Hitachi external voltage regulator - resistor help

[Uiseag]

I've become the custodian of a 47 year old Yanmar YSE12 with a 25A externally regulated alternator.
The charging system functions after a fashion; the charge light extinguishes but the voltage only reaches a steady 2.7v.

On the reverse of the Hitachi TLIZ-51 regulator one of the resistors (rightmost in the picture) has burned so I'd like to replace it and do another test before I give up and by a modern alternator.
The only markings on the resistor are '10 Ohm JB' and it's between the Ignition and Field terminals of the regulator.
How would I calculated the wattage required, please?
I was looking at aluminium shell power resistors on Ebay which seem to come in 25, 50 and 100w versions - but I'm out of my depth here.
Size is unimportant as I can mount it outside of the case if needs be.

Suggestions as to which resistor I need to buy would be much appreciated.

Pic:
r2.JPG

 

[superheat6k]

I would not expect the control current will be much at all so I would try a 5w and see how warm it gets or put it in series with the ammeter on your multimeter as these normally measure up to 10a. Once you know the current then measure the voltage across it then the power is the product of the two values.

 

[Uiseag]

Thanks. A 10w will fit nicely in the original location so I'll order one of those and see how it goes.

(I meant 12.7v charging voltage rather than 2.7v by the way)

 

[Uiseag]

Just for info - I fitted a new 10 Ohm 10w resistor to the regulator but the situation wasn't improved.
I also noticed that upon turning on the ignition there is a marked drop in starter battery voltage, from say 12.6v down to 12.3v, so I presume this might be a good indication that the alternator diodes are shot.

I'll now consign this 1970's kit to the bin as a new internally-regulated 35A alternator is only £85.