Autopilot Servo

[Bob Cook]

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 ?


Bob

[Ron Ball]

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]

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.

bob

[Paul Sharp]

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]

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?

yes

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. ..

bob

[Paul Sharp]

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?

[GMAs]

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...

But,....

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]

Gmas

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

PAUL
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.

Secondly
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.

so
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!

Bob

[Burt Benson]

Paul,

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.

Burt

[Paul Sharp]

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).

Thanks.

[Burt Benson]

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

Burt

Burt:

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

Keven
________
Ecigarettes

[Bob Cook]

Paul

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.

regards

bob

[Burt Benson]

Keven,

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.

Burt

[Paul Sharp]

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.