In the upper right area of the board is a 14 pin chip. Clean it off and look for the chip number, (an example number would be LM3900).
Google the chip number and data sheet. A data sheet shows chip pin functions,including the positive and negative supply pins (aka "rails"). Careful use of a voltmeter will tell you what voltage is on these pins, compared to each other (for a single suppply, e.g. +12 volts), or compared to ground for a two sided supply (plus or minus 12 volts, for example).
I would be very cautious in doing this. I didn't see a transformer which would supply the chip voltage(s), which would typically be maximum 15 volts for a single sided supply.
Some boards rectify the AC line voltage of 110 volts, and regulate this to lower voltages with zener diodes. This can cause serious hazards (electrical shock) because power is not properly grounded. If there is no transformer, do not go poking around the board, get advice from an electrician or electrical engineer.
If there is a transformer, low voltage AC is rectified into pulsating DC, and an electrolytic capacitor smooths out the pulses. If this is 12 volts, you are in business. If it is higher you would likely need a 12 volt voltage regulator and a few other bits to get your tach going.
I am not an electrician or engineer, and there may be errors in the above, so proceed at your own risk.
Grant
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