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saba: 7; Meersburg Automatic -- Automatic tuning jitte

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » saba: 7; Meersburg Automatic -- Automatic tuning jitte
           
James MacWilliams
 
 
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16.Nov.09 21:51

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I have restored a Saba Meersburg 7 and it works and sounds beautifully.  My question concerns the "normal" amount of tuning jitter (or noise, or vibration) one should see and feel in the tuning knob when the automatic tuning is turned on and a good strong station is present.  In my set there seems to be a significant amount of vibration felt and seen in the tuning knob.  It is a function of the station, but is still significant with a good strong station.  It is the same whether the station is FM or AM.  After a while the motor warms which I would expect since it is constantly driving the tuning.  But I feel the motor is too active, that more filtering would reduce the strain on the motor.  Does anyone have experience with what should be normal?

Jim

Bernhard Nagel
Bernhard Nagel
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18.Nov.09 17:24

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Hello Jim,

The effect you described is based on the AC driven motor (50 Hz) and especially on the modulation method for the FM band. For FM (frequency modulation), the center frequency of the carrier wave is changing with the modulated af, ie language and music. The motorized mechanism of the Saba radio has the tendency to keep the tuned station always perfectly matched, it's a "motor AFC".  But this control circuit is also affected by the (lower) af-frequencies,  the motor will vibrate (jitter) in rhythm with the af signal. Every deviation of the center frequency causes a motion of the motor. For AM reception, this effect should be less noticable.

This is a normal behavior, not a fault of the device. The jitter does not reach the tuning elements (FM variometer or AM ganged variable condenser), the ever-present mechanical play and the spring effect of the dial rope (Saba uses here Nylon cord) prevents this.

Please try it with a non-modulated rf signal from a generator. No more jitter beside a residual 50 Hz hum should be there. If you tune the generator slowly, the motor tuning of the receiver will follow accordingly.

A warming of the motor is normal, even when not tuned to a station. It's advised to switch off the automatic tuning when using the radio for a longer period. This will lower the consumed AC power and prolongs the lifespan of the tuning motor.

A similar thread in German you will find here: Bässe beeinflussen die Automatic in Saba Schwarzwald.

Bernhard Nagel

This article was edited 18.Nov.09 17:27 by Bernhard Nagel .

James MacWilliams
 
 
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18.Nov.09 18:22

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Thanks.  This is exactly what I was looking for.  I did notice that when I aligned the automatic tuning that the motor drive was much quieter, but it still had some vibration (the 50 Hz).  I had not realized that the jitter would not reach the tuning system due to the mechanical play.  So it's nice to understand better the whole system.  The radio works nicely and sounds great.

Regards, Jim

  
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