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MG Midget and Sprite Technical - rev counter

The rev counter on my 65 midget starts at 2500 rpm then goes up when i rev the engine and will go to o when i stop the engine think some one replaced it in the past as max revs 7000 rpm any ideas how i can fix it
b bithell

Very likely one or two specific capacitors have failed with age.

Easy to fix if you can solder.

If somebody doesn't post the fix before me, I'll fire up my laptop and find the pictures of what you need to know. I've fixed a few of them, so I know it's an easy fix.

Meanwhile, here's a wiring diagram. Don't worry about polarity unless you are changing it.


And a pic' of the likely failed cap. Open up the rev counter, and it's often easy to spot -- maybe split open.






anamnesis

Here you go. One I fixed earlier.

The white one is new the black one is the old failed cap'.






anamnesis

Black old.

White new.

anamnesis

Anam, agreed. Almost certainly electronic fault. The tach is a voltmeter whose circuit converts the number of ignition pulses per second into a variable DC voltage (proportional to rpm), which is applied to the meter movement. The components are old and capacitors are inevitably the first to fail. There are also dated germanium transistors (hard to source now) which can fail. Silicon transistors will not work in the RVI circuit.

In the OP fault a dc voltage within the tach circuit is somehow getting to the meter when ignition is on.
Bill B

Tanks Anamnesis and Bill i will remove it and have a look inside and get a capacitor see how i get on Thanks
b bithell

Just to echo this. Mine wasn't working either. Luckily for me one of my good friends is an electrical engineer and knows a thing or two about electronics.
We used the above diagram and tested the whole circuit and replaced a few caps and the thermistor that had come loose.

All tested and working fine for a good few years now.

Better to fix the original.

Good luck
James Paul

Be nice to know where you go to get such replacement capacitors for our ancient instruments
Tim C

Tim
It used to be Maplin but now either ebay (although you'd probably have to buy 10) or cpc Farnell maybe. Modern capacitors are fine (better in fact) in the old circuit.
Bill B

I don't know if it's that important for the rev counter but for modern stuff when replacing the likes of electrolytic capacitors you need to check the specs of the original and find an equivalent, not just voltage but ESR (Equivalent Series Resistance). One of my monitors was playing up and when I opened it I found most of the electrolytic caps in the power supply were leaking, replacing them got it running again as good as new.

https://en.wikipedia.org/wiki/Equivalent_series_resistance
David Billington

This thread was discussed between 07/09/2026 and 10/09/2026

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