

Bob Cook
07-09-02, 05:18 PM

You should be able to turn on master, pull knob and turn the knob left or right in which the ailerons should follow. The servo motor should operate incrementally. When AP is on you should not be able to turn the yoke. This is a manual overide and has nothing to do with the AI. The autopilot has feedback from the attitude indicator and turn info from the heading select bug on the DG. This test is basically putting a correction voltage from the pot directly to the servo. The outer ring is used to compensate for fuel load or trim and centres the should be trimmed so the heading bug aligns with top dead centre.



Paul Sharp
07-10-02, 10:27 AM
Well, this morning I turned the A/P on and I hear the servo motor going, so I took off that inspection panel and looked at it. I couldn't read the model or part number plate because it was too close against a panel and impossible to read in the small space wihtout removing the unit.

Whenever the A/P is on, the motor is running - still trying to command a left turn, I suspect as that's what it does when I'm in the air flying. It doesn't matter whether the knob is in or out or what any other knob positions or settings are, it stays running trying to make that turn.

The case was split diagonally, held in with 4 screws so far as I could tell. The whole unit, i.e. the whole case, was mounted in the wing, of course, but if I were to try to undo the "electronics" portion, or what appears to be the outer hal fo the split case, I'm not sure what I would have found and I was leery of messing with that if it might leave the plane where I couldn't fly it while getting it checked or fixed.

So I'm not sure what to do next at this point.

BTW, Bob, when you mentioned "so that the heading bug aligns with top dead centre" what do you mean? Do you mean that when you set a course with the A/P "Heading" knob that the bug on the DG should center (or else you adjust it then with the little outer lever)?

Bob Cook
07-10-02, 11:39 AM

re autopilot. I meant that the DG heading bug should determine the direction the autopilot steers the AC.

I found the manual. You have either a follower potentiometer in the actuator that failed or the transistor (s) that have quit.

Just remove the four fillister head screws. The electronics can be removed without removing the motor and clutch assembly. You can still fly the aircraft but placard the autopilot and disconnect fuse or breaker.

While you are at it you should check and clean brushes in the motor.

The transistors are
Q1,2 2n1040
Q3,4,5,5,6,7,8 2n1546
CR1 1n2972
CR2 1n2970

left turn is Q1,3,5,7
right turn is Q2,4,6,8

What is tricky is all the above transistors are DC coupled and constitute a DC amplifier.

One of the transistors /2n1546 most likely / is likely shorted and needs to be replaced. A local radio shop or ham can test the transistors and replace the defective unit.

Check the potentiometer with an ohm meter and make sure it is okay if transistors check out okay. The pot provides positional information to the autopilot computer to electrically show the deflection of the aileron. If the pot is open the autopilot thinks the aileron is where it isn't <G>.

pin out of plug (small round one) is
A = voltage CCW
B = voltage CW
E = DC return
C = 10 VDC(-) reference volt when ap is on.
J,H +24 vDC
k Follower on potentiometer
D effort (motor winding)
F effort (motor winding)
Ground = aircraft.

The removable portion is a PC board with heatsinks. Rotate collar on connector and pull gently. The connector is polarized.
It is a small round blue body that supplies connections to the controller.

I can scan schematics etc. but almost guarantee this is the problem.

This is a PA-510A Actuator. there are different transistors based on a 12 or 24 v actuator! Do not get these mixed up.

If this is okay then check to voltages in the connector / -10 / +24 to make sure they are present. The 24 volt is for sure based on the motor running. The -10 ref voltage if not present could cause similiar problem by biasing the amplifier in one direction.

The elevator servo in the port tailboom is exactly the same and if you wanted to do a quick test you can substitute the electronic portion above (swap out). Your choice.

In the end, you should be able to pull out the turn know and move the ailerons left or right on the ground with the AP engaged as a test for the servos.



