Magic Box for 12V - from the late 90's

[Ian Watson]

Hi all, back in 1992 while living and working in the UK we bought a Chris Gould Glass Fibre A7 Ulster styled special. We attended Beaulieu, Gaydon and other events and many weekend trips away on the B-roads. The car came back with us to New Zealand in 1997.

The car when built had a 12V battery and electrics for lights etc. I understood that it also had 6V output for dash lights and other items (now I am not so sure). To achieve this, they used what was referred to as a black (or Magic) box. Allegedly this took the 12v input and provided output for both 6 & 12 volt. I did have a wiring diagram that came with it but I cannot find it now. Photo of the unit is attached, silver, about 2” square and about 1” thick and I believe were reasonably popular in the mid/late 90’s.

The issue I have is that I have taken out the mechanical tacho that was never connected (I don’t know if it worked or was for show only). I have a Smiths RVI 1000/19 electric tacho, rehoused in the correct looking bezel. I am now trying to find the 12V power supply (via a fuse) to run a line for the tacho light. As far as I can see from crawling upside down under the dash, there are 3 wires on the unit. Brown – thru fuse to the A terminal on the switch panel (and via the circuit board to lights, ignition etc). The blue cable and green with yellow trace both connect to grey braided cables into the wiring loom. I put a meter on the green/yellow cable and this appears to connect to head and side lights. No idea about the blue.

I am hoping that someone may be familiar with this unit or have the installation/instruction sheet that was provided with it when the unit was purchased. Any help would be appreciated.

[John Cornforth]

Hello Ian

I am not familiar with this box, so let's hope someone else had one and has saved the paperwork.

In the meantime I can offer the following:

The wire colours are standard for households in the UK, Brown for Line, Blue for Neutral and Green/Yellow for Earth/Ground. I don't know how these were used in this application, but they don't accord with any car wiring standard.

What I would expect to see is a passive semiconductor regulator IC, which takes a +12 volt input and drops it to a regulated +6 volt output. This will also need a ground connection, and since grounding via the metal case is a bit tenuous (especially if its mounted on glass fibre !) there is likely to be a dedicated earth wire.

The most likely allocation is therefore Brown for 12 v in, Blue for 6 V out and Green/Yellow for Earth, but don't rely on this educated guess alone !

Since the "wasted" 6 volts in a passive regulator means a certain amount of heat dissipation, this sort of unit would most likely be used for relatively low consumption items like the fuel gauge. I doubt it would be man enough to power lamps or motors. There is a possibility that its a more advanced design called a switching regulator, which is more like 95% efficient and has more capability.

If you still get no joy, take the lid off. If its not full of resin then a photo will allow further thought.

[AustinWood]

Use a voltmeter to measure which wire is carrying 12V and which 6V.
I would guess the 3rd wire is earth.
Alternatively take out the box and put 12V bulbs in the dash lights.

[Malcolm Parker]

I had a similar unit on a special back in the 1990's.  I seem to think that it was supplied by Nick Plevan who was active in the SA7C at the time, but i may be wrong.  IIRC when you took the cover off the aluminium box it was full of resin to hide the electronics within.   I recall that I nearly cooked a dynamo whilst using it.

[John Cornforth]

Hi

Its possible that this is an example of the Nik Plevan unit, used to convert the Dynamo to 12 volt operation without cooking the field coils. If that's the case, my comments above are irrelevant as they apply to 12 to 6 volt converters which are an entirely different animal.

This old thread has more information: https://www.austinsevenfriends.co.uk/for...p?tid=5677

[Stumpi]

I had a Nick Plevan unit and it looked like that. It lasted over 20 years of trialling pre Led days.
When the dynamo finally died i had it rebuilt by Smartpower who also supplied a more upto date cut out which is quite small and will fit inside the original cut out for anyone who wants the car to look original.

[David Cochrane]

Your unit looks like one of Nik Plevan's regulators. Scans of paperwork attached.
You should put a reverse diode across the 15A fuse as if it blows the spike can destroy the regulator .

[Ian Watson]

Thank you all, as soon as I saw the of mention of Nik Plevan I recognised the name, and thanks David for the instruction sheet, that was the one I had.
The link provided by John shows is the same as mine.

About 15 years years ago the dynamo packed up and I replaced it with a rewound for 12V. There were some issues and a local old auto electrician who did his time on old cars installed a 12v regulator, it has not given any problems since. It would appear from the instructions that this is purely 12v output, and the solid state unit and Regulator don't appear to be in conflict.
With the diagram it will be a guide to locate an appropriate power source for the tacho. My car is running a coil engine and an SM5 switch panel, but they are essentially thee same.

Thank you all for the help, very much appreciated. Back to the garage now to sort it.

[dickie65]

I have a brand new unused one in the garden shed complete with instructions.
Well its from the 1990s when I bought it but it has not found its way onto the car yet.