Conference terri::cars_uk

    If the box is an inverter it might be capable of producing +12V from a
    -12V source. It would need to have a ground and 2 leads to do this (an
    input from the car electrics and an output to the radio), but you only
    mention 1 lead. This would mean that you could keep the radio chassis
    connected to the car chassis - definitely a good thing, otherwise you
    are likely to introduce noise into the radio signal. There might be a
    problem of overloading the device. A modern radio is likely to have a
    higher output power (and so require more current) than old one.
    
    Is it possible to convert the car to -ve ground ? Is there anything which
    relies on the polarity of the electrics ?
    
    Andrew

    A clock might rely on the polarity.... mind you a 67 MG does take you
    back in time..
    
    Does this MG have an alternator or a dynamo? If you convert to -ve
    earth they would probably need replacing as well.
    
    Stu

It's easy to convert a dynamo to the opposite polarity, you just 
have to flash the field!

	A 67 MGB in original form can easily have its polarity
	changed. Only the generator minds and this can be repolarised 
	by applying a reverse polarity to the field using the battery.

	-John

    I had an old Morris Minor which had +ve earth.
    
    I tried installing an old 8 track tape machine (remember them) ....and 
    guess what???the continuous tape unwound itself!!!
    
    I had to insulate the unit completely, and wire it directly to the
    battery...that solved the problem!
    
    Tim

The MGB does have a dynamo, but I had not considered changing the polarity of
the electrics. It would appear to me that the fuel pump, starter, guages and 
coil would have to be rewired (or at least the connections reversed). Given the
'delicacy' of the electrics on this car I think I'll go for isolating the
stereo, it may be safer.

I think I'll try using the Motorola box and make sure everything else is
isolated, including the speakers. My guess is that this box has some kind of
current limiter which cascades the voltage down to 0 if there is a short,
without blowing anything. My guess is that based on the P/N of the box (KM56P)
that the current output is 5.0, 5.6 or 6.0 amps, of course, I may be completely
wrong there, its just a guess (it sure ain't 56 amps though, the black wire is
only 18 guage).

I agree with the reply about the current draw on newer stereos, but the one that
somebody had used this box on did have four speakers. I may end up with a
problem where the stereo cuts out if I turn the volume up to much (maybe a good
feature for some of these people who have 1000 watt stereos in their car, and
insist that you should listen to what they are listening to). 

    Most things are usually insensitive to polarity
    including
     - starter motor
     - fuel pump (if its the diaphram kind)
     - voltage and fuel gauges (which are often thermal. If not the wires
       can be reversed).
     - coil (although the polarity of the HT circuit can affect the spark,
    	     so you might want to get a new coil - should be easy to find)
     - voltage regulator for the dynamo (I think this is the case with the
       older relay based regulators)
    
    Things that might be sensitive
     - electronic things
     - clock (you can probably reverse the wiring)
     - heater fan motor (you can probably reverse the wiring)
     - electric engine cooling fan (if fitted)
     - other permanent magnet motors
     - Ammeter, but it may not matter if the needle goes the other way,
       it should be easy to reverse the wires anyway
     - rev counter (if electric)
    
    Andrew

    Hi,
    
    Nice to hear from another '67 MGB owner, I have thought about exactly
    the same problem, the "recommended" solution is to reverse the
    polarity, there is a book available by Lyndsay Porter "The MGB
    Restoration Guide" that has a section on doing this. The main areas
    covered (from memory) are.
    
    	Re-polarize the Dynamo
    	Switch battery Leads
    	Rewire the Revcounter (Not just a wire switch)
    	Swap some wires over
    	
    I will bring the book in tomorrow and list the rest but items such
    as the starter, coil etc. are mainly unaffected although some may need
    the +ve and -ve wires switched.
    
    Another solution is to fit a "period" radio which are now available in
    "restored" condition. Look in Practical Classics, if your car is fairly
    original this would probably look better and deter the casual radio
    theif from taking much of an interest.
    
    Hope you get the radio fitted, my 'B' is currently in about 10,000,000
    bits all over the house and garage so fitting a radio is still a fair
    way off. 
    
    Keep me informed of your progress
    
    Good luck and Regards
    
    Graham Naylor
                                           

    From latest Practical Classics:
    
    	Vintage Wireless Company
    	Britannia Garage
    	8 Britannia Road 
    	Sale 
    	Cheshire
    	M33 2AA
    
    Tel: 0161 973 0438 
    
    They specialise in restoration/repair and supply of period
    Radios/Cassettes/8 track players for cars from 1930 - 1975.
    
    They may be able to help with either the conversion of the car,radio or
    supply a suitable unit.
    
    Richard Huntley
    

RE: .8

Graham,

	Did you convert your 'B' over to -ve earth before you disasembled it?

I'm still a little leary of doing the conversion as I have recently finished the
restoration and would like to have some fun with it before I spend more hours
working on it. It really will be a lot less work to isolate the stereo than to
do the conversion, plastic edging around the dash orifice, and plastic brackets.



RE: .9 

Richard,

	thank you for the information regarding the vintage wireless company.
Unfortunatly I'm in the U.S., but I intend to contact them next time I'm over in
the U.K. (hopefully April). I'm planning on restoring a 1960 MGA next. 

    Richard,
    
    I left my B as +ve earth before the strip down, and currently I intend
    for the sake of time/cost/originality to keep it this way. The proper
    title for the book I mentioned is "MGB, Guide to Purchase and DIY
    Restoration" by Lindsay Porter.
    
    If you are over in the UK in April and are near the Reading (London)
    area let me know and if you have any spare time and fancy a beer we
    could arange a meet. Also there is a nearby MG dealer here who can get
    most parts which I could order in advance for you if needed.
    
    Regards
    
    Graham
    

	Surely there's another problem with the radio in that the 
	aerial earths to the radio and to the car bodywork. If
	you don't do this then reception is poor and interference
	high. 

	They used to have dual polarity radios, but with these they 
	were rewired internally.

	-John

RE: -.1

I did think about the problem of the aerial and I'm going to try one of those 
that does not connect both inner and outer of the coax to the aerial, just at
the stereo connector. This provides screening where the cable runns alongside
all of the wiring behind the dash. There could still be a problem though because
the chassis of the stereo will not be earthed.

I'll post how things go. I had not planned to take the MG out of its winter
wraps, but we are having a heat wave. The temperature is supposed to get up 
to 70 degrees F tomorrow.