Paul Sharp
07-10-02, 03:25 PM
Cool stuff. I really appreciate the details. I'll try pulling out the electronics guts and see if I can find someone to check the transistors. I'm a ham and have elementary knowledge but never did much building and never tested transistors.

My potentiometer in the Control head sets the heading. But it obviously follows the DG because if I move it with the precession knob it changes the heading the A/P is taking accordingly. And I was in error in referring to a "heading bug" as my DG doesn't actually have one. It has simply the adjusting knob for precession. So it would seem that my outer adjusting lever in the middle of the A/P just allows things to be centered for balance/fuel, etc.?

Also, if there's something in the tail I wasn't aware of that - mine is only a one-axis A/P and I don't have electric trim. So I don't believe I'd have anything in the tail unless there is something on the rudder (seems unlikely), would I?

The guy at A/P Central in Tulsa said it was probably either the servo (as you've been saying) or it could possibly be the turn/bank indicator which might have lost it's sending to the control head; that apparently makes sense from a standpoint that it helps keep the plane straight-and-level?

The potentiomenter - it seems that if it's nothing other than a pot that it would be unlikely to fail, wouldn't it? I would more suspect the transistors, from what you've said, shouldn't I?

When you said clean motor brushes, etc. - does that mean they're accessible once the electronic half is out?

If I need a transistor all I have to do is replace with same model number, I assume? I mean mine is a 24V system although it may be stepped down to 12V for the A/P it sounds like not so, but if you replace a transistor with the same model number the voltage will be right, won't it?

Bob Cook
07-10-02, 05:42 PM
A) if single axis then there is no altitude hold. A servo in the rudder is called a YAW damper and no you do not have one.

It could be the sensor in the AI which is giving the AP reference, HOWEVER, the acid test is moving the manual potentiometer on the ground which OVERIDES and does not use the AI input. It is taken out of the circuit.

The semiconductors are for the 28 volt model. since the transisitors are DC coupled, you may need to take them out of the circuit for testing.

There is a way of testing the servo while in AC but I would leave that to a knowlegeable person (techie) or a person like GMAS.

You can get at the brushes when the elect module is out. Probably okay. Brushes may be worn. lube the chain and gear mechanism but no lube on the clutch.

I have seen these units show up on EBAY but transistors are worth about 3 dollars apiece. Cheaper than postage!.

[B] Never take your autopilot out and send it away. You are asking for trouble!!!!!!!! [B]

These transistors are TO3 cases and easy to test and replace.

Be careful!


Ron Ball
07-10-02, 05:58 PM
Bob, does the same basic set-up apply to a 400 auto-pilot? I have a T-337-B 1967 that half the time the auto-pil;ot when turned on turns to the right. I turn on and off 5-6 times then most of the time comes up and works. Some times comes right on, if do not use for awhile 2 weeks or so, then does that turn to right situation.

Bob Cook
07-10-02, 06:40 PM
The autopilot is basically the same. The actual servos are very similiar, however, there is a little more complexity.

The acid test is flying with the steering know on the ground or in the air. You isolate the heading reference portion of the system. Yours problem if intermittent is more likely a dirty follower pot in the servo. You can measure with an ohmeter while moving aileron on the ground. i can give pin numbers.

Other problem would be the reference voltage (-10 volts DC) see pin numbers. Other causes can be broken wire or dirty connectors..

The 400 computer is more complex and the power supplies on the 400A were not very stable but once working they seemed to work forever. Once stable with the turn pot controlling the aircraft and problem still exists then it is more likely to be the Pickoff coils in the AI. Need more information as to how the AC reacts before I can give you help.

My 400 B with IFCS works like a charm and is rock solid. Does full intercepts with ILS hands off. They just need a little TLC and when working are far better than the S-TEC which is a roll rate device.... ARG!

get me the computer number and I probably have the manuals for it here. Does it have three switches on the right side one above the other ?


