Testing a Tacho

[bob_rich]

Hi Vince and others

I think the tacho you have may have been be a RVI with an external current loop and I think the note provided by
Jon is for a version with an external current loop and as you point out yours appears to have been modified. My experience and the test I conducted is for a RVI with the internal current loop type.

It would need someone with a greater experience than mine on Smiths Instruments but `I believe that the part number that can be seen of the very bottom of the dial is usually of the form RVI 1xxx/xx for the external current loop type and RVI 2xxx/xx for the internal current loop. Where xxx/xx are number options that relate to the specific application such as + or - earth, added dial indicators, red line, scale limits, and so forth.

The test I dreamt up I believe will work fine with any internal loop RVI that is coded RV1 2xxx/xx. I think that most original ?lan and +2s had the internal current loop type. Incidentally the Internal current loop units can be very noisy , caused by the behaviour of the saturable reactor, but you don't usually notice it in the car but on the test bench it is quite surprising.

A lot of tachos have been modified be like the RVC type which is voltage pulse triggered but may retain the original dial markings. I suspect this applies to yours Vince.

Hope this is helpful but don't think it fixes your unit sorry !!

regards Bob

[Quart Meg Miles]

[quote="bob_rich"]
It would need someone with a greater experience than mine on Smiths Instruments but `I believe that the part number that can be seen of the very bottom of the dial is usually of the form RVI 1xxx/xx for the external current loop type and RVI 2xxx/xx for the internal current loop. Where xxx/xx are number options that relate to the specific application such as + or - earth, added dial indicators, red line, scale limits, and so forth. /quote]

My car has RVI 2402|00x or similar and has the external coil i.e. a couple of turns of the wire around a metal U embedded in plastic which is then attached to the back of the meter. Just for information.

[bob_rich]

Hi Meg

I did look up a lot of different Tachos on the web and it suggested my idea might be realistic but clearly your unit suggests I am wrong ! You indicated your car is a 1965 S1 (0.5) and one thing I am confident in is that the external current loop you have would be an older unit that the Internal current loop type.

Not sure if the test I proposed would work with an external current loop type but if you want try and see.

with 45+ years old units many will have been repaired ,modified changed out from other cars (not necessarily even Lotus) and may not now be representative of how it was first assembled.

Perhaps someone may chip in and explain the part number system !

cheers

Bob

[Quart Meg Miles]

JonB, the Tacho in your link is a two-transistor model and is a simple RC decay circuit, which may have been used in some later Loti.

Bob, in your download you descibe the single transistor blocking-oscillator type that I have never seen the circuit of but have traced from my own car and also from a "spare" tach which was -ve earth, of later dial and with an internal coil. Although mine works it has not been stable since I replaced the transistor with a silicon type when the original Ge failed (a long time ago). I built an almost identical test circuit to yours to check the spare (which then got lost in the post) but my external-coil one is not so easy to test without slicing out the wiring so it's never got fixed.

Have just located the post that both Bob and I contributed to in 2012 on this calibration issue!

lotus-electrical-f38/tach-issues-t26256.html

[bob_rich]

Hi Meg

yes I remember that post. It was a suggestion and I did not try it. The idea was that the chopped up waveform from the SCR speed controller would provide the sharp edge to the current . Since then I have repaired a couple of tachos and I have made a proper circuit with a variable oscillator and a few other refinements and this will trigger most type of current loop tachos.

But my recent note was to try to simplify it as much as possible and to obtain a reliable 100pps without to much effort.

I have the circuit of the internal current loop tacho and I have attached it here hope it helps
It is a very simple circuit-- I like that. Other posts are quite critical of it however. It should work with a silicon transistor but I suspect it will require some value changes
the most critical part is the saturable transformer which, if it has to be repaired, should keep the turns number and ratio exactly as is.

best of luck hope this helps

Bob

[Quart Meg Miles]

At last, a circuit diagram! Thank you, Bob, it differs from my back-trace in that your capacitor looked like a diode and the thermistor looked like a capacitor. The transistor should be a MT59A so I'll see if I can find one for sale and get back to a proper reading tachometer, it's not restful cruising with 7000 rpm showing!

[bob_rich]

Hi Meg

glad to help. Most parts are easy to get but if you ever get any info on the thermistor that would be useful

best of luck

Bob

[vincereynard]

In the end I tested it the easy way by getting a set of test leads - https://www.ebay.co.uk/itm/272282003013

and simply connecting the tacho to + / return / coil - Nothing happened.
I tried with the leads connected to the Dolly tacho (also a RVC) and that worked.

So I assume the +2 tacho is bust. Its been send to Speedy Cables.

I may pop the Dolly tacho in as a hole filler and to confirm the actual wiring.

Thanks for the responses. (Even if much of it was way beyond my limited grasp of electrickery.)

Vince