Radiomobile 4200

[innellan House]

Last year, I acquired a 401. It is negatively earthed. The original radio and clock are not connected, presumably because they are positive ground. The radio is a Radiomobile 4200, and I cannot see any provision provided to switch its polarity. However, I understand that one can use a DC-DC converter, as a device to reverse polarity......so that the original radio or clock may still be used. Does anyone have practical experience of installing such a device?

[Geoff Dowdle]

Vincent ,
an easy solution to your problem is to change the polarity back to the original positive to earth.
Geoff

[Julian Caples]

Vincent

the easy solution, is as Geoff suggests, revert to positive earth - there are few real issues with this.
remember that the generator is polarised, and you will need to repolarise the generator if you do this (i.e. it is not quite as simple as changing the battery terminals, but may well be simpler and safer than messing with HT supplies)

the early valve radios all had vibrator power supplies - effectively an electromechanical DC-DC converter, so to make the radio work on negative earth you need to swap the polarity of the output transformer from the vibrator

DO NOT DO THIS UNLESS YOU KNOW WHAT YOU ARE DOING - the output is high voltage and you (may or) may not live to regret a mistake

the only thing the radio actually uses 12v for is the valve heaters and the lights (and they don't care what the polarity is)

exchanging the vibrator for a solid state DC-DC converter is commonly done because modern solid state DC-DC converters are more reliable than vibrators, you wire them according to the polarity that you need - it is not necessary to change the vibrator to a DC-DC converter to change polarity

does the radio work (if you take it out of the car and power it up with the correct polarity)? if it does, then changing polarity is straightforward (see above), if it doesn't, then the radio needs somewhat more detailed problem solving (I suspect the latter, I know your car)

regards

Julian

[penman]

Hi
There maybe reasons why what I suggest below can't be done but:-

Would it not be more simple to change over the feed wires to the clock and the radio?
Just connect the positive wire to the negative terminal and vice-versa.

If the radio has an asymetrical conector plug which can't be rewired then cut each wire and reconnect the ends to the other wire.

[BRIGHAM G]

Probably the main reason why these sets are polarity conscious is the use of electrolytic capacitors which are themselves polarity concious. In the case of the radiomobile 100 as fitted to the 400 Bristols,there are three all in the 12 volt supply circuit and are used to filter interference from the cars 12 volt system and that generated by the vibrator itself.These can be replaced with modern capacitors which are no longer polarity concious.I have so modified a radoimobile 100 which now works well on both +ve or _ve earth. It is now surplus to requirements as I no longer have my 400, so if any one would have a use for it please phone me on 01845 567 519 N.Yorkshire.

Best wishes, Gerald.

[innellan House]

Thank you for your replies. Julian, I've had the control unit/amplifier removed from the car and tuned to ABC radio Brisbane, powered by a spare car battery (everything works). It is possible to run the radio as an electrically independent unit, with lightweight rechargeable 12V DC NiMH or Lithium batteries that can store 15Ah. With a 3.7A current draw for the B amplifier, this provides a few hours of listening. The commercially available power inverters only convert positive earth to negative, so this is no use to me. There is a chap on the Sunshine Coast that does the basic AM/FM plus auxiliary input upgrade, with integrated amplification (and conversion of the radio to negative ground) whilst preserving visual and functional integrity on the outside. But this does destroy originality and there is something special about the cackle and timbre of an old valve radio. Because there is no reference to this particular radio in the 401 owners manual, I will try to send (1) the service manual (2) users instruction for tuning (3) a period advertisement....regards, Vincent.

[innellan House]

the service manual

[innellan House]

the service manualhttp://www.bristolcars.info/forums/images/attach/jpg.gif