Ron Ball
07-10-02, 07:21 PM
The unit I'm referring to is an 400, not and A. It is on my other Skymaster. 1967 T-337-B. My P-337-G is a 1973 with the 400A. the problem with the 400 is intermitant. When cycled on , sometimes wants to turn to right ,no matter what is done with the knobs. After 5-6 times comes up and works. Sometimes comes up on first time, a few times would not come up at all[ I mean stays in the right turn mode].

Bob Cook
07-11-02, 08:45 AM
400 and 400A have basically the same computer and both are two axis I believe.

In any case, by pulling using the knob only steering, everything is eliminated as far as directional control.

If you cannot HAND steer it with the knob and pilot engaged then there are a few things to look for.

1) power supply +/- voltages at the computer.
2) reference voltage (going to feedback pot at the servo) which is -10V.
3) broken or bad connection between the feedback pot and computer or a dirty and intermittent feedback pot at the servo.

Sounds like dirty connectors or dirty pot. You can swap the electronics from the elevator servo to the roll servo as they are the same.

Give me a computer part number and servo part number and I will give you more feedback.

As mentioned earlier the power supplies gave problems. It could also be dirty contacts on the printed circuit boards in the main computer itself. The computer should be located at the back of the aircraft, in the headliner behind the pilot and behind the flap actuator motor.

The engage switch is in fact a solenoid that is actived by an archaich series of "and gates" one of which is power. If you are having problems engaging the switch and staying latched it is most likely the power supply.

check the connectors.


Paul Sharp
07-11-02, 02:55 PM
Bob, here are some additional questions (sorry and hope I'm not being a PITA):

1 - If the clutch were bad, how would it make a turn either way? I probably don't understand the function of the clutch.

2 - If there are CW and CCW voltage leads, who two 24V leads?

3 - What do the "Effort" leads do? What should they read?

4 - Does the description of the leads mean there are two totally separate circuits in play for each direction (left and right) and that when measured, the appropriate leads should show voltages and the opposite ones won't?

5 - Assuming the -10V reference is there properly, then if I fire up on the ground, pull the center knob and turn to the left, will the CCW and matching 24V and "Effort" leads all read and the others not? And if I then turn to the right, will things be reversed?

6 - When measuring the voltages, will they always be referenced to A/C ground, or to their own side of the circuit?

I'm wondering once I've got the thing out why not just replace all the transistors? They seem cheap and why mess with them? If the control head is sending the correct signals and the potentiometer is good, seems I should just replace all the transistors, or at least the seven 2n1546 versions as you mentioned.

(I think I understand your point about the center knob and the turn coordinator: if it overrides that then and there is still the malfunction, then a potential loss of turn coordinator signal isn't the problem anyway.)

Bob Cook
07-11-02, 03:41 PM
Bob, here are some additional questions (sorry and hope I'm not being a PITA):

1 - If the clutch were bad, how would it make a turn either way? I probably don't understand the function of the clutch.

Clutch allows you to overide AP. Also slips so motor does not stall.

2 - If there are CW and CCW voltage leads, who two 24V leads?

These leads > should see voltage (control input)

3 - What do the "Effort" leads do? What should they read?

feedback for current monitoring to computer.

4 - Does the description of the leads mean there are two totally separate circuits in play for each direction (left and right) and that when measured, the appropriate leads should show voltages and the opposite ones won't?


5 - Assuming the -10V reference is there properly, then if I fire up on the ground, pull the center knob and turn to the left, will the CCW and matching 24V and "Effort" leads all read and the others not? And if I then turn to the right, will things be reversed?

should see voltage on cw and ccw inputs from computer (control voltage). small voltage on effort (current). -10 is input to pot and each end wiper goes back to computer

6 - When measuring the voltages, will they always be referenced to A/C ground, or to their own side of the circuit?

to gnd

I'm wondering once I've got the thing out why not just replace all the transistors? They seem cheap and why mess with them? If the control head is sending the correct signals and the potentiometer is good, seems I should just replace all the transistors, or at least the seven 2n1546 versions as you mentioned.

its up to you but not necessary. may be easier for you in the long run.

