The meter reads zero, even on its highest sensitivity. There is unfortunately no tone defeat switch. I'll crack open the service manual and see if I can bypass the board completely.
If it requires service above my (limited) capabilities, would this model even be worth professionally servicing? It's the TX-2500 MKII
I know nothing about them, but from this post, unless the repair is super cheap, it doesn't sound like somewhere to put a bunch of money, only because you can just buy another for what you might need to get a basic repair done on that one.
Now, if you had a TX-8500 II, that would be different...