Rectifier Fault or Something Else?

[Dilkris]

I have a 1975 T160 with a possible charging anomaly - I have been steadily working through a list of faults on this bike and hopefully I believe this is the last thing....

Taking the battery conditioner off the bike before starting it I have a voltage of 13.8V - all good I believe.

With the engine running at 1500 rpm I have a battery voltage reading of 15.75V - increasing the revs to 2500 rpm results in a battery voltage of 16.32V - this surely must be overcharging yes?

Before I start spending money can some body confirm that I should be replacing the rectifier, (It is the original Lucas unit), or should I be checking for something else?

 

[colnerov]

You might be better checking the regulator.

 

[Seadog]

It won't be the rectifier and I don't think that there is a regulator proper other than a Zener diode across the output from the rectifier. My bet is that that has failed or has a bad earth connection.

 

[Dilkris]

You might be better checking the regulator.

True - but I can't find any reference to a voltage regulator in the manual - or on the bike....

 

[Seadog]

True - but I can't find any reference to a voltage regulator in the manual - or on the bike....

Read my post.

 

[Dilkris]

It won't be the rectifier and I don't think that there is a regulator proper other than a Zener diode across the output from the rectifier. My bet is that that has failed or has a bad earth connection.

Read my post.

Yes - Seadog - I did - sorry.
I can only find a rectifier, Heat Zinc and Zener Diode in the charging circuit. Is there a way to test the Heat Zinc/Zener Diode?
(Sorry this stuff is out of my my comfort zone.... )

 

[Morrisman]

I ran my SR500 Yamaha with just a diode on the alternator output. I fitted a 12 volt battery to regulate the current, as there was no regulator in the system, the original had been stolen years before.
Without the battery it would blow bulbs as the rpm rose, as the voltage would go wild. With the battery there was no problem.

 

[Dilkris]

I can only find a rectifier, Heat Zinc and Zener Diode in the charging circuit. Is there a way to test the Heat Zinc/Zener Diode?
(Sorry this stuff is out of my my comfort zone.... )

OK - I have found the test procedure in the manual for the alternator, zener diode and rectifier - will come back later if I can make sense of it.

 

[paulcar]

From memory British bikes had a Zener Diode to control the charge voltage. It use to be mounted under the fuel tank at the front of bike.3

 

[Dilkris]

From memory British bikes had a Zener Diode to control the charge voltage. It use to be mounted under the fuel tank at the front of bike.3

Indeed it is - it is buried within the rear of the heat zinc which is mounted forward and under of the fuel tank between the fork stanchions - I'll post a photo later.

 

[Dilkris]

Heat Zinc and Zener Diode as discussed above.

 

[paulcar]

I would disconnect the diode un screw it from heat sink try to identify it, numbers on it, etc. Order one from R. S. or CPC. it should not cost a lot.

 

[Chippie]

Last picture shows a stud mounted diode…should be replaceable…Sourcing one might be challenging.. it’s not exactly a 500mW rated device..
Testing with a bench power supply should show that at its rated zener voltage, it should clamp any further rise in input voltage.

 

[Seadog]

Heat Zinc

Heat SINK

 

[Seadog]

I would disconnect the diode un screw it from heat sink try to identify it, numbers on it, etc. Order one from R. S. or CPC. it should not cost a lot.

From memory they only have a Lucas part number

 

[Seadog]

From what I can see, they're rated at 50W.
The Norton electric start had two in parallel to cope with the higher output alternator.

 

[Dilkris]

To all the above - many thanks for your input and comments.
The Lucas Part Number for the Zener Diode is ZD.715 and they are seemingly unobtainable - the trend appears to be to replace the Zener Diode, Heat Sink (Thanks Seadog) and rectifier with a solid state assembly which performs all functions.
I have no idea as to the capacity of the Zener Diode.
I am attempting to run tests as described in the Triumph T160 manual to confirm that the Zener Diode is at fault - apparently they are quite temperamental things and are prone to failure.

 

[ajlelectronics]

A 5A regulator isn't a difficult proposition from modern components.

 

[Seadog]

If you've a variable dc power supply you can easily test the zener.

Ebay has several replacement regulators for around £16.

 

[colnerov]

Heat SINK

Thanks for that, I've been dying to do it but didn't want to be that man.