How can you fit a modern radio to a positive earth spitfire!!
Posted By : Jason Chinn, 21-Dec-2004, 09:13pm
Carefully with complete insulation eg put the thing in a box! 
It's far easier to change the polarity of the
car to be honest.
It's far easier to change the polarity of the
car to be honest.
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How can you fit a modern radio to a positive earth spitfire!!
Posted By : Ian Fraser, 21-Dec-2004, 09:58pm
I'm also the owner of an unrestored mk2 and as the idea of a positive earth system is something I have never dealt with I was considering changing it to negative earth, so whats involved, does much need replacing?
Ian-
How can you fit a modern radio to a positive earth spitfire!!
Posted By : Van Hamlin, 22-Dec-2004, 10:20am
This is what I can think of:
Remove dynamo and control box and replace with lucas ACR alternator from 70's/early 80s BL car.
Swap round wires to coil.
Swap round wires to battery!
Wiper and heater motors - this is where it gets tricky. If they are the round-bodied type you might be able to unscrew the 2 long bolts and turn the body 180 degrees. This saves you messing about with wires. If you have a single speed motor, you could also do it by swapping the wires round. If it is 2 speed, you have to swap the wire to the unswitched side of the motor with the wire to the input side of the switch and it starts getting messy.
Starter motor will be OK.
No idea about electric rev counter but if it is a cable one then it doesn't matter.
Not sure about dashboard voltage regulator but the guages themselves will be OK. You could find out about regulator by comparing part numbers, or someone else might know.
Van
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How can you fit a modern radio to a positive earth spitfire!!
Posted By : Philip Rosenberg, 22-Dec-2004, 10:33am
Okay, I know this might sound really stupid, but what is stopping you from connecting the positive radio terminal to the car body and the negative terminal to the battery (via ignition switch etc). I guess if the radio was earthed to its case then this is why Van suggested putting it in a box to isolate it, is that correct?
Phil-
How can you fit a modern radio to a positive earth spitfire!!
Posted By : Van Hamlin, 22-Dec-2004, 12:18pm
Jason suggested it not me and no it isn't stupid.
It's a problem though because most radios have metal cases. So you have to make sure there is no risk of radio touching car body, which usually means putting the radio in a box. You then have the problem that radios produce some heat and the box/mounting must allow for this. So you come full circle back to the idea of converting the car to negative earth, which also allows you to fit alarms and stuff like that some time in the future if you want to.
Van
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How can you fit a modern radio to a positive earth spitfire!!
Posted By : Jason Chinn, 22-Dec-2004, 01:18pm
I tried the radio in a box idea many years ago when I had a Morris Minor - the problem is that you have to be very careful to keep it all isolated. In the end (after a fried radio) I changed it over to negative earth - I say "I", a mate did it with no parts and seemed to just swap a few wires over and touch the dynamo to "re-energise" it, this worked a treat and everything else was fine. Wipers didn't go backwards or anything else daft, it just worked fine. I don't think you can just connect the raidio up in the opposite way, they all had big warnings on them so there was sure to be a reason for that!
Now if I were to do it again I'd convert to an alternator as that seemed to be the only component that was sensitive to polarity (I could be wrong here as I didn't do the work, just watched in awe!) - this tech article is pretty good for that http://www.northcotts.clara.net/tsscdevon/technica...-
How can you fit a modern radio to a positive earth spitfire!!
Posted By : Colin Wake, 22-Dec-2004, 01:30pm
Isn't there supposed to be something laid down in the annals of Classic car history that says Positive earth cars rust faster after conversion???
I beleive you can actually get a few 'decent' radios that are dual polarity.
Colin
p.s. you could always buy a personal radiogram (I think the youngsters of today call then Eye lids or something)
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How can you fit a modern radio to a positive earth spitfire!!
Posted By : Steve Brooks, 22-Dec-2004, 06:51pm
I followed the following instructions to the letter (Yes I know it's from a MG website!)
http://www.mgcars.org.uk/midgetspriteclub/sprite/s...
I did this on my 1965 12/50 herald and have had no problems since ( about 5 months)
Flashing the Dynamo just changes it's polarity as far as I know, but has to be done.
The things I found were
1) You may have to twist the coil around 180 degrees so the wires fit
2) There may be a problem with the battery terminals as in you may have to cut off the terminals and buy new ones ( about a fiver) because the pos and neg terminals are different sizes.
3) because the battery has been rotated through 180 degrees, you need to make sure there is no possibility of the live terminal (+) earthing through the battery clamp strip. (this is because before, the bat terminals were near the bulkhead, but now they are facing away from the bulkhead.
It worked fine for me and I'm glad I changed to neg earth.
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How can you fit a modern radio to a positive earth spitfire!!
Posted By : Kevin Rochfort, 22-Dec-2004, 01:36pm
Changing the generator for an alternator is not strictly necessary, and will reduce the originality of the vehicle.
Its more than 15 years since I last reversed the polarity of a vehicle - an Austin 1300, and from memory there is only a few things to do - once you have ensured that there are no positive earth "electronics" fitted.
1. Disconnect the battery and turn it around in the battery box
2. Disconnect the wires from the Generator
3. Swap the Low Tension (CB and SW) connections at the coil - not strictly necessary, but maintains ignition efficiency.
4. Connect the positive terminal of the battery to the old negative lead
5. Connect the negative terminal of the battery to the old positive lead (earth), watching out for excessive current draw, which would indicate other problems.
6. With a length of wire, connect one end to the POSITIVE terminal of the battery, and then "Flash" the other end onto the terminals on the back of the generator - this will generate some sparks!! - this will reverse the polarity of the generator. Then remove the "flash" wire from the battery.
7. Reconnect the wires to the Generator
I do not recall having to change the wiring to any of the motors, such as heater or windscreenwiper, and the car was not fitted with an ammeter. If the car is fitted with an ammeter, it will of course have to be reversed.
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