One of these days when I upgrade my computer and iPad to something a bit more current and friendly, I'll put together a thread about adding indicator lights to the tach side......
Here's a link to some of the inanities that I've done to my bike(s) over the years....peruse at a risk of boredom. In there is a shot of the clocks and the voltmeter.
http://www.xs650.com/forum/showthread.php?p=66518#post66518
The one tach shown is from an SR500 that came with the neutral, high beam and turn signal indicators stock from the factory. The fly in that ointment is that the SR's redline is 500 rpm off from our 650's.....no big deal.
On my '77 I wanted to go the same route as my '83 so I picked up an SR500 tach to do it again-but like Skull pointed out, the illumination on the '77 stock clocks is green, whereas the SR and the '83 is yellow. Wanting to keep the green illumination I had to build the neutral and turn indicators into the '77's tach. Easy enough as the plastic clock housings are manufactured with punch outs in the back to do that........just have to drill the faceplate big enough to apply the plastic indicator lenses-which I had from the SR tach.
This all may not make a lot of sense but once you start to break these tachs down it becomes clear.
Once the light tree in the middle is no longer needed, I move the ignition switch to the front side of the bike by swinging it down and mounting it to the frame (another story) and then trim the light tree mount down, removing the upper part and leaving the lower that has two mounting holes that were for the ignition. These are spaced wide enough for mounting the type of voltmeter that I prefer.
If and when I convert my '78 over, I'll make a tutorial thread.