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Hello brethren, I’m talking about a 1972 Triumph T120 here. For those familiar with the wiring diagram for post ‘66 models you’ll find a simple circuit connecting battery to rectifier, number 1 terminal main switch and Zener diode.
After a straightforward battery change this circuit has developed a short circuit. By a process of elimination I thought I’d tracked it to the rectifier, but a NOS replacement has not fixed the problem.
The circuit is fine until the Rectifier is connected. When connected, the fuse will blow. I’m puzzled now as this is a simple circuit and it seems the rectifier is the villain in the piece but not so. Any ideas ??. Thank you. RR
 

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Hi RetroRod, You do have battery hooked up correct way round? Sorry just have to ask.

Do you have an ohm meter? You can bench test the rectifier quick & dirty with ohm meter. All wires must be removed. If you have a shorted rectifier it will show on this test. Easiest to test with rectifier removed.

In your elimination tests did you disconnect wire from Zener?

The brown/blue wire should be on the center "disc" of rectifier. Is that where you have it?

Just to be clear, the connector under bolt head of rectifier is earth. It may have red wire connected to it. Red wires are earth. (ground) Not shown on most of diagram.

So center wire brown/blue is removed, all is ok? Lights work & bike will start? Fuse doesn't blow? Is that correct?

What happens if you disconnect the 2 alternator wires & then connect the brown/blue?

Also very closely look at brown/blue wire from ignition switch to rectifier for any damage to insulation that could be touching & shorting to earth when wire is in the position it's in when connected. Saw that once.

Here's a rectifier test sheet I made. The boxes are your ohm meter. The leads are red & black. Black goes to common on ohm meter. The figures in the boxes are from a good rectifier I tested recently.

A better rectifier test is using a battery & test light connected to rectifier in a specified way. Light must light brightly one way & not light the other.

Test old & new rectifiers. They could both be shorted or both good. In any case don't buy any more parts until they actually test bad.
Don
 

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Hi RetroRod, You do have battery hooked up correct way round? Sorry just have to ask.

Do you have an ohm meter? You can bench test the rectifier quick & dirty with ohm meter. All wires must be removed. If you have a shorted rectifier it will show on this test. Easiest to test with rectifier removed.

In your elimination tests did you disconnect wire from Zener?

The brown/blue wire should be on the center "disc" of rectifier. Is that where you have it?

Just to be clear, the connector under bolt head of rectifier is earth. It may have red wire connected to it. Red wires are earth. (ground) Not shown on most of diagram.

So center wire brown/blue is removed, all is ok? Lights work & bike will start? Fuse doesn't blow? Is that correct?

What happens if you disconnect the 2 alternator wires & then connect the brown/blue?

Also very closely look at brown/blue wire from ignition switch to rectifier for any damage to insulation that could be touching & shorting to earth when wire is in the position it's in when connected. Saw that once.

Here's a rectifier test sheet I made. The boxes are your ohm meter. The leads are red & black. Black goes to common on ohm meter. The figures in the boxes are from a good rectifier I tested recently.

A better rectifier test is using a battery & test light connected to rectifier in a specified way. Light must light brightly one way & not light the other.

Test old & new rectifiers. They could both be shorted or both good. In any case don't buy any more parts until they actually test bad.
Don
Hi Don, Because the harness is double connected at the Zener, Rectifier and Main Switch, the circuit is maintained even when all these components are disconnected. It’s simply a matter of connecting each unit and watch for which one blows the fuse. I will try do the test suggestion tomorrow. Thank you. RR
 
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