(I think I understand your point about the center knob and the turn coordinator: if it overrides that then and there is still the malfunction, then a potential loss of turn coordinator signal isn't the problem anyway.)

right. you should be able to roll ailerons left and right with knob on gnd and pwer to autopilot. it is a simple gnd test to insure computer and servos are working properly including motor. The clutch allows you to overide with considerable force in the event there is an autopilot runaway.

hope this helps and no you are not a pita. ..


Paul Sharp
07-12-02, 10:45 AM
OK, here's what I found this morning - first the wiring to the plug which attaches to the servo:

A - White/Green
B - White/Yellow
C - White/Red
E - Green/Dark Blue
F - Green
H - Red
K - White/Grey
(7 wire bundle)

With the A/P running, center knob pulled and turned to the left, only pin with any voltage reading on it was H. That's it. Whether i turned the knob to the right or pushed it in, that was the only pin reading any voltage, positive or negative, of any in the connector.

So, where am I then? Any suggestions of what I should do next?

I called around and checked on the transistors; they all cross over now to NTE numbers, but are listed as available (some here in stock, others would need ordering). But it seems from the readings I'm getting that I may have a control head problem?

07-12-02, 11:20 AM
Been reading the mail... and it seems you all are looking in the right place... replacing all the transistors is not a good thing to do... as the new ones are not the same as the old ones... they have more gain and a different set voltage... 0.7 while the others are 0.3... so you will lose some of the sense on the operation...


More its the pot in the servo that gives you the feed back... and they are AB pots that can be gotten from Newark Electronics... they are a carbon type... and were never designed to be used like the ones are... and yes the carbon wears thin by the movement of the controls... weather its on or off... to the point where it is no more... and you get the symptoms of it commanding one way or the other... another problem is that it gets oil on it... and that will give you the same problem... WD-40 and some of the other lubes are penatrants... and they make their way into everything... and in this case will mess up the electronics..

Hope this helps... you guys are doing a good job of going thru the operation and parts of the thing... GMAs

Bob Cook
07-12-02, 01:43 PM

there should not be any difference in semiconductors as they are all silicon. Secondly, the gain is not a real issue in this circuit.

Get an ohmeter and check for shorts in Q7 and Q8 to start. If there is a short then replace. Lots of radio shops still have the original part numbers in stock including autopilot central.

The potentiometer. We do not know if it is any good. put an ohmeter between pin K and H and then move the aileron. You should see the DC resistance change!
We are trying to determine the problem here and we are not eliminating anything.

There should be a reference 10 volts at pin C (when the electronics are plugged in. Can't measure voltages without these being back in place. It should be seen on top of r18.
CR1 is a zener diode.

If the pot does is bad then the same symptoms can occur.

1) check q 7,8
2) check pot R13 for continuity and value.
3) check CR1 / and check for 10 volts on CR1 adn top of r15.
4) you should see a voltage on pin K which is potentiometer output (all electronics back in place).
5) if no voltage in 4 then CR1 is shorted and i will give you the value.

Now lets go back and systmatically resolve these questions or get a technician and have him check these things out. Lets not guess and waste time.

I will scan the necessary diagrams or if you have a technician call me I will discuss the problem with him. Print out this email and give it to him as well..

We will get this fixed...................... somehow!


Burt Benson
07-12-02, 04:42 PM

I have two servos (roll and pitch) out of our 1976 337G. We removed the ARC 400A in favor of an STEC 55. The servos are in good condition.

ARC Roll Servo p/n PA500A
ARC Pitch Servo p/n 3570-0028

If you are interested in either of them let me know.


Paul Sharp
07-12-02, 04:48 PM
OK, let me see if I get what you're saying correctly:

1 - Put the plug back onto the servo since if it's not plugged in things won't read right? I assum then to check things I'll have to undo the back of the plug again so I can get a meter on the leads.

