Installing a Radio Smith with positive ground

Someone know how install a radio smith 500t positve ground to a jaguar 3.4 negative ground??

The 500T was a positive ground only radio. I won’t say it is impossible to install it in a negative ground car but it would require you to isolate the radio chassis from the car’s chassis. The antenna would also need to be isolated from the car’s body. When you are done with that any exposed metal would have 12V on it, so there is some danger. Also, it might not work well at all, since the chassis, which is supposed to be grounded, will then be connected to the positive side of the battery, which is also connected to the ignition system and the generator, all of which create electrical noise.

Well, I’ll have to take out that radio and look for another one… I think it can fit in that casing…

Thanks mike…Well, I’ll have to take out that radio and look for another one… I think it can fit in that casing…

The warnings @Mike_Eck gave are good point.

If you isolate the radio from ground, it is easy to run it ‘positive ground’ in a negative ground car.

Neither the radio nor the car knows what voltage the chassis is or what voltage or polarity the ‘power’ conductor is … with “isolated from ground” being key.

Run separate negative conductor (black wire) and a separated positive conductor (red wire) from the battery to where the radio will be.

The voltage and polarity of those two conductors will be 12 volts between them, and the polarity will be based on which terminal of the battery each is connected to. With the back wire connected to the negative battery terminal, that will be the negative wire (-) and with the red wire connected to the positive battery terminal, that will be the positive (+) wire.

For ‘positive ground’, connect the positive wire the radio’s ground; which leaves the negative wire to be connected to the negative power wire to the radio.

Correct. And being as the antenna cable is shielded, with the outer shield conductor being ‘ground’, one way to solve that dilemma would be to mount an antenna connector to the isolation material used to isolate the radio, then from that connector, add a jumper to the shield conductor and a jumper to the center conductor, swapping the connection of the two going to the radio connector (you will have just swapped the radio antenna shield to be ‘positive’ and the radio antenna center conductor to be ‘negative’. That allows the antenna to be mounted to the metal body/chassis of a negative ground car without additional concerns of protecting the antenna and it antenna cable from crossed polarity and shorting out.

The positive ground radio will see ‘positive ground’ and ‘negative power’ as it expects to see.

The key, then, will be (again): (and to clarify, that “12V” will be reverse polarity from the chassis and body of the car. When you touch the two together (radio and body of car), you tend to see 4th of July fireworks and blown fuses, just as a starting point of what will likely happen)

If I really, really, really wanted that positive ground radio in a negative ground car … it can be done … but isolation from the metal chassis and metal body (fiberglass bodies would likely not be an issue unless reinforced with metal plates which were grounded and you screwed the radio to one of those grounded metal plates … July 4th again).

There is another possibility that may work, depending on how much power your radio consumes. If you were to use an isolated 12V to 12V converter you could leave the radio chassis grounded and feed -12 Volts to the radio’s power terminal. This unit may work, depending on the power requirements of the radio:

That will still leave the metal radio chassis and all exposed radio metal at +12V (instead of at -12V) … with all surrounding body/chassis metal being at -12V (related to a -batt+ source).

I understand the attraction of having the rare period HMV radio in the car. Assuming that radio is still functional, you would be lucky if it has only several watts of power to run the aging 6x9 speaker mounted above. Why not mount a modern unit perhaps in the glove box to power two 5 inch speakers in the the ‘A’ post panels and a pair of 4 inch spekaers on the rear parcel shelf? I’ve watched dozens of videos on you tube regarding fitment of sound systems in Mark 2 cars. If sound quality is a factor to be considered, even if you solve the positive ground dilemna the result would be of minimal value. If you open a window and drive at freeway speeds, the sound quality on that HMV radio would be practically nonexistant. The choice is yours I suppose.

On another thread somewhere, it was under XJSs as I recall, there is a company that, basically speaking, uses the front panel of old style radios and ends up with a new modern radio controlled by the old faceplate and its contols.

You get the old original look with modern functionality.

No, the metal radio chassis will be bolted to the car’s chassis. On a negative ground car, the convention is to call the power source +12 volts while the chassis is at Zero volts. My suggestion is to employ a positive-to-negative voltage converter to generate negative 12 volts with respect to the car’s chassis. That would then be connected to the radio’s power terminal to make the radio chassis 12 Volts more positive than the power terminal. That simulates a positive ground for the radio. My worry is that, since the radio uses tubes (valves) it may pull more current than the example converter could provide.

For a time, I had a neg-ground radio in my pos-ground MKII.

To fit it, I wrapped the radio body in a sheet of gasket material. It was thick enough that nothing was going to inadvertently ground it. The biggest challenge was isolating the two stalks, vol and tuning, so they didn’t “ground”. I ended up using hard plastic tubing cut to size and slipping them over the stalks. Fiber washers kept the nuts from grounding on the center panel. I didn’t need a proper antenna, a short piece of wire did fine, so the body-mounted one was just a prop.

It worked perfectly until I found a period pos-ground radio.

Yes … an … no.

Yes, on a negative ground car, the chassis is 0V … as measured relative to the postive battery terminal.

Just as on a positive ground car, the chassis is also 0V … as measured relative to the negative battery terminal.

When a positive ground appliance (the radio) is wired with power to the negative terminal of the battery, the chassis of that appliance, while it is “ground”, “ground” must be kept in mind that it is the +12V terminal of the battery, even though it is 0V “ground”, with “power” being connected to the negative terminal of the battery, i.e., -12V.

The only thing that appliances, lamps, equipment connected to “power” see and work on is “12V” difference between the two conductors (“ground” is a conductor). And they only work if the polarity is correct.

Negative ground is chassis to -battery+ is power.

Positive ground is chassis to +battery- is power.

Once the polarity is ‘reversed’ to allow a positive ground appliance to work in a negative ground circuit, the “ground” of the appliance (metal chassis of appliance) is +battery, with the “power” being connected to battery-.

When installed in a negative ground (-battery) system, the positive ground (+battery) “ground” needs to be isolated from the negative ground (-battery) “ground” as there is “12V” difference between them.

Take a screw driver and touch the +battery terminal to the -battery terminal and see what happens. Same thing as putting a positive ground ground appliance in a negative ground system … you need to isolate the “+” from the “-” to keep from putting that screwdriver across the battery terminals.

Thanks for the refresher course, Jerry, but I knew all that long before I ever became an Electrical Engineer. From your response, I don’t think you fully understood my posts about purchasing an isolated positive-to-negative voltage converter in order to generate a negative voltage with respect to the car’s chassis to power the radio.

Mike, I do understand your post, however, using an isolated positive-to-negative voltage converter does not alter the end result that the radio’s chassis “ground” is +12V on relation to the 0V reference of the negative ground chass.

The end result is that the radio chassis “ground” is not the same voltage as the vehicle chassis “ground” voltage is.

Edit: In my posts above I clarified that the postive ground radio chassis ground (all of the metal) would need to be isolated from the negative ground chassis … I left the “isolated” part out in the above, but it has been part of the discussion, and still is when you are dealing with a “grounded” metal faceplate and any other metal part which touches “ground” radio-to-chassis.

Thus my “screw driver across the battery terminals” as that is what is being done when anything metallic touches the two “grounds” which are at different voltage polarity.

Thanks to everyone for going deeper into this topic…nightmare!!! :grinning::grinning:…I better go install a [negative] radio…and the matter is over.:smile:

but I have learned something else…