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Old 22nd Aug 2010, 4:21 pm   #1
ex 2 Base
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Default Old car radio - how to change polarity?

My friend who is a vintage car enthusiast has a car radio with POSITVE earth and he wishes to run it in a car with NEGATIVE earth, is there a simple way of converting the polarity or is there a gadget available.
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Old 22nd Aug 2010, 5:15 pm   #2
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Default Re: Old car radio - how to change polarity?

You don't say what model radio it is. Most had some provision for changing polarity by means of switching or straps. 6V/12V switching may be incorporated too.
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Old 22nd Aug 2010, 7:51 pm   #3
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Default Re: Old car radio - how to change polarity?

If thears no switch it could be quite complicated unless you could isolate the radio from sed car inc the ant , as the screens usaly coneted to the body ,
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Old 22nd Aug 2010, 9:53 pm   #4
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Default Re: Old car radio - how to change polarity?

Hi
It is easy to reverse the polarity of the car. It must have a dynamo as alternators are for negative earth.
Disconnect the battery and the dynamo leads, reverse the battery leads and reconnect it then flash the "F" connection of the dynamo with 12v positive from the battery. Disconnect the earth from the battery, reconnect the dynamo and reconnect the battery, hay presto a negative earth car.
Gauges should still work ok but if it has an old electronic taco beware, do the radio instead.

As to the first post are you sure its the car with the negative earth, as most old cars are positive earth, all modern radios are Negative earth.
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Old 22nd Aug 2010, 10:25 pm   #5
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Default Re: Old car radio - how to change polarity?

I'd Goggle DC to DC convertors rather that alter an historic vehicles wiring or the radio. I came up with several.
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Old 22nd Aug 2010, 10:29 pm   #6
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Default Re: Old car radio - how to change polarity?

Quote:
Originally Posted by murphyv310 View Post
It must have a dynamo as alternators are for negative earth.
Except for a few Lucas 10AC or 11AC types with separate control gear. I don't know about foreign makes......

BTW the procedure described is for converting a dynamo from +ve to -ve earth..... I think the opposite would be required here if the car does have a dynamo.

Knowing what the radio is would be the biggest help, as already commented.

The big picture is not clear, but it's not unlikely for a vintage car enthusiast to have an old radio (+ve earth) that he wants to use in a more recent car.
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Last edited by Herald1360; 22nd Aug 2010 at 10:35 pm.
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Old 22nd Aug 2010, 11:37 pm   #7
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Default Re: Old car radio - how to change polarity?

As others have said, it depends on the model. My Philips 493VT hybrid from 1961 has a polarity panel and the service sheet gives instructions as to what links to change on that panel to go from positive earth (as supplied) to negative earth to suit more modern cars. If no panel is fitted, then a DC to DC converter may be the way forward. It is possible to completely isolate the radio and its aerial but when I did it some years ago I found it a bit of a fag insulating the radio, its mounting screws and coax outer.




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Old 23rd Aug 2010, 8:52 am   #8
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Default Re: Old car radio - how to change polarity?

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by murphyv310 View Post
It must have a dynamo as alternators are for negative earth.
Except for a few Lucas 10AC or 11AC types with separate control gear. I don't know about foreign makes......
Ah - but the diodes in the alternator are still the same way round, so what murphy310 still stands.
Quote:
The big picture is not clear, but it's not unlikely for a vintage car enthusiast to have an old radio (+ve earth) that he wants to use in a more recent car.
AFAIK there were no vintage (that's pre-1931) cars fitted with radios.

Back to the original question, easier to change the radio, and the fact that quite a few old car radios allowed the polarity to be reversed.
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Old 23rd Aug 2010, 9:48 am   #9
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Default Re: Old car radio - how to change polarity?

Either you can change the radio with a built in switch, or you can't.

It's maybe 20 years (more like 40) since cars had dynamos and could be changed to -Ve from +ve or vice versa. I think I may have done this in the mid 1970s (+ve to -ve for a new car radio).

It the radio has no internal switch, you need a DC to DC converter as the aerial and case is very awkward to insulate and you risk shorting the supply.

It would be really bad idea nowadays to swap a car to +ve earth, even if you could. I'd only swap a vintage or classic car to +ve, that was originally +ve and converted to -ve, as part of a restoration.
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Old 23rd Aug 2010, 10:42 am   #10
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Default Re: Old car radio - how to change polarity?

Many years ago, I had an old Morris Minor (+ earth) and a new radio (- earth). The Minor had a hardboard shelf below the dash board which seemed like good enough insulation so I mounted the radio on that. Worked very well. Unfortunately I didn't notice that the shelf had a steel reinforcing bar in it. (I guess it would have been a bit wobbly if it hadn't). Somehow I had managed to miss the bar when I drilled the mounting holes. Then one day whilst transporting a young lady, she knocked her knee against the radio. It moved it enough to short out the mounting bolts and the rod. Result - dead short across the battery, lots of sparks and smoke and young lady disappearing down the road never to be seen again. The moral is, if you're going to insulate the radio from the car, make sure you do it properly. If you can't switch the polarity of car or radio, I would tend to get another radio.
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Old 23rd Aug 2010, 12:54 pm   #11
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Default Re: Old car radio - how to change polarity?

Quote:
Originally Posted by Mike Phelan View Post
Ah - but the diodes in the alternator are still the same way round, so what murphy310 still stands.
I wouldn't know about that, though it seems rather unlikely that the (admittedly not exactly common but it did exist) positive earth 11AC alternator would have its diode pack the same way round as the negative earth version.

http://somerford-mini.co.uk/pdfs/Cat...lternators.pdf

has a reference to them.