2 - Check between pins K & H with ohmeter for resistance change while moving flaps to make sure potentiometer is working OK.

3 - Turn unit on, check for voltage (not resistance) on pin K.

4 - With unit on, check pin C for -10V.

Now, as to checking other things - I'll need to remove the unit to check the following, won't I?

Q 7, 8
10V on CR1 or R15

I have CR1's equivalent. From what you've said I can use the new NTE equivalents anyway so if I have a bad Q7 or 8 or CR1 then I can order the parts from the local store.

I have a brother-in-law who's got more smarts than myself (remember I'm just mostly an applicance operator), so if you can fax the schematic I'm sure it would help. I'd be glad to repay you for any phone call costs. Here's my voice number: 801-463-7012 (home where I am on Fridays and evenings/weekends).


Burt Benson
07-12-02, 04:56 PM
I should have mentioned that I have all the parts for the ARC400A.

ARC A/P Computer Amp p/n 37970-0328
ARC A/P Controller p/n 37960-1028
ARC Trim Servo p/n PA520B
ARC Trim Sensor p/n DT-520A
ARC Trim Regulator p/n 1270723-6
Autopilot Relay assembly


07-12-02, 05:34 PM

I've got an inop 400A in my 337A. How much do you want for yours?

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Bob Cook
07-12-02, 05:35 PM

1) YES
2)yes, you can also put ohmeter right on the pot between centre on one side and centre and the other side. To check pot you do not need board plugged in or power on,.
3) see above
4) yes.. 10 volts not minus (-) 10 volts
5) check transistors for shorts with ohm meter. in or out of circuit. there should be a forward and backward value like a diode between collector emitter and base.
6) I will scan and send schematic but unable to do so until sunday. If you do the above I am sure you will narrow it down to one or the other.

if the voltage is missing (10v) then check the 10v zener diode (cr1). it can be replaced locally as well. This is the reference voltage for the pot and is dropped in the circuit from 24v to 10 volts. You should see the voltage on the wiper output as you move the ailerons with the autopilot on.

btw the 400 servos are different than the 300 as mentioned before.



Burt Benson
07-13-02, 09:18 AM

I'll send you the whole lot and a manual for $900 plus whatever it costs to ship. You will also probably end up with some customs charges. The computer might need a bit of work. When the altitude hold is engaged, it will start to climb after a short while. Otherwise everything works fine. Let me know if you want it.


Paul Sharp
07-14-02, 11:17 PM
Bob - I didn't see anything on the FAX machine. I sent the FAX number (both the toll-free and the 801 area code versions) to you n an email message. If the 888 didn't work, I've heard occasionally from someone not getting through on it from the east coast, but the other will work, and I'll be happy to return the $ for any phone calls.

My brother in law is willing to go out with me Wed. AM early to trouble-shoot it - he has electronics expertise and is good. So if we can get the schematic that would really be helpful. I've printed all the emails and will take them with me for him to look at, too.

Burt, sorry but I didn't notice your original message to me - I don't know if your models are compatible with mine but I want to try to fix mine first before looking or a replacement; sounds like Devin may be interested.

07-16-02, 03:48 PM
The inop autopilot on my 1966 337A is a "Navomatic 400" Is this the same thing as an ARC 400 or ARC 400A? Where do I look on the unit itself (logs and original equipment list is with A&P making entries)?


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Bob Cook
07-16-02, 04:46 PM

There is the 400 and 400A, also the 400A with IFCS. The one you have should have three switches down the right side. In the original equipement list is the part number for the computer. should be a 535-xxx mod ?

Then I know which one you are talking about. The computers are all basically the same with the same servos. There was a major change around 75 when they upgraded the system. All autopilots were made by ARC / American Radio Corp. Believe Cessna owned these guys at the last. they made good stuff (some of it anyway).

What's the problem and I will tell u where to look.


