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Join Date: Feb 2003
Location: Charlottesville, VA
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Porsche Crest Clock voltage problem

I'm having trouble with my center console clock keeping accurate time. I had it adjusted perfectly running on a AC/DC converter where it was keeping perfect time on 11.84 volt power source.

However, when I plug it into the center console in the car, it runs much faster since the voltage is 12.6 at rest in the car and something slightly higher when the motor is running.

I have already tried loosening the tension on the clock's spring, but I have to adjust it so loose that it will stop keeping time as it nears the point where the contacts touch and the spring gets wound again, which I guess can only be expected after 30+ years of service.

I need some help from you electrical gurus: what's the easiest way I can regulate the voltage behind the clock so that it will be around 11.8 at all times so that I can keep my clock keeping accurate time? There must be some way to do this with supplies found at Radio Shack. Any help would be greatly appreciated on how to do this. Steve
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Steve Bomeli
74' 2.0
Old 03-24-2005, 01:52 PM
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Steve,

I don't think these clocks need voltage regulation. The time is regulated by the movement and all the voltage does is fire the winding solenoid. Maybe it was running good inside the house on the bench where it was warm and constant temp? Does the mechanism need a cleaning and lube, something a watch shop might be able to do? I may be all wrong here as I've never opened up the VDO clock but I have worked on old GM clocks.
Old 03-24-2005, 06:50 PM
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That's interesting, as I had it set up with the AC/DC converter right on the bench in the garage, at the same temperature, etc.

The voltage is the only thing that changed, so I'm thinking it might work again if I find some way to limit the voltage. Anybody know how I can do this fairly simply?
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Steve Bomeli
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Old 03-25-2005, 07:11 PM
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Probably the simplest way is to use a 11.5-12V zener with a small value series resistor. The clock doesn't draw much current so power isn't much of a concern.
Old 03-26-2005, 05:44 PM
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Porsche Crest

What is a zener? I'm not familiar with this device.
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Steve Bomeli
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Old 03-26-2005, 08:05 PM
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A zener diode. Your local Radio Shack has 12V, 1W devices in stock. Put the diode across the clock with the diode's band to the + terminal. Then connect your 12V to the clock + through a resistor. The resistor value should be just high enough to supply current ffor the clock and the diode will clamp the max. voltage to 12V. The zener shouldn't draw any current with the ignition off since the battery voltage will drop back to about 12V.
Old 03-27-2005, 06:39 AM
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Thanks Guy. Why do I need both the zener and the resitor?
I was thinking I could just use a rheostat that I could wire in series with the + electrode, and just adjust the voltage down to 11.8 or so by turning the dial. In that case, I wouldn't ever need the zener, correct? Thanks again for all of your expertise. Steve
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Old 03-27-2005, 12:37 PM
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Th zener diode will clamp or limit the voltage to 12V. The resistor ir required to limit the current to the zener when the battery voltage starts to go above 12V, which is the normal condition when the engine is running and the alternator is working. Without the resistor, the zener would burn out connected directly to the battery. If the clock was drawing a continuous current (I don't think it does) you could lower the voltage with a series resistor. That voltage would still vary with the battery voltage it would just be lower. If the clock only draws current when it winds the spring you'll measure pretty close to the battery voltage regardless of the resistor value.
Old 03-27-2005, 05:36 PM
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Guy: Thanks again for all of the help. I'll have to get to Radio Shack and then go at it with my voltmeter to see what I can accomplish. I think I'll give the zener diode a shot with a rheostat so that I can adjust it if need be to achieve the desired voltage because as you say, the clock only draws current when the spring is being wound. I will probably give it a shot later this week, but I'll keep you updated. I figure between some combination of a voltage step-down and tampering with the spring tension, I can get a clock that will hold the time (even when it is in the car). Steve
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Old 03-27-2005, 06:40 PM
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Well, before adding more components from radio shack to the car, I decided to open up the clock and adjust the mechanism just to make sure that I hadn't missed something easy. Nothing remarkable inside, but I did loosen up the spring by bending it ever so slightly with some micropilers...now the clock is in the car holding pretty close to real time. Just a few minor adjustments to the screw on the back and it should be as good as new. Steve
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Old 03-30-2005, 02:01 PM
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