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swapping needles and dial faces on rev counters


mpbarrett
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In my Herald I have a Smiths speedo (original) and a Smiths electronic rev counter which I think was from a Spitfire 1500.

I also have a mechanical rev counter made by Jaeger. The dial design of the Smiths electronic rev counter does not match the Speedo.
What I want to do is to remove the needle and dial from the mechanical Jaeger rev counter and put it onto the Smiths rev counter. I can adjust the calibration in the drive software to the rev counter to match the new dial. I am assuming the shaft diameter are the same on the smiths and Jaeger gauges.

I have tried pulling the needle on the Jaeger gauge but it does not want to move. Before applying more force do they come off the shaf and if so is there a trick to getting them off?

Pictures below showing Smiths Speedo and Smiths Taco and Jaeger mechanical tacho.

cheers

mike

 

 

speedo and tac1.jpg

tacho.jpg

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yes,   needle fixing on the electronic may just be quite different  to the mechanical unit    sorry dont know the answer 

smiths and  jaeger are identical badge engineered  units  from  Smiths Industries     ( caerbont instruments now )

Pete

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got the needle off. the trick was to hold the drive to the shaft (metal disc) and then twist the needle slightly and then it came off very easily.
this was on the jaeger so not worried about tis position, will mark the electronic one when I do it (thanks Pete).

Might have to drill new holes in the face and use the old screws to block the unused holes.

Mike

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well that's didn't go so well...

The spindles on the electronic rev counter are 0.5 mm dia and the mechanical rev counter about 0.8mm.

So had to drill out the old boss and  machine up a new boss to fit the needle and glue it to the needle. Drilling a 0.5mm hole in a big lathe was interesting, glad I brought plenty of 0.5mm drills... Pics below.
I have drilled the new fixing holes for the dial but am now thinking about adding a tell tale  light that I can use for the full beam (on the original Herald instruments this was in the combined fuel and temp dial). I have a couple of old speedos that I can robe for bits.
Will play with it  tomorrow morning before watching the GrandPrix, cant drive the car as I realised that the insurance ran out on Thursday! Had a reminder in June but we were away and I forgot to renew it..... :(

Mike

 

speedo1.jpg

speedo2.jpg

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One last thing...
Does anyone have a cct diagram for the electronics in the rev counter?
I could reverse engineer it, it seems to be a ceramic substrate with resistive elements on it and a selection of capacitors and a 8 pin DIL IC. Plus a large electrolytic capacitor that looks like a late 'fix'.

Cheers

mike

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56 minutes ago, yorkshire_spam said:

I think, but I'm not sure that it might be the same as the volvo one...

https://www.sw-em.com/Smith's Tachometer.htm

I slightly doubt that. The page you linked to refers to a current-sensing tacho that connects in series with the coil. While Triumph did use them on some cars (Mk1 PI?) I'm fairly sure the Spitfire electronic tacho is a voltage-sensing type with one wire that connects in parallel at the coil -ve.

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4 minutes ago, NonMember said:

I slightly doubt that. The page you linked to refers to a current-sensing tacho that connects in series with the coil. While Triumph did use them on some cars (Mk1 PI?) I'm fairly sure the Spitfire electronic tacho is a voltage-sensing type with one wire that connects in parallel at the coil -ve.

yes it is voltage sensing and has an IC rather than discrete transistors.

 

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that cct looks very similar to the one in my Rev counter but does not have the variable resistor.
I have calibrated it by scaling the tacho output from the Megasquirt ECU so didn't have to adjust the circuit components.

Got it finished and fitted to the car, better match for the speedo but not perfect as the font is slightly different from the speedo. At the needles match!

The high beam 'jewel' is lit by a small blue LED with a connector on the back of the rev counter body.
Need some White Lectraset (can you still buy it) to make a label for the indicator.

Cheers

mike

 

 

WP_20180709_12_52_53_Pro_LI.jpg

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just one more picture.
The rev counter meter movement is very neat, its a moving coil around a magnet. To get the connections to the coil they feed the voltage thru two opposite direction wound hairsprings! There is also an adjustment which I think helps set the zero rev position.
Sometime its nice to admire the neatness of 1960's engineering..

mike

 

coil11.jpg

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As said needle on the electronic takes a larger shaft. However, main problem is the two scale do not match up and the once the conversion has taken place the rev counter has to be re-calibrated.

This requires a matching transformer low to high impedance, 600 to 10,000 ohms, and an accurate, frequency, AF generator. The high side of the transformer feeds the rev counter.

Once the pointer is correctly set against the torque spring then the final calibration is by moving the metal tags that surround the magnet in or out. 

The Spitfire/2500S rev counter is a voltage pulse sensing type, most are. The change from 4 to 6 cylinder is made by changing the timing circuit components.

The chart from my rough notes from many years ago! shows the RPM on the left with the frequency, in Hz on the mid and right for 4 and 6 cylinder engines.

A nice piece of work Mike. 

Dave

Rev_Fre_ Chart.jpg

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I do know the calibration of the two dials as I used a pulse generator (and scope). Its fairly linear so just needed a multiplier value for the Megasquirt tacho output.
Interested comments about the metal tabs I did wonder what those were for, I guess they shape the magnetic field to ensure the response is linear.

for the original scale (0-7000 rpm) tacho the measurements were

Frequency     RPM
0                     0
29                  1000
112                3000
188                5000

cheers

mike

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Mike, Yes you are right  the tabs change the magnetic field presented to the moving coil. They are cut up into segments so each section of the scale came be calibrated. Starting with the low RPM and working up.

The frequencies in my chart are the pules produced by the ignition at a given RPM for a 4 stroke engine.

Dave

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