The Sinclair QL Forum

If the on board RTC is working - then the I2C from the GAL is working - so it's not your MK2 that's gone duff.

Pr0f wrote: Sat Sep 20, 2025 8:15 pm If the on board RTC is working - then the I2C from the GAL is working - so it's not your MK2 that's gone duff.

I’ve replaced all the cables and devices & vcc/gnd are good but only the NVRAM/RTC respond/found

The external pins for data/clock run directly off 2 pins of the PCF8583 RTC so in my mind this IC could be damaged?

That chip is just a another 'target' for the I2C bus to find - so the fact you can read from that chip suggests the issue lies with either the pins for the I2C output, the resistors R1 and R2 - the 2K2 resistors, or the small PCB track that exists between the connection on the NVRAM/Clock chip and the pins for external devices.

I suspect what may have happened is that connecting to these pins on and off may have caused mechanical damage on the PCB.

Will check continuity between IC legs and the pins to start with then

Odd thing is, if it’s mechanical failure, is that yesterday was the first time I’d unplugged/re-plugged cables directly to the pins since I first fitted it as I use an external connector

I guess the other possibility is a failure of my soldering!

It might be worth 're sweating' the joints in that case - it could just be a dry joing on that 5 pin header.

Pr0f wrote: Sun Sep 21, 2025 7:59 am It might be worth 're sweating' the joints in that case - it could just be a dry joing on that 5 pin header.

Looks like a duff solder joint on the CLK pin, now to find the time to re-solder it

Many thanks
Tony