positive earth and radios

[cscott]

Anyone help with a small problem I have with my morris. It's positive earth. I,ve recently bought a car radio that says it is both positive and negative earth, although someone has also scratched on it the word "negative". I can't see that there is a switch inside or out that makes it positive or negative earth. Anyone know what would go wrong if the radio was truly negative earth ?
Is there anything I can do to get round any problems with this. It is an early Sharp solid state radio with only medium and long wave capability

[mike.perry]

Are there any stations still broadcasting on medium and long waves?
Easiest solution convert the car to neg earth

[DAVIDMCCULLOUGH]

I have one that has a screw underneath for cahnging from + to - but this is clearly marked. I remember my Dad fitting one to a block of wood to get round a polarity problem.

[bmcecosse]

Yes - swop the polarity - if you connect it wrongly (although it should be protected internally) you may cause irrevocable damage.

[chrisd87]

I would be extremely wary of trying to insulate the radio in order to connect it the wrong way round - it would only take something like a key to bridge the -ve radio casing and +ve bodywork to wreck the radio and potentially cause a fire. Either find a way to change the radio polarity or convert the car to -ve earth.

Medium and Long wave are still useful - Radio 4 is on 198LW (1515m), and there's plenty still on MW.

[LouiseM]

I would be extremely wary of trying to insulate the radio in order to connect it the wrong way round - it would only take something like a key to bridge the -ve radio casing and +ve bodywork to wreck the radio and potentially cause a fire.

Yes, I can vouch for this. Tried it in my first car, a Triumph Herald, and the radio obviously wasn't as well insulated as I thought as the car filled up with smoke when I was driving As others have said, best to change the polarity. It's an easy job - see here: http://www.mmoc.org.uk/index.php?page=322

[cscott]

Thanks for all the responses. I suppose my next question is how do I know it is definitely negative or positive ?

[LouiseM]

Just take a look at the terminals on your battery. If the lead from the negative terminal is attached to the earth lead eg the one which is fixed to the body of the car then the car is negative earthed. If the lead from the positive terminal is earthed to the body then it's positive earthed. If you're still not sure post a photo up (and welcome to the messageboard )

[cscott]

Sorry I should have been clearer on my question. It was more the radio that I was concerned with. How do I know if that is set up as negative or positive

[washer-bottle]

that says it is both positive and negative earth

There should be no short between the metal casing or exposed screws of the radio and either supply lead for it to work like that. If there is a short on one lead then that is the earth for the radio, ie black lead shorted to case means radio is only negative earth.

Al

[bmcecosse]

It's likely a dual polarity radio like that will be 'floating' - so checking black lead to case may not work. You can only really go along with the marking on the case. But also look carefully - there must be a selector screw/switch to make the change , or perhaps a multi-pin plug that is fitted one way for -ve and another way for +ve use??

[MarkyB]

What make and model is the radio?
A picture would help too, someone here may have the same one, there may even be information on that new fangled interweb thing .

[cscott]

Well the make is Sharp. I think the model is atr 933. Hopefull the picture has been attached ok. I have looked again in the guts of the thing and I cannot find anything that looks like a switch or plug that can be altered. This may not mean there is not one though. Its just that I cannot see it.[frame][/frame]

[MarkyB]

http://www.ebay.co.uk/itm/Car-Radio-Vin ... 0878317051

This says "Car Radio Vintage SHARP ATR-933 MW-LW 12V + or - Earth "

You could ask the vendor how to change the polarity.

[cscott]

I have managed to locate a tag that you pull out and put into a different position to change the radio from positive to negative and vice versa. Now not getting any sound from the speakers. Does anyone know if you have to get different sort of speakers for old radios. I had bought a pair of cheap new ones to get glorious mono from, but I am getting nothing from them. Any ideas ?

[bmcecosse]

No a speaker is a speaker - should be fine .

[GPO_Van]

Be carful, speakers need to match the output of the radio e.g. 4 ohms, 8 ohms etc.
If you connect too low resistance speakers onto the radio it can damage the radio.
The correct resistance/impedance is normally written on the radio or on a tag on the speaker wires, if not it will be in the instruction manual.
Regards
Reg

[chrisd87]

Are you sure that the radio actually works? Perhaps set it back to negative earth and try connecting it up to a battery off the car to see if you can get any sound out of it then. If it's a jumper-type system, were all the contacts clean, and have you bridged the correct ones? A nice big photo might help us get a better idea.

Normal impedence for car speakers is 4Ω, so if you have 4Ω speakers and want to use both for one channel, you'll need to connect them in series (to get 8Ω), not in parallel (which would result in 2Ω and probable damage to the radio). Best to check the manual if possible, however, just to make absolutely sure it's not designed for a single 16 or 32Ω speaker, or something like that!

[bmcecosse]

It would surely still make some kind of noise - pretty much no matter what ohms the speaker. These old radios were pretty tough!

[chrisd87]

Absolutely, and any damage would normally take quite some time to occur if you did connect speakers of too low an impedence.