Ron Ball
07-16-02, 05:10 PM
The 1966-A would have, if stock the 400[ No 3 swithces down the right side] Has center turn knob for directional control. I have a 1967-B with this 400 unit. Also, have a 1973 -G [pressureized] with the 400A [ has the 3 swithed, white rocker switches, down right side. They are not the same.

Pete Somers
07-16-02, 06:11 PM
I have been reading the various messages about the 300 nav-o-matic in Paul's 67 T337B.

The servo is the old version which you undo 4 screws, the 2 at the bottom are easy but the 2 upper ones are hard to get to, then the servo splits at and angle and you remove the whole servo which has the electronics,motor, followup and clutch leaving just the drive assy in the wing so that the a/c can be flown with this part removed.
That does make fault finding alot easier!
The computer is part of the control head (the 2 boards that were mentioned).
I pretty sure that the you will find the drive transistors u/s you will be best to replace them all.

You will need to recenter the servo before you secure it back in the wing.

If the servo is not the problem try diconnecting the plug on the AH and see if that cures the problem.


07-16-02, 11:17 PM
Ron, Bob, et al,

Here's the way my 66' 337A "Navomatic 400" appears on its face:

From left to right and top to bottom:

Top left is a vertical dial labeled "up down" and below that is a yellow light labled "trim"

Next and to the right, top is a round dial which on top of the dial reads "push heading omni" and the dial itself reads "pull turn"

Below this on the bottom of the panel are three toggle switches from left to right which read "Alt On", "Omni Hdg" and "On Off"

And to the far right on top is a round dial which reads "Heading" and has numbers on the dial itself (like a DG).

From what Ron said in his last post, it appears that I probably have an ARC 400, not the ARC 400A.

Bob, in response to your last post to me, my problem is that when the headliner was off, I discovered that I had NO SERVO, that, naturally, poses a problem for using the AP.

The question now is can I use Burt's Servo (and other parts if necessary) from his 400A on my bird?

Bob, you'll be able to see this first hand next week if all works out well for OSH. But any help from you all before then to get the wheels in motion would be most helpful.

Let me know.


Voyeur Xxx (http://www.fucktube.com/categories/43/voyeur/videos/1)

Bob Cook
07-17-02, 12:18 AM

Are you telling me you have no computer? There are no servos in up in the headliner, just the flap motor. The elevator servo is in the port boom at the back and the trim servo is in the starboard boom (copilots side). The aileron servo is in the wing.

If you do not have the computer then you must be very careful trying to mate a new one in it's place. You will need to go thru a complete alignment of the servos and computer in the aircraft including flight test.

This is not good news. I can probably find someone with the expertize to help do this reserection.

Why not meet me in Buffalo and fly to Osh together? Why take two aircraft? Its less than 2 hr flight north of Greensboro to IAG.
I am rather open when I come and go. Maybe drop in and Grab Larry on the way <G>, if nothing more he can buy us lunch...... there are 3 empty seats.

It would be nice to know the serial number and model number of the computer from the original equipment list. BTW there are differences in the wiring harness between revisions (yrs).



07-17-02, 09:43 AM

Yes, I THINK that's what I'm telling you that I don't have a computer, not servo. An A&P (not my normal one) who helped with chasing down a leak when the headliner was off mentioned to me that he noticed that I didn't have a servo while he was poking around. That's where I got that info. He obviously mispoke, and I obviously was too ignorant to inquire further.

However, the fella I bought the plane from told me that there was no computer because he had removed it to have it looked at, and never put it back because he didn't know what he was going to do, repair/replace, etc. Then he lost his medical, and couldn't find where he put it in his warehouse (he owned a Chevy dealership) so it was lost in a sea of parts. That's the story I got, don't kill the ignorant messenger;)

Concerning flying in to OSH together, I'm all for it! Problem is that I'm just not sure that we'll be able to coordinate. I'm meeting a client in Cinci before I head on to OSH. My original plan was to have the client meeting, spend the night with my sister and her family in Cinci, and then get up early and head straight to OSH. Of course, I have not even spoken to my sister yet to see if she's going to be in town. If not, I was going to head to OSU to see my dad. I don't know how far out of your way Blue Ash or OSU is for you. I'd be at one of those two places.

