mount tracking too fast

[lesbehrens]

hi. i am asking this q on behalf of my work mate. he has an old eq mount and his problem is it seem to track to fast on an object. he thinks his motor drive mite not be good. he only has a motor drive in Dec. there is a place for a hand control. if he had one could he change the speed of his tracking? or do you think it mite be his motor. i hope you understand what i am trying to say.
les

[jjjnettie]

He'd definitely need the hand controller to change the speed. Sounds like it's trying to track the Moon instead of the stars.

[DavidU]

Some older EQ mounts are driven on motors that are Hz dependant. Thus if it is a US made mount it will be designed to track with a 60hz AC supply. If you run it here it will have 50hz supplied and be 20%out.
Is it an older US made EQ drive?

[lesbehrens]

thats true. he does have a 60Hz motor. if we changed it would it work better.
les

[bojan]

60Hz motor would go slower at 50hz, and that would be 20% slower, the HUGE difference in speed. And if it is AC motor there is no option for changing speed (because motor is connected directly to 240V AC).
I saw quite a few mounts like that in the past.. for example, older Celestron scopes had AC motors with gearbox so the output rotational speed was 1rpm/

Long time ago I helped a friend of mine by making for him a variable frequency oscillator (50-70Hz), and I also had to rewind the motors (to be able to run on 6VAC, from amplifier).
Actually, his mount had two 110VAC motors, connected in series, and both coupled onto the same large gear. I removed one of them, and he was very happy with the results, this scope probably still works as it was modified by me.

Perhaps the best option is to try and find 1rpm AC synchronous motor for 240V AC (ebay?) or to fit in the stepper motor with driver.
I may be able to help here, but I need more information.

[DavidU]

Thats what I did with my Meade Starfinder, a variable 555 timer, voltage doubler and a Mosfet to drive it. Worked like a charm (all off 12V DC)

[bojan]

Exactly right !

I am glad there are people here on IIS who appreciate this kind of approach to amateurism :thumbsup:

[lesbehrens]

i wish i could give you some more info but thats all i know. it is an old meade mount that uses a magellin hand control but he doesn't have one. i think he got it in the 90's. i remember reading his manual and it said it was a 110 VAC 60Hz motor.
does this help at all?
les

[DavidU]

Schematic ( HZ via variable potentiometer)
http://electronics-diy.com/electronic_schematic.php?id=609

[bojan]

Hi Les,
I am not sure if this is enough for me to be able to help..
If there is Magellan handpad, then it means the tracking speed is variable, which means it does not matter what kind of motor is used because most probably the motor is driven via electronics...
Anyway, if you find out more from him, let me know. We will think of something :-)

[bojan]

Yep, this or similar will do the job without re-winding of motors.

My solution was 555, followed by 4017 (Johnson counter) and resistor ladder network, generating quasi-sinusoidal signal, which was amplified and buffered by PNP-NPN transistor output stage, and no transformer (that's why I re-wound the motor, it was easy job, 200 turns of 0.3mm lacquered wire (if I remember correctly).
It worked fine, frequency was stable enough for visual, and he was able to do photography by guiding (pot mounted in handpad).

These days I have microprocessor solution (for stepper motor) for this kind of problems :-)