Yes, "vintage" strictly means pre 1930 but my issue was about what a vintage enthusiast might have radio wise, not whether the radio came from a "vintage" car.

"ex 2 base" hasn't provided any further curiosity satiation, but I think he's been provided with suitable advice by now..... "If you can't just switch it over by design, look for a polarity inverter for the live feed to the radio"- that way the radio case can stay connected to the body of the car.
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Old 23rd Aug 2010, 5:24 pm   #12
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Default Re: Old car radio - how to change polarity?

Your link won't work, Chris, but the +ve earth alternator probably had a different set of diodes with the cases and leads of opposite polarity.

Back to the question, though, neon indicator has pretty well answered it in his post #9.
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Old 23rd Aug 2010, 10:35 pm   #13
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Default Re: Old car radio - how to change polarity?

An alternator is AC. The diode pack is wired differently for +V or -V earth. REALLY Old alternators had external diode pack and separate regulator (which is different for + or -). Modern alternators have diodes (usually 6) and regulator built inside case. Also regulation is different on a Dynamo and Alternator.

You can't feasibly change the polarity on a Modern car (too much electronics and engine management) and even 15 year old 20 year old with mostly only relays has a modern "all in one" Regulator -rectifier - Alternator.

We are flogging a dead horse.

[OT] In late 1970s I fixed car electrics and motor bike timing electrics as well as repairing radios and TVs after school. Eventually I got a 200cc Scooter. Magneto Ignition and direct AC drive to lights and horn from coils on stator. Flywheel of crankshaft had magnets. No battery. manual adding of oil to petrol (Two Stroke).

Recently I was helping my son fix a S/H scooter he bought. broken exhaust stud needed drilled and tapped. It's got ONLY battery starter. No kick starter at all. Fuel injection. Auto lube oil injection to crankcase. Electronic CDI for spark. All microprocessor controlled including choke. Water cooled 50cc two stroke with varator continuously variable transmission. AKA "Twist and Go". I'd not want to convert that to +ve earth either.

Google OBD port and car WiFi hijack ...

Last edited by neon indicator; 23rd Aug 2010 at 10:45 pm.
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Old 23rd Aug 2010, 10:38 pm   #14
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Default Re: Old car radio - how to change polarity?

Quote:
Originally Posted by neon indicator
We are flogging a dead horse.
We certainly are until such time as ex 2 base tells us the make and model of the radio and a bit more about the "Vintage" car.
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Old 23rd Aug 2010, 10:46 pm   #15
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Default Re: Old car radio - how to change polarity?

The only way to put a positive earth in a negative earth vehicle is to completely isolate the earths of the car and the radio.

The earth lead on the aerial must be isolated too. The performance of the set may not be as good because of this, but it's the only way. You can add a ground plane on the aerial but it must not touch the rest of the car.

Run the car radio straight from the car battery. Because the radio and car are separated electrically then it will work.

Cheers,

Steve P.
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Old 24th Aug 2010, 2:18 am   #16
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Default Re: Old car radio - how to change polarity?

Steve_P has it
I use T03 mounting bushes and neat L-brackets made from aluminium angle mount the radio to that and MAKE sure its isolated !!!!
ALSO Isolate the aerial from the car body ( should already be so if the car aerial is mounted correctly) then fit an isolating washer to the UNDERSIDE of the car aerial, where its mounted to the body, THEN connect direct to battery ( thats easy and DOESNT need Ford USA type 300 or so eye tags screwed to the battery posts directly.

NEGATIVE, or the previously " hot" lead goes to the body of the car
POSITIVE can go to the ACC(sesory) lead for the car
I have done many like that, and with a lil grinding and filing of the aluminium L-brackets you wont even notice

BUT beware !!!!!! IF the radio is mounted using the extra nuts on the volume pot and tuning shaft, that ALSO needs insulating, I use power diode bushes and fibre washers

Joe
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Old 24th Aug 2010, 6:54 am   #17
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Default Re: Old car radio - how to change polarity?

"THEN connect direct to battery" May I suggest, a fuse inline with each power lead too.
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Old 24th Aug 2010, 12:08 pm   #18
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Default Re: Old car radio - how to change polarity?

Quote:
Originally Posted by Steve_P View Post
The only way to put a positive earth in a negative earth vehicle is to completely isolate the earths of the car and the radio.
But not if you use a polarity converter..... as suggested several times already.

The polarity converter generates an isolated battery voltage supply which can have its positive output earthed, thereby providing the correct negative supply for the radio with its chassis connected to earth in the usual way.

Think of it as a separate battery with its positive terminal grounded, just for the radio, with the advantage that it doesn't need to be periodically recharged.
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Old 25th Aug 2010, 6:08 pm   #19
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Default Re: Old car radio - how to change polarity?

Very many thanks to all who responded.The new car is a 1947 Riley fitted with a dynamo, my friend read the replies and has decided to buy a DC - DC converter, as suggestet. He has been on to Google and ordered.I had offered to flash the dynamo field but he is a purist old car renovator.
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Old 25th Aug 2010, 6:23 pm   #20
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Default Re: Old car radio - how to change polarity?

Actually, I'm surprised the 1947 Riley isn't positive earth itself.

Let us know how the DC-DC converter works - it may generate interference unless carefully located and screened.
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