Early RVI Rev Counter

[bob_rich]

Hi again

Pretty sure the tachos are RV1 (one) not RVI (eye) though with fonts and stuff could be either I guess. In my earlier post assume that you had a tacho with an internal current loop-- i though that most +2 used this type --but I could be, and looks like I am wrong. The internal current loop uses a single transistor and has a 2.2uF capacitor across the base and emitter --the only capacitor in the unit. The 2.2uF is a polarised electrolytic or tantalum part so has to be connected the correct way round .

The external current loop has two transistors and has 3 capacitors in it. The current loop is visible on the outside of the tacho secured by a small clip with a knurled nut. The 0.25uF is the key part that determines the tacho pulse length so if it is smaller lower reading and if higher higher reading 0.22uF and 0.27uF are electronic industry standard values and close to 0.25u so should be possible to tweak the adjustment at the rear of the tacho to get calibration back.

for the 0.25uF (0.22uF or 0.27uF) any voltage(>12V) ceramic, polyester, or plastic film capacitor should do. The voltage rating does not matter in this application higher volts makes it cost more and it will B bigger. A polarised type electrolytic or tantalum is not required for this application.

Hope this helps

regards

Bob

[ardee_selby]

Hi again Pretty sure the tachos are RV1 (one) not RVI (eye) though with fonts and stuff could be either I guess.

Have always thought RVI (eye)

Ref: post149020.html

Cheers - Richard

[AHM]

Bob,

Great thanks for the details - I'm sure others will find this useful in future. I went to maplin and a very knoledgeable assistant helped me ?1.50 for all 3.

In my earlier post assume that you had a tacho with an internal current loop-- i though that most +2 used this type --but I could be, and looks like I am wrong.

Maybe most +2's did use that type - I don't know.... I don't have a +2

[simonknee]

It's an I (eye) and I always thought it stood for current as this is denoted with the letter I.
As opposed to the RVC version where the C stands for ... erm... volts

Which has lead me to lookup why is current is the letter I and according to the gospel of wiki:
The conventional symbol for current is I, which originates from the French phrase intensit? de courant, or in English current intensity.
Well there you go

[bob_rich]

Hi Again folks

The marking on my +2S tacho is RV1 2324/00. I only claim this is RV1 (one) on the basis of similar tachos that use the same circuit. Smiths made many such tachos and used varients to give manufacturers special dials and cases for the different indicator lamps they put in them. By and large the internals were similar as there were ( I Think!) 3 basic types RV1 with internal and external current loops and the RVC which is voltage triggered.

Attached is a photo from the dial of a MG Midget ( or Healey Sprite?) with the code RV1 2417/00 on the basis that on my +2 tacho the second part of the code is 4 numbers and that the 1 (one) is the same font style both parts of the code on the midget one suggesting 2 me that it is a 1 (one). Could not get my microscope camera near the

despite all this pedantry(!) whatever they are it is useful to know that the innards of a wide variety of tacho's of the RV1 series are identical so a spare could be source from a variety of other cars if that became necessary and tweaked to work with the Elan range of cars if required

Hope this helps and does not annoy!!

regards

Bob

[AHM]

I had assumed that the I stood for inductive, since that is how it is triggered. And surmised that C stood for Capacative...

I read somewhere in the last couple of days that I stood for Impulse, nothing was offered for C and I discounted the explanation as they are both Impulse.

Any guesses for the RV part? Residual Voltage?!

[ardee_selby]

I had completely forgotten about my letter from Smiths Industries! D'oh

Note the capital "I" typeface used (in the days before fonts ) and the three Part Nos.

RVI 2402/04 ( +ve earth)
RVI 2410/04 ( -ve earth)
RVI 2419/00 ( -ve earth)

Cheers - Richard

[AHM]

So I connected the rev counter earlier today and it worked fine - really pleased!

Finished putting the dash back together and.... nothing!

The engine runs so there is power through the white wire to the coil. I get 12v when I check the green wire.

When it was working turning the ignition key caused the needle to flick now it doesn't even do that.

Any ideas?

[types26/36]

Doesn't the casing have to be earthed? (normally by a ?eye? connection on the mounting stud) ....... if so is the connection on and defiantly going to an earth point?

[ricarbo]

Does sound like there is no power to the instrument, if it worked before. Either no supply or no earth. As only other connection is the supply to the ignition and that is 'loop through' to the coil. So if the engine runs it can't be that. Does the internal illumination work when the lights are on? if not, it suggests no earth connection made.
Regards
Richard

[AHM]

Brian, Yes earthed ok.

Richard, yes my line of thought also..

I wrongly assumed that the flick of the needle with the ignition switch was caused by power to the instrument....

I have just rechecked everything and noticed that the legs of the "coil loop carrier" were going the wrong side of the legs on the instrument, preventing the carrier from going fully home - corrected this and it works again.

so the flick of the needle is caused by power on to the coil.

Thanks all,

Simon