I would further complicate your life by needing to be back in SC (GSP) to pick up a friend around lunch on Friday to head to Charleston. So I'd have to get back to my plane Friday a.m. sometime (a little over 2 hours for me from Ohio to GSP).

It would be nice if it could work out, but I understand if you can't accomodate my bizarre schedule!!! It would be particularly nice to have Larry buy us lunch:)

I'll post or e-mail the info on the serial and model # on the computer, hopefully later today, so we can see what I'm looking at.

Thanks again,

Ship Sale (http://ship-sale.com/)

Bob Cook
07-17-02, 10:32 AM
I go into Blue Ash all the time or Lunkin when the wx folds.
Could meet u there but KIAG would be same or shorter for you and you could leave the AC in my hanger.

I was planning on going to Quincy (UIN) so I might need to change plans. Let's see what unfolds by Friday.

Re autopilot, think you will need a computer to start. I know we could probably get it running again but how long and how much is the 1M$ question.

Need to look at the servos, connectors and make sure everything else is still in place. Also the control head is compatible with the computer. There are differences. I am inclined to suggest an STEC if the existing harness and control head is not compatible. Depending on who is doing the work you can pay for the STEC in labour.

There are tons of these computers out on the market and only a handfull of people that know the system. TOOO bad they lost the computer.


Ron Ball
07-17-02, 12:54 PM
Yes, you have a 400, not 400A. They are completely different. I have both. Probably alot of units out there, collecting dust. Probably call Auto pilot central or others and get one they have gone thru and repaired.

Paul Sharp
07-18-02, 04:49 PM
Well here's the latest: Went to the A/C this morning with my brother-in-law, who's good with electronics. Armed with the info from Bob, we checked and did have voltage on the reference lead (C - 10 volts). The potentiometer seemed to be OK. But we found 2 bad transistors (Q6 and Q7 if I remember rightly now - anyway I tossed them so I can only replace the bad ones by filling the empty sockets). Picked up 2 new transistors which are now NTE121 (or whatever those 3 letters are) of the newer brand.

Bob helped with schematics and much other advice, including via phone. I really owe him a lot more than a steak dinner but would be willing to buy one of those whenever he's ready as some sort of small thanks for a whole bucketload great expert help! And thanks to others here who've chimed in and helped, too.

[Bob, BTW re: the two screws that seemed to be shorting out the two transistors that were insulated from the heat sink - when I tried to put them back I found that what was happening was that the collectors of each of them were supposed to be connecting via the screws, which went through insulating shoulder washers on the sink and into studs on the back side which had wires on them; so while the cases are still isolated from the heat sink they are making contact on the back as part of the circuitry. Anyway, I'm putting it all back together the way it was with the 2 new transistors, and plan on reinstalling in the A/C tomorrow.]

I use the A/P regularly, although not heavily. But there are times when it is really a boon, such as when plans change in IMC and you have to study a new chart, long straight courses that you don't want to hand steer, etc. Without at least a "heading George" (which a single-axis A/P amounts to), I would tend to like any particular plane a lot less.

Paul Sharp
07-21-02, 05:33 PM
Well, plugged in the servo electronics and it promptly blew the breaker, twice. So took stuff back out and came back to the bench. Found a couple of fried resistors (they might have caused the problem originally, although can't tell). Getting some new transistors (expensive fuses....).

Noted that the turn coordinator isn't working. Don't know if it's related to the servo not being connected or not. Mine does seem to be a little different since the O.E. List says something to the effet that the A/P is interconnected with the gyro and etc. Also I suppose there is supposed to be a breaker for the turn gyro but I couldn't find one (other than the A/P breaker as mentioned above) that was blown. Could it have one inline or something? Or is it possible gone south?

Sigh.... This might turn out to be an expensive project.