Amiga 4000 RTC not found

Sometime a RTC doesn’t work on an Amiga 4000. It can be a trace or via, but if they are all connected and test good the most common issue with this is a “floating” ground (pin 8) of U177. Or the RP5C01 RTC chip is dead 🙂

If we check the ground pin we see a DC offset >0V, in this case 1.720V

If we check the output of U177 pin 7 (Q3), we notice that the offset on the output is ~1.5V.

U177 provides clean address bits to U178 (RP5c01 RTC). If the DC offset is too high the address decoder of the RP5C01 is not able to sample the bits, resulting in a wrong address for he RP5C01.

I added a jump wire from pin 8 (ground) to another ground point of the Amiga 4000 mainboard.

You could use pin10 of U891, or the negative pole of C192. Those are most close to U177.

As we can see, the DC offset is gone, so the bit sampling should be good now.



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