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FAB Cruiser
Joined: 03 Jul 2008 Posts: 77
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Posted: Tue Nov 23, 2010 3:14 pm Post subject: Smiths Clock - Converting to Negative Earth |
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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
Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh
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Posted: Tue Nov 23, 2010 4:17 pm Post subject: |
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Hi David,
I'm surprised that it is polarity sensitive. Have you tried running it with negative earth? Does it run backwards?
Peter _________________ http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon |
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Penman
Joined: 23 Nov 2007 Posts: 4756 Location: Swindon, Wilts.
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Posted: Tue Nov 23, 2010 4:21 pm Post subject: |
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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. _________________ Bristols should always come in pairs.
Any 2 from:-
Straight 6
V8 V10 |
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FAB Cruiser
Joined: 03 Jul 2008 Posts: 77
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Posted: Tue Nov 23, 2010 4:27 pm Post subject: |
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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
Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh
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Rick Site Admin
Joined: 27 Apr 2005 Posts: 22446 Location: UK
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Nic Jarman
Joined: 05 Oct 2008 Posts: 1031 Location: Stoke by Clare, Suffolk
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Posted: Tue Nov 23, 2010 8:37 pm Post subject: |
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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. _________________ 1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush |
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peter scott
Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh
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Posted: Tue Nov 23, 2010 9:03 pm Post subject: |
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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 _________________ http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon |
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FAB Cruiser
Joined: 03 Jul 2008 Posts: 77
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Posted: Tue Nov 23, 2010 11:40 pm Post subject: |
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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
Joined: 03 Jul 2008 Posts: 77
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Posted: Tue Nov 23, 2010 11:47 pm Post subject: |
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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
Joined: 05 Oct 2008 Posts: 1031 Location: Stoke by Clare, Suffolk
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Posted: Wed Nov 24, 2010 12:33 am Post subject: |
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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. _________________ 1936 Morris 8 Series 1
1973 MGB roadster
1977 MG Midget 1500
Dax Rush |
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