Early RVI Rev Counter

[david.g.chapman]

My 1970 +2 has an early type RVI rev counter, and has been over-reading at high revs. For 4000 rpm the reading would fluctuate from 4200 to 4500, for instance. This effect was worst in summer, and on a hot day the rev counter would read double revs at low rpm as well.

Well if your rev counter does the same, this might fix it.

1. Remove rev counter, and carefully turn the bezel until the tags line up with the slots in the case. The bezel may well be gunged up with a black pitch-like substance, which hopefully has turned brittle and does not give too much trouble.

2. Two screws at the rear to undo - be ready to catch the inner assembly in your hand as it makes a break for freedom. Have clean hands and be careful not to damage that pointer, and the thin winding wire now exposed.

3. Take a look at the vintage printed circuit now on view, You should see a black component with 2uF 16V written on it, and a + sign. This is an input decoupling capacitor which is supposed to filter out the high frequences in the current pulses. It is this component that loses capacitance over time, leading to erroneous readings.

4. Find a replacement electrolytic capacitor - I used a 2.2uF 35 V item - make sure the value is 2.2uF, and the voltage rating 16V or more.
Maplins or Tandy should have one.

5. If your original capacitor is a different uF value, go for the closest match you can - it should work.

6. Replace the capacitor, making sure the + terminal of the new component goes the same way round as the old component. You will need basic soldering iron skills - and a soldering iron (or know someone else with one).

7. At this point, if you have a multimeter, it is worth checking the resistors in the circuit as well. Change them if they have drifted in value more than 15% or so. If you don't have one - don't worry, just re-assemble the rev counter and give it a whirl...

Dave Chapman.

[Kiwi elan +2]

Great advice. Thanks for the detail.

[AussieJohn]

David, do you think this could cause under-reading too? cheers, John.

[david.g.chapman]

Under reading could be caused by:

1. One of the resistors drifting in value by a lot (50% say), resulting in the transistor part of the circuit not always switching. You would need to check the values with a multimeter against the colour code on the resistor body. Note that you would need to remove each resistor before checking, as the surrounding circuit could affect the result. If you get all the resistors within about 10%, then you might get away without re-calibrating the rev counter - adjusting the potentiometer on the printed circuit. Avoid disturbing the potentiometer while working on the circuit, of course. As soon as the workings is removed, mark where the pot. position is so it is not lost.

2. If the resistors are OK, then someone could have fiddled with the pot (potentiometer) in the past. Try adjusting the pot, loosely fitting the rev counter back again and running the engine. You get to know what 800 rpm or 1000rpm sounds like, and a few goes around the buoy might get you near enough (If you have a pulse generator and 12V power supply you can do it accurately, but I you have those you probably won't need to read this).

Note that if the transistor is blown the rev counter will probably not read at all.

Dave Chapman

[AussieJohn]

Thanks for the tips Dave, cheers, John.

[AHM]

It sounds like this is what I need to do.

However, I see no 2.2uF capacitor but I have a disc with a wire soldered to either side (possibly this is it but the coating has fallen off)

Does anyone have a picture that I could refer to?

Thanks,

Simon

[AHM]

Seen in the attached picture.

0.25uF 150v Capacitor red/brown colour and looking a bit tired - I can't seem to find an equivalent on RS's website any suggestions

To the left of the big red/brown capacitor is a disc with no markings - it looks like the coating has fallen off. Is this the 2.2uf capacitor refered to? If not what is it?

Also does anyone know where I can get the rubber gaskets that go between the bezel and the gauge body, and also between the dash and the gauge.

Dash in bits - not going anywhere until this is fixed!

[ardee_selby]

Seen in the attached picture.

0.25uF 150v Capacitor red/brown colour and looking a bit tired - I can't seem to find an equivalent on RS's website any suggestions

To the left of the big red/brown capacitor is a disc with no markings - it looks like the coating has fallen off. Is this the 2.2uf capacitor refered to? If not what is it?

Also does anyone know where I can get the rubber gaskets that go between the bezel and the gauge body, and also between the dash and the gauge.

Dash in bits - not going anywhere until this is fixed!

Simon / Guys,

I wouldn't know a capacitor, or transistor, if it bit me...

However, a while back, I found this guy had put a circuit diagram together for an inductively coupled tach (maybe help to ID components?). He went on to talk about a calibration tool.

http://www.classictiger.com/techtips/motach.html

Apologies if it isn't relevant to you...but the idea of using a PC soundcard as a signal generator sounds cool!

Cheers - Richard

[ardee_selby]

Also does anyone know where I can get the rubber gaskets that go between the dash and the gauge

They are often listed on ebay. Like this for example...

http://www.ebay.co.uk/itm/Lotus-Elan-In ... 20be81c893

...but no doubt available elsewhere.

I, too, would like to know about seals between gauges & bezels Are they also square section or would some o-rings be OK?

Richard

[Jason1]

http://www.maplin.co.uk/components/capacitors

Maplins are all over the UK now.

I would take the old one off and pop down to them. The guys on the components counters are usually very helpful.

Looking at the above link I expect you will get one for under ?1

[bob_rich]

Hi Folks

The circuit in the Classictiger link is not the circuit used in the RV1 tacho. the RV1 only has a single capacitor in it and this should be 2.2uF across the base and emitter of the single transistor in the unit. The item to the left of the capacitor in the picture by tired looking 0.25uF capacitor is the thermistor. This is a resistor with a temperature v resistance characteristic to compensate for the copper wire coil on the tacho moving coil meter. Dont have any info on this.

there is a lot on the RV1 tacho on this web site a search should find you some pictures and further info. I dont have a picture I can post

Hope this helps best of luck

Bob

[holywood3645]

Duplcate

[holywood3645]

It's an electroic capacator and should have a positive and negitive wire. They go bad with age.and often indicated as going bad by a slight bulging of the component. Also the big 'O' ring goes over the outside of the instrument and between the wooden dash in the slight instrument recess. It offers a sort damping cussion to instrument movment when it tightened from behind.

The capacitor should be availiable in an electronic component store

I bought the large O rings in Marshalls Hardware in the US.

Good luck

[AHM]

Bob,

My tacho (67 S3 +ve earth) says RVI rather than RV1 and the circuit appears to be similar to the Tiger one. Also I have seen reference to early and late tachos.

re thermister - Thanks error averted!

Richard,
thanks for the links - I particularly wanted the square section gaskets, as you can see them and I'm fussy!

All,
I have RS and Maplin but no 0.25uF 150v capacitor listed.
The Tiger article suggests 0.22 or 0.27 uF are ok ...there are hundreds! all different voltages and materials - does it really need to be 150v? does it matter what it is made from?

I should add that the problem is that at tickover it reads 4000rpm (standard ignition etc)


Thanks,

Simon

[AHM]

update:
The number or the Rev. Counter is 2402/00B.

There is another useful post here:
elan-f14/smiths-tachometer-problem-t16197.html