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Smiths Clock - Converting to Negative Earth
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FAB Cruiser



Joined: 03 Jul 2008
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PostPosted: Tue Nov 23, 2010 3:14 pm    Post subject: Smiths Clock - Converting to Negative Earth Reply with quote

I just bought an original Ford Zodiac Smiths Clock on eBay which I want to use in my '57 Zephyr. The bakelite mounting is great, but the clock itself is positive earth and my car is now negative. I'd like to know how to convert it.
There's a lot of advice on this subject on the web but much of it is contradictory. I'm not interested in a mod which leaves the case or chrome bezel 'hot'. I could get a slightly newer clock which is -VE earth but I like the 50's detailing of the original.

All advice appreciated.

Cheers
David
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peter scott



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PostPosted: Tue Nov 23, 2010 4:17 pm    Post subject: Reply with quote

Hi David,

I'm surprised that it is polarity sensitive. Have you tried running it with negative earth? Does it run backwards?

Peter
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Penman



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PostPosted: Tue Nov 23, 2010 4:21 pm    Post subject: Reply with quote

hi
peter scott wrote:
Hi David,
I'm surprised that it is polarity sensitive. Have you tried running it with negative earth? Does it run backwards?

Peter


whther it does or not, just try conecting the wires to the "wrong" terminals.
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FAB Cruiser



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PostPosted: Tue Nov 23, 2010 4:27 pm    Post subject: Reply with quote

Hi Peter,

I haven't tried that because the terminal is labelled as negative and I've read of one going pop when simply connecting the other way around.

If you look at this post on the Cobra forum, there is a picture of the back of a clock very similar to my one. http://www.the289register.com/phpBB3/viewtopic.php?f=3&t=862

Thanks
David
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peter scott



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PostPosted: Tue Nov 23, 2010 6:49 pm    Post subject: Reply with quote

Hi David,

My apologies for the careless suggestion. The only electric car clocks that I've come across were free from semiconductors but it seems I'm a bit behind the times: http://www.agua.nu/ds-files/tony-jackson/red-cd/Electrical/Jaeger%20clock%20repair.pdf

Rather than opening up the clock (which is not recommended in your link) you could use one of these 12volt to 12volt converters http://www.maplin.co.uk/Module.aspx?ModuleNo=98174&OrderCode=N83CN

The inputs and outputs are isolated so you can connect the negative input to the positive output and you will get -12volts out from +12 volts in.

http://www.aimtec.com/site/Aimtec/files/Datasheet/HighResolution/AM1S-Z.pdf

If you don't use one of these then you would need to isolate the clock case but the bezel would be at +12volts. Not nice. The above chip should avoid this horror.

I haven't checked the power consumption of your clock but I suspect it won't be a problem. The chip can supply up to 83mA.

Peter
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Rick
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PostPosted: Tue Nov 23, 2010 8:16 pm    Post subject: Reply with quote

Sorry I can't help with your question, does that clock mount in the roof above the screen? - just curious as I might have one somewhere. Are they hard to find?

RJ
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Nic Jarman



Joined: 05 Oct 2008
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PostPosted: Tue Nov 23, 2010 8:37 pm    Post subject: Reply with quote


Let me reiterate from the website that I borrowed the picture from.
Remove straplink from term 1 then meter to the case. If isolated then connect +12v to term 1 and 0v to term 5. Make sure you have someone that has a good knowledge of electricals with you when you do this. Additional metering may be required. From the picture it looks as if it may be possible to remove all the screws on the back and remove the case for a visual inspection.
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peter scott



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PostPosted: Tue Nov 23, 2010 9:03 pm    Post subject: Reply with quote

peter scott wrote:
Hi David,

My apologies I'm a bit behind the times

Peter


Never do anything the easy way if there is a harder way

doh!

Peter Embarassed
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FAB Cruiser



Joined: 03 Jul 2008
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PostPosted: Tue Nov 23, 2010 11:40 pm    Post subject: Reply with quote

Rick wrote:
Sorry I can't help with your question, does that clock mount in the roof above the screen? - just curious as I might have one somewhere. Are they hard to find?

RJ


Yes it is roof mounted. A Bakelite 'bowl' with Smiths clock inserted. They come up on ebay about twice a year but not particularly valuable. I paid £27 but I'm missing the metal mounting assembly which sits behind the headlining and attaches to the windscreen frame. I will have to fabricate something suitable.

Cheers
David
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FAB Cruiser



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PostPosted: Tue Nov 23, 2010 11:47 pm    Post subject: Reply with quote

peter scott wrote:
Hi David,
Rather than opening up the clock (which is not recommended in your link) you could use one of these 12volt to 12volt converters http://www.maplin.co.uk/Module.aspx?ModuleNo=98174&OrderCode=N83CN


Peter,

I've heard about this option before. Usually in connection with radios. Is this what is known as as an 'inverter'?

In the US, they sell them to handle much higher currents (sealed boxes with flying leads).

When you have the time, perhaps you could explain the principle in a bit more detail and give us a wiring diagram?

Regards
David
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Nic Jarman



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PostPosted: Wed Nov 24, 2010 12:33 am    Post subject: Reply with quote

If one starts with DC it can be chopped to make a sort of castlated waveform (squarewave). This can be done with a pulse width modulator IC or more simply with a 555 timer IC. This can then fire a MOSFET transistor on one side of a transformer. The transformer would be 1:1 and a bridge rectifier and smoothing caps would give a floating output. If the +ve output is tied to 0v then the negative side is, yep you guessed it -12v. This is a VERY general overview and not meant to be accurate.
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