Thread: A new car!
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Unread 05-25-2019, 06:31 PM   #204
Workin' on it
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Default Fixed analog clock

Here's a non-essential one: fixed the analog clock. These VDO clocks are basically the same across Porsche 928, 944 and 911 of the time.

This took some fiddly de-soldering and soldering work. It would have gone easier if I'd have had some solder wick. The results were good in the end.

So, the clock in the console didn't work. It's kind of a neat thing once I figured it out. There's no motor spinning that gets divided down. That'd take a lot of current, I suppose. The trick is that they have a high-frequency crystal and divide that down to pulse the motor once per second. There's an ancient-looking IC chip that does the dividing. Then there's a magnetic disk on a shaft. Every second a field gets applied to it and that gets reversed on alternate pulses. Hmmm... I'm not sure why it doesn't just go backwards sometimes.

Looking on-line, the common failure is that one or two of the two large capacitors go bad. These electrolytic caps are 100uf and 16V. Actually, there's two different caps but I could only figure out one so put two on like that. Probing the circuit, they're charged the same.

First, gotta get the clock open. A few screws and de-soldering the ground pin. Then remove the caps and then put in the new caps. These caps have a polarity and the "-" is marked on the board. That's the short leg of the new caps. Easy enough.

The clock didn't start up well, though. If I tapped on it, it'd tick-tock a few times and stop. I sprayed it with some DeOxit as that'll evaporate. There was something making the motor stick but some spraying, blowing that off and working it finally made it run smoothly. Bench tested it for a day and it was keeping good time.

I like the old-style clock and want to keep it in. It's handy to have when on the track as DE and practice sessions are all timed.
Attached Images
File Type: jpg ClockCaps2.jpg (237.5 KB, 0 views)
File Type: jpg ClockCaps3.jpg (135.7 KB, 0 views)
File Type: jpg ClockCaps.jpg (177.4 KB, 0 views)
1980 928S Euro
1987 944 N/A

Last edited by GlenL; 05-25-2019 at 06:55 PM..
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