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  #1  
Unread 07-08-02, 12:30 PM
Paul Sharp Paul Sharp is offline
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Autopilot Servo

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?

Thanks.
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  #2  
Unread 07-08-02, 12:39 PM
GMAs GMAs is offline
George M. Amthor, Jr.
 
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Unhappy SERVO REMOVAL...

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
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  #3  
Unread 07-08-02, 01:21 PM
Bob Cook Bob Cook is offline
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re autopilot

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.

regards

Bob
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  #4  
Unread 07-08-02, 06:03 PM
Paul Sharp Paul Sharp is offline
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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.
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  #5  
Unread 07-08-02, 09:26 PM
Bob Cook Bob Cook is offline
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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.

Bob
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  #6  
Unread 07-09-02, 12:08 AM
Paul Sharp Paul Sharp is offline
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OK, Bob. THanks for the input. I'll go down and have a look at it and see what I can do.
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  #7  
Unread 07-09-02, 01:22 AM
Kevin McDole Kevin McDole is offline
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Paul,

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.
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  #8  
Unread 07-09-02, 03:28 PM
Paul Sharp Paul Sharp is offline
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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.
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  #9  
Unread 07-09-02, 03:34 PM
Paul Sharp Paul Sharp is offline
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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.
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  #10  
Unread 07-09-02, 05:18 PM
Bob Cook Bob Cook is offline
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re autopilot

Paul

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.

fyi

bob
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  #11  
Unread 07-10-02, 10:27 AM
Paul Sharp Paul Sharp is offline
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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)?
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  #12  
Unread 07-10-02, 11:39 AM
Bob Cook Bob Cook is offline
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autopilot

Paul

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.

fyi

Bob

Last edited by Bob Cook : 07-10-02 at 11:43 AM.
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  #13  
Unread 07-10-02, 03:25 PM
Paul Sharp Paul Sharp is offline
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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?
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  #14  
Unread 07-10-02, 05:42 PM
Bob Cook Bob Cook is offline
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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!

Bob


Last edited by Bob Cook : 07-10-02 at 05:45 PM.
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  #15  
Unread 07-10-02, 05:58 PM
Ron Ball Ron Ball is offline
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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.
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