Battery charging problem

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When was the welding done in relation to the current problem?

No, it was done because the old switch was broken and the new one was a different size.

 

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Looking at the diagram it appears that there is no separate regulator. It may be contained in the inbuilt rectifier. From the other switch diagram, it shows the connection of the white and yellow together when the light switch is on. This would boost the dynamo output when the lamps are on. Not certain, but this may be a fault in your switch. Do you have any pictures and circuit of the coils in the dynamo?
Can you test the "no load" output of dynamo at 4000 rpm?

Since my original post I have found that what I initially thought was the regulator is a relay so I assume that the regulator and rectifier are both contained in one unit. As regards the yellow and white wires, at no time are they connected to one another. Effectively, when the lights are off, the charging current is supplied by the brown and yellow wires. When the lights are on the charge is via the brown and white wires. Looking at the coils in the alternator, some look to be different in size/ number of windings to the others. From this I conclude that the windings supplying the white wire are capable of higher output than those supplying the yellow wire. This would be to compensate for the extra load from switching on the lights.
I have tested it up to 2500rpm. Its output never goes above 10/4/2 volts in each phase no matter what the rpm.
The biggest problem I have is that I have found it impossible to trace any technical data on this particular model of tractor.

 

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Further update. I found a second hand alternator on ebay but unlike mine it was a 2 wire not 3. It came in the post today and I have fitted it to my tractor. I connected it by joining the white and yellow wires together with one from the alternator and the other one connected to the brown wire. The battery is now being charged with lights on or off so I can confirm that my original alternator was the problem.
I'm not going to let the company I bought the tractor from off the hook though. They insist it is the regulator at fault and are sending me a new one. When I get it I will call them and tell them it has made no difference. It will be interesting to see if they will finally supply a replacement alternator or come up with another implausible excuse to avoid doing so!

 

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Yet another update! After running the tractor for about 15 minutes the red charge light started to glow again. It was not fully illuminated but no matter what rpm it stayed on. I checked the output from the alternator which was 20+ volts but the battery was not being charged. I tried it again this morning. Same result, charging at first but then drops away. I suspect that the regulator diodes are breaking down as they warm up so I look forward to receiving the replacement regulator promised by the supplier to be able to confirm this.