Roger Weichert wrote:
> Hi Guys,
> 
> 
> SUPPLY . Crudely, the boards only use a 5.6v zener to regulate the supply to 
> the original micro and two other housekeeping IC's . so I assume that needs 
> to be addressed.
> 
> The board runs off 12V ac, via a rectifier into a 22000uF cap. Then it has a 
> 82 ohm resistor, a zener to drop it to 9 volts . then another resistor (120 
> ohm) and the 5.6 volt zener mentioned above  ..  crude !! .. but they've 
> been running for 20 + years.
> 

Just in case... you should check that the board will run fine at 5V 
instead of 5.6V; modify one and verify that they work at 5V.  Check FET 
gate drive levels and look up the specs to be sure you will be able to 
turn them on completely.  If everything works fine at 5V then you might 
consider the PIC16F616 - it has an internal shunt regulator.  I think 
there are others in that series as well.  It would make the mod simpler.

Alternatively, if the board does work fine at 5V, you can probably 
change out the 5.6V zener for a 5.1V unit and then use any 5V rated PIC.

Other hardware issues to watch for are PIC pins that float during reset 
- is that compatible with the existing circuitry?  You may or may not 
need to address that with pull-ups/pull-downs, etc.

It sounds very do-able!

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