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MG MGB Technical - 1973 MGBGT Tachometer

My 1973 MGBGT Tacho has worked only intermittently since I bought the car. I have always suspected that it was a bad connection. However, I have recently fitted a pertronix ignition module and now it does not work at all. I have stripped the unit, and it does not rely on an input transformer. The impulse line appears to be connected directly to a capacitor and the electronics seem to have only one active component which is a Texas integrated circuit. I have checked the archives, but all references seem to be for the transformer-coupled type. Does anyone know where I could get hold of a circuit diagram for the more modern unit?
C.S. Barrow.

See the books, Classic British Car Electrical Systems or MGB Electrical Systems by Rick Astley for circuit diagrams. Cheers - Dave
David DuBois

I also have a '73BGT, and went through a phase where every year in the spring, as soon as the weather turned warm, I had to remove the tach and hose it out with an electronic cleaner - the needle mechanism was sticking - usually somewhere around 3000rpm, but sometimes not returning to idle rpm. I always guessed that it was getting gum/dirt inside, and the warm weather made it more difficult for the needle to rotate. Anyway, cleaning seemed to work - until the next spring season.

Patrick Callan

OK, I donít have access to the books mentioned by David. What I really need is to find out where I could purchase a replacement integrated circuit chip. The existing chip is a Texas device and the number appears to be either MIC7309 or MIC S/C or MIC 1S/C. However, none of the Texas catalogues refer to any of these numbers. Alternatively, if anyone anywhere has a circuit diagram that they could let me have, I could possibly figure out an equivalent.
C.S. Barrow.

Is it the electronics? Or the meter? You should connect a voltmeter to the actual instrument before rushing out and buying a new chip, the instrument is known to stick. Electronic triggers often cause problems for the earlier (used to the 72 model year RVI tachs), not usually for the later RVC used from 73 on. These letters are on the dial, could be RVI/2430/00 or RVI/1439/00, or RVC/2415/00AF or RVC/1410/00AR.
PaulH Solihull


I don't know anything about the later Smiths tacos, but am surprised that a mid 70s unit has a semiconductor chip, unless it is a basic unit that has been cleverly finangled to work as a frequency to voltage converter (tacho).

Available today is an LM2917 F to V converter for $A8, and the datasheet gives some cool tacho circuits. Google LM2917.

The impulse type (RVI series) will work reliably with electronic ignition, if wired such that the electronic module does not take its power from the hot end of the coil, but from a separate switched ignition point, ie before the current transformer on the tacho. What happens is that the electronic module smooths out the current pulses, because of its internal capacitor across supply, thus reducing the pulse which then is insufficient to trigger the tacho.

Herb Adler

Yes, Smiths commisioned a custom circuit for tachometers in the 70s. It was in the bipolar technoligies available then and Texas Instruments enjoyed the business.
Stan Best

air filters for the input! In the meantime, I have been doing some research of my own (and yes, I did check the meter first). Under a magnifying glass, I found a few soldered joints which looked a bit suspect so I re-soldered them. I again checked the tacho on the car and it now worked after a fashion, but was very sluggish. At that point I had a bit of a brainstorming session, and decided to try a
C.S. Barrow.

This thread was discussed between 07/03/2011 and 14/03/2011

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