tachymeter modification in masse negative earth

[geni]

How do I change the positive earth of a tachometer to negative earth and how do I do it?

[ericbushby]

Hi geni,
I will assume for now that the Tacho is an early RV1 type.
It is not difficult to change the polarity
Remove the mechanism from the case by easing the bezel tags and rotating the bezel to where it can come off.
Screws at the back of the case will release the mechanism.
There is a large 150 ohm resistor and a green wire which have to be changed over where they are soldered to the circuit board. On mine they would both reach the new position.
It is written up quite well on the lotus Cortina website.-- search for lotus-cortina.com/electric/convert.htm
There is a small mistake in the write up. It describes the resistor as a diode, but it is a 150 ohm resistor-(brown/green/brown).
While you are there the capacitors should be replaced as their life expectancy is about 20 years
The above website gives details and a circuit diagram.
When you are finished the wire which is looped into the clip on the back of the case needs to be looped the opposite way so that the tacho gets a similar signal to before the change.
best of luck
Eric in Burnley
1967 S3SE DHC

[Craven]

Slight hiccup there Eric, brown green black = 15 ohms.

[ericbushby]

Quite right Craven,
I must be losing it at last. I have read colours directly as numbers since I was about 14 and never need to think about it. Edited.
Thanks
Eric

[Slowtus]

Quite right Craven,
I must be losing it at last. I have read colours directly as numbers since I was about 14 and never need to think about it. Edited.
Thanks
Eric

I always used that (now unacceptable) mnemonic

[ericbushby]

I am not with you, what mnemonic?. I just new the resistor values because I was interested and using them continually. No method needed to remember them. Now I have got one wrong, is that the slippery slope, oh dear.!! It is now nine years since I took a late retirement so I might be losing my grip.
Eric

[nmauduit]

I am not with you, what mnemonic?

I suppose this may be a colorful Canadian version of "Bad Boys Rape Our Young Girls But Violet Gives Willingly"

[ericbushby]

Thanks, new to me, maybe it is after my time.
Eric

[Craven]

Eric, sounds as if like me, so familiar with resistors in every day work you used pattern type recognition to get the value, no need to work it out. They tell me reading Morse Code is similar you know the pattern not hear dot & dashes.
You will then of course remember the old Body Tip Spot code and mfd on ‘condensers’.
Cheers.

[ericbushby]

Craven, yes, I remember there was a code but I wasn`t as good at that. I didn`t find it intuitive.

Eric

[Slowtus]

I am not with you, what mnemonic?

I suppose this may be a colorful Canadian version of "Bad Boys Rape Our Young Girls But Violet Gives Willingly"

No, it was a racist, sexist, misogynist British version...worse than the above.

[Slowtus]

Thanks, new to me, maybe it is after my time.
Eric

Heard it the early '60s when I joined the RAF, stuck with me all my life after that - not something to use in mixed company etc but once burned into the hippocampus, it stayed there