I have three of these single transistor negative ground Smiths RVI tachs so I sat down today and penciled out the schematic. I then drew it in AutoCAD. Here it is:

Needs to go in the wiwi.

Good idea. I have some DC resistance measurements of the coils and the meter coil to add as well. Unfortunately, I am unable to regester as the random code generaqtor is not working with either IE7 or Firefox.

When you get it complete let us know and we will do our best.

In th' ~wiwi~ ?!?! ....eeeewwww.

Tom, R1 & R2 and pot in ohms, yes?

I had a schematic to "convert" the Lotus one from pos-to-neg long ago but can't find it now. And want to do it with the B tach... any "hints"? An RVI 2401 /008.

Ah, yes the single transistor one. I have one of those. I was going to fix it but it had a broken spring and at that point it becomes like a watch.

Yes. 2.2r is 2.2 ohms. I will change that to ohms to make it simple. While I am updating that, I will add the DC resistance measurements I got for the input coul and the meter coil.

I don't have any prior knowledge about these or knowledge of the other types and I can't help with polarity conversions. I got into these because of necessity. Mine is still acting up. I plan to replace the cap and see what that will do. After I located and fixed the intermittant ground, it worked OK once. Now it reads high at idle. Grr.

According to my copy of Astley's book (MGB Electrical Systems), the potentiometer should be 150 ohms and the resistor 22 ohms. The color codes can get dark on old resistors, then a brown band looks black and so on. Then they can be quite hard to read. Especially in car electronics, which get hot and are not in a clean environment.

The coil resistance measurements should also be useful to have--that's the kind of thing that you usually can't find unless you have a known working unit to measure. But, when you want to know it, it's usually to check a meter you aren't sure is working.

Nice! Only, the trim pot in the three that I have are 50 ohms, not 150 ohms. Still it should not matter since one end of the resistance is not connected to anything. Thanks!

Of course, there is no guarantee that Astley is correct, either--I've found a couple errors in his book. However, it seems clear to me that the 22 ohm resistor is right, and the 150-ohm pot could be OK. If you have 50, and it hits the stops before you can get the tach to read the right value, just add another resistor of 50-100 ohms or so in series with it.