I guess the final check would be to lift pin 26 and see if the 5 volts comes up on the print.
VLSI chips such as the System Control uP are normally very reliable and the last suspect of all, however in this case pretty much everything else has been proved good.
Pin 26 is an input port and so should not be pulling down a logic level supplied to it.
Only you know whether everything else works OK. If it doesn't then it would be worth going around ALL the pins on the uP and seeing if anything untoward shows up. 98% of the time issues like this are not the chip itself but rather some input, reset or clock problem external to the device.
VLSI chips such as the System Control uP are normally very reliable and the last suspect of all, however in this case pretty much everything else has been proved good.
Pin 26 is an input port and so should not be pulling down a logic level supplied to it.
Only you know whether everything else works OK. If it doesn't then it would be worth going around ALL the pins on the uP and seeing if anything untoward shows up. 98% of the time issues like this are not the chip itself but rather some input, reset or clock problem external to the device.
I have read that the clock is the heartbeat of the system. How would I check the clock and where on the chip. I have checked pin 1 for the oscillator crystal and it's dead on point at 4mhz 3.5v p2p. Also can't remember if I said earlier that pin 26 is 650 ohms to ground. And I checked a few other pins and pin 18 labeled sp ry is missing the 4.6v. I guess that means speaker relay. So I'm thinking like you something on display board is dragging my input down even though pin 26 is an input. Do you think it would be safe to inject 5v to pin 26 at some low current to see if it will come out of protection, or at the least see what gets hot and find the issue that way.
Ok update. I took 3 AA batteries and applied 4.75v to pin 26 and it stayed out of protection. And speaker relays clicked on as well. But I haven't ran a signal through it yet just wanted to let u know where I stand at now.
It does sound like the chip is pulling the 5 volts down but you still really need to prove it as I outlined above. If the pin26 has become low impedance in some way then it could still be functional but drawing more current than the 5v source can supply.
I would expect the input to be a CMOS type and so draw an unmeasurable current.
I would expect the input to be a CMOS type and so draw an unmeasurable current.
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