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Does anyone have a circuit diagram or schematic of the 75 speedo? In particular the circuit board attached to the motor?

Anyone know how it works?

There are 4 cables that go into the board. The Red and Black are obviously 12v and GND, but what about the other two? can I assume they are the signal from the sender in the gear box?

I’m trying to find out what the signal looks like, is it a sine wave or a digital pulse?

Are here any electronic geeks out there that have pulled apart one and may know?
 

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Post #6 in this thread shows the pulses, displayed on my oscilloscope: http://www.alfabb.com/bb/forums/milano-75-1985-1993/254185-speedometer-amplifier.html

They are more or less square wave. That signal is carried on a single wire, the return is the common ground. I don't know the function of the 4th wire.

This is what I learned about the pulse rate for the GTV6 speedometer
I then tested it out on the bench with my drill motor and gear to generate pulses while I monitored the pulses with the oscilloscope. It appears that the Veglia speedometer requires 100 pulses per minute per indicated mph. In other words, a pulse rate of 5500ppm will make the speedometer read 55 mph.
 

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Signal is digital pulse

Hi

" I’m trying to find out what the signal looks like, is it a sine wave or a digital pulse? "

The signal is a digital pulse.
Search this forum and you'll find several threads on the speedometer operation. One of them has a link to the chip used in the speedo.

You can find a datasheet for the chip, and it describes the output signal.

I have a dismantled 75 speedo. PM if you want pictures

Bye
 

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You can find a datasheet for the chip, and it describes the output signal
The signal from the "amplifier" is from a transistor which boosts the signal from the chip to a 12 volts square wave. I posted a schematic in the thread that I referenced in post #2
 
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