Zitat : perl hat am 29 Nov 2010 14:36 geschrieben :

I cannot decode the marking either, but these are small ceramic disc capacitors for the prevention of RFI. Such devices typically have capacitance values of about 5nF.

Would 47nF be too high?

The optimal capacity depends somewhat on the operating frequency of the antenna.
The higher the frequency, the lower the capacitance should be.
The sole purpose of these capacitors is, to attenuate the small interference that the motor produces as long as it turns.
Obviously this is useful only with a receiving antenna. For a transmitting antenna you might ommit these capacitors at all.

There is a resonance frequency which is determined by the value of the capacitor and the lenght of its leads.
At this resonance the capacitor works best to remove the the interference caused by the sparks at the brushes of the motor, but the value is rather uncritical.
Dependent on frequency used, typical capacities lie between 820pF for VHF and 4.7nF for the AM-Bands.

This looks like an actuator to move a large or older satellite dish. The motor should work without the capacitors.
What happened to it? Lightning strike?


_________________
-=MR.ED=-

Anfragen bitte ins Forum, nicht per PM, Mail ICQ o.ä. So haben alle was davon und alle können helfen. Entsprechende Anfragen werden ignoriert.
Für Schäden und Folgeschäden an Geräten und/oder Personen übernehme ich keine Haftung.
Die Sicherheits- sowie die VDE Vorschriften sind zu beachten, im Zweifelsfalle grundsätzlich einen Fachmann fragen bzw. die Arbeiten von einer Fachfirma ausführen lassen.

[ Diese Nachricht wurde geändert von: Mr.Ed am 29 Nov 2010 22:51 ]

Yes it's a satellite dish actuator from Kathrein, it's 20 years old or something.

I have problems with it, it never comes back to the exact point when i have stored the position, it always looses some pulses.

I thought the problem might be the broken capacitors?

Zitat :

the problem might be the broken capacitors?

Probably not.
Rather its a worn out position sensor.
If a potentiometer is used as a position sensor then some dirt may habe been deposited inside.
Thats essentially the same effect that makes the volume controls of some old radios crackle.

Mirto from Italy just wrote me a private mail regarding the function of the Kathrein actuator.
I'll try to translate it:

Zitat :

The Kathrein system has permanent magnet on the shaft of the motor activating a Reed contact in order to count the revolutions. When the desired position ist reached, another reed contact switches of the power of the motor and, at the same time, the motor terminals are shorted circuited in order to perform a breaking action. It was then as well known problem that the contacts which were used to short the motor used to burn out. Therefore the positioner overshoots the target position. Annoyingly, after cutting off the motor, the impulses from the reed switch are no longer counted, so that sooner or later the syncronization between the antenna and the control terminal gets lost.

[ Diese Nachricht wurde geändert von: perl am 30 Nov 2010 21:31 ]

So this is not so good motor then, it is good when it's new but fails after a while.

No reason then for me to look for a new one as they all have the same problem.

Thanks for the information.

Reed relays aren't particulary good switches for high currents, but they are fast.
I think this is the main reason why Kathrein hasn't provided a more powerful relay.
Obviously the motor must come to a stop within 1 revolution, but this breaking action creates a very high current, which burns the contact.

Therefore one might consider to replace or to augment this reed contact by a Power-MOSFET. Bipolar transistors probably have a somewhat too high saturation voltage.

Here are picture from the PCB with the Hall sensor.

Anything i should repair to get it work properly again?

[ Diese Nachricht wurde geändert von: Likvid am  7 Dez 2010 12:05 ]

[ Diese Nachricht wurde geändert von: Likvid am  7 Dez 2010 12:06 ]