I was doing a search to try and find out what was wrong with my tacho and I came across this site:

http://mk1-performance-conversions.co.uk/technical_electrical.htm

I don't know whether this has been linked to from Ausmini but there's some great documentation on Smiths Instruments etc.

This one looks particularly good
http://mk1-performance-conversions.co.uk/pdf/smiths_care.pdf

Cheers,
Simon

The hassle is there are a number of different variations for the old smiths tacho and getting the correct drawing can be a challenge, although there are plenty out there in google.

The most irritable issue with the old tachos is the germanium transistor. The germanium mineral grows as it ages, little hairs push forth from it which eventually shorts the transistor out, or in the least changes the way it operates. They have a different bias voltage normal transistors (think of bias as the amount of pressure you might need to turn on a water tap to get a better idea) and therefore can't be swapped directly for a modern BJT transistor.

They are available from specialist joints in the US, they're used in old audio amps for the audiophiles. Although its simpler to get a modern board insert from specialist instrument shops. It replaces the original electronic components (not the movement however) and has the side benefit of being absolutely accurate, which may take some getting used to.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I'll have a look at the wiring when I get a chance and if that all seems okay, I might have to look at getting an appropriate donor tacho and put the guts into my Smiths guage.

Tspeed Jem did this recently and it works beautifully. Just need to find an appropriatly sized tacho which has the same swing arc (i.e. 6000 directly opposite 0) and buy that. Tspeed only spent around $60 for a brand new VDO tacho.

It wouldn't be an RVI type by any chance, would it?

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

Yep it is (is that bad?)

edit - I don't have electronic ignition

Not necessarily. I have a very nice 100mm RVI tach here which I had fitted to my car for a couple of weeks, worked perfectly (wired in series with the +12V feed to the coil, detected the charge current before each spark) for a while, then driving along one day it went full deflection and stayed there.

Now, if I apply the +12V supply and earth (out of the car, on my bench supply) it just flicks over to 7,000 RPM and stays there

I've not yet found a circuit diagram for it, but physically it looks fine internally. The components are so old I don't know exactly what they are.. but I'm guessing a capacitor or the germanium transistor has given up.

What's wrong with yours?

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

yep that would be the transistor. The capacitor is there to level out the ac pulses from the pickup, but since there is no pulses on the bench....

Measure the voltage drop across the transistor on the bench, if it is 0 Volts across the emitter and collector with 12 volts applied to the tacho, then it is the transistor which is dead.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I bought it off ebay about a year or so ago and only recently got around to trying to hook it up.

Basically it does nothing. Every now and then the needle will give a little ****** and then go back to sleep.

I may have wired it incorrectly although I think it's right. I'll check again now that I've located some better quality wiring instructions.

Cheers,
Simon

Try changing the direction the coil wire loops through the pickup.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Yep, I'll give that a go.

btw - Ausmini objects to the word f l i c k e r (without the spaces) and turned it into asterisks instead. weird

The RVI tach is a little different to nearly all modern electronic tachometers, which are triggered by a voltage level pulse from the distributor side of the coil. Instead, the RVI has an external (early) or internal (later, like mine) inductance loop which senses current, either in the charge side of the coil (+ve) or the distributor trigger (-ve) side. I think the RVI is supposed to be wired on the charge side, but I've heard of it being wired on the trigger side, in series between the coil and distributor.

Because it's an inductive sense coil, the direction of current flow through the sensor is important. If it's wired backwards the car will still run fine, but the tachometer will basically detect a negative charge current and not register anything on the needle. This might be what's happening with yours.

Mick, do you have any tips for a source for replacement germanium transistors? It's been many, many, many years since I've had one of those in my bits box.. I'd be thinking Radio Parts?

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

I've just compared the wiring diagram which I used when fitting the tacho to another diagram which I found on the net this evening.

They are completely different of course!

When I wired it up, the inductance loop wires are joined to the coil (side not attached directly to the dizzy) and the switched power. I got the diagram from one of 9YATAHs posts - http://www.ausmini.com/forums/viewtopic.php?t=23156&highlight=smiths+tacho

In the new diagram that I found, the inductance loop wires are joined to the dizzy and the coil so that it is basically wired in line between the two. see page 21 of http://mk1-performance-conversions.co.uk/pdf/smiths_care.pdf. The tacho shown in this manual has the same model number as mine (RVI 1000/00) so I will rewire this way and see how it goes.

Cheers,
Simon

Yep, that's how I had mine wired, and it worked perfectly (for a short while).



I've seen that version around the traps as well. From a signal point of view I'm not sure it makes a great deal of difference which way it's wired, except perhaps that the tacho sensitivity (i.e. the number of coils in the inductive pickup) might vary between the two. The trigger frequency will be the same in either case.

My installation involved a 123 electronic distributor, which from some accounts causes the coil to discharge more fully than a points dizzy (and therefore induce a deeper charge cycle). In this case, the account I read suggested that the magnitude of the charge current would cause saturation in the inductive pickup of the tach causing it to over-read, but like I said mine worked perfectly while it worked at all.

Is yours the external loop version, or internal? If external, you may be able to increase the tach's sensitivity by looping the signal wire around the core again.

_________________
Simon

The adventures of an owner builder in the Tallarook Ranges

Flip the wires around at the coil to change the current direction through the pickup on the tacho. If that does nothing try connecting it between the coil and the dizzy instead. Flip the connections here to change the current flow as well.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I don't have the part number anymore, but they are available out of the US. They are used in old Guitar amps, and are a bit of a legacy item to keep the old Marshall and Fender amplifiers going. There will be a number on the diagrams you can punch into google which will take you to the most mind numbing websites ever, and you can pick them up from there for about 2 bucks each or so. There will be plenty of instrument techs over here who will be able to fix them however or at least supply you the transistor if you want to save yourself the hassle.

Like I said above however, they can replace the electronics with a new board which uses a modern fixable and reliable BJT and small PIC to give absolute accuracy through all the rev range. Cost is about $110 when I last asked 3 years ago. Add up all the frigging around, and it's not bad value....

With the original electronic gubbins, even when working correctly, when you calibrate the counter at the bottom of the rev range, it is completely out at the top end. Smae goes for vice-versa, so calibrate it at about 2500 rpm to make the error half the size at both ends of the rev range.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.