1961 A50 blows main fuse when ignition turned on

[blackeyeian]

Hi Guys
My 1961 A50 has been unused for 6 months, which ran perfectly well before placing in the garage
After charging the battery with the battery in situ, the main fuse blows every time the ignition switch is turned on.
The lights work without the ignition turned on.
The original fuse wa 5 amp - it has blown instantly fuses including 35 amp.
Can anyone help me by offering a procedure and any suggestions as to How to track this Dead Short down
Regards
Ian

[limeyrob]

So you know its not on the lighting circuit and its not between the battery and the ignition switch. Pull all the connectors off and try it, in case the short is inside the switch, then try each wire. Its odd that it was fine before, either something is stuck - check the points, corroded or eaten by mice (it does happen) or charging the battery melted a wire somehow, perhaps the zenner or rectifier.
Thinking out loud here, try disconnecting the zenner and each wire off the rectifier as both of these can break down.

[idie]

Take one lead off battery and fit a bulb in series from lead to battery turn on ign bulb will light because of short. Then pull things out or move wiring until the light goes out. A good safe way of finding a short.

[blackeyeian]

Thank you for your replies
I have a Boysen ignition fitted, which was never a problem - so no points.

I have pulled all the wires, cleaned and replaced.
I have used a bulb to see if the light goes out when each connection is unclipped, Zenner, rectifier, each coil, boysen connections etc. - the bulb only goes out when the ammeter is disconnected.
I have obviously not tried the bulb trick with the wires removed from the switch - Is this the next thing to try, remove each wire 1 at a time?

Regards  Ian

[iansoady]

I would suggest either the ammeter has shorted out - modern repro ones are poor quality - or there's a short on the wire going from the ammeter to the zener diode / rectifier. Does the bulb light when that wire is disconnected?

[idie]

I'd go with amp meter shorted. This happened on my Shooting Star. Try joining the wire on amp meter to confirm as mentioned by iansoady.

[blackeyeian]

HI
When I join the ammeter wires together, the light stays on
when I disconnect the Zenner wire, the light goes out
Does this confirm anything?
Regards Ian

[iansoady]

Sounds like the Zener has gone short circuit. Easily checked with a multimeter. It should conduct in one direction* but not in the other (unless the threshold voltage of 14.5 volts or so is exceeded).

*depending whether your bike is negative or positive earth

[limeyrob]

The zenner breaking down makes sense. They do do this and the only thing you did was charge with the zenner in circuit as the bike was not in use. Charging with the zenner connected would have got it very hot as it would be dumping the excess volts / current with no airflow over the fins.

[blackeyeian]

Thank you for all your valuable help.
it looks like the Zenner Diode has failed

Does anyone know where you can buy a new replacement Zenner?  They seem to be in short supply or out of stock

[limeyrob]

It may be worth investigating electronic regulators. These replace the Zenner.  Zenner prices seem silly and the regulators are under £40.  I have no experience of these regulators but they make sense; the Zenner is a blunt instrument that just burns off the excess so regulation is more efficient.

[iansoady]

Al Osborn from the Norton Owners Club often has them. If not he stocks a decent regulator.

http://www.aoservices.co.uk/index.html

[DAVE BRADY]

Hi,

As limeyrob. 
A bit of a long shot but standing for 6 months may have resulted in some corrosion in the switch that could be shorting it out.
Have you cleaned the ignition switch or try just by-passing it to eliminate from the equation.

Dave.

[rhyatt]

When I have electrical diagnosis to do, I made up a test light , using 1 headlight bulb , 2 lengths of wire soldered to the bulb to make it work and 2 spade terminals.
I plug it into the fuse carrier and if it lights up , then something is on or shorting to earth.
Then unplug components 1 x 1 until the light goes off , you've found the duff component

Saves blowing fuses

[limeyrob]

You can see if its the Zenner it by unplugging it and seeing if everything works. You can run the engine and ride the bike but if you rev up it will over voltage whatever is on.  I seem to recall back in the day these were quite a common replacement and usually kept near the parts counter.