......now the post just above this is part-fixed..as follows IYAI........I just took off R7, and replaced the CD74HC123M, and it is now giving exactly the right output.........
.so the next thing is to connect it back to the UCC28070A..... the first time i powered up, a voltage
must have coupled through the UCC28070A pin capacitance-to-VCC, and then blew up the CD74.....so , yes, i will use a
1N4148 in there (in place of R7).....must admit i would still like some resistance aswell.....but the
track goes on an inner layer after R7, so not so easy to add an R.......maybe i can do a "wigwam"
type thing with a 1N4148 and a
1k resistor in series between the vias either side of the R7 pads.....
Must admit, i would also now like a signal diode on the CD74 output pin, (just in case it rings below ground) but i'll struggle for room.
I never realised how utterly delicate these CMOS chips are (CD74HC123M). Just because the 12V powered up first...and then capacitively coupled through the vcc-to-pin capacitance of UCC28070A, ignored the R7/input_pin_capacitance time constant of CD74, and then killed the CD74, with what was probably a tiny overvoltage for a tiny amount of time!
*********************************Now not working again*****************************
Just put 1k and a 1n4148 (anode to CD74), instead of R7, , and it now doesnt work......or should i say, when i probe at the mid point between 1k and 1n4148, it pulses for a second or two after power-up, then just goes flat at ~5V.
So it now goes CD74.......1K......1N4148.......UCC28070A
So i am wondering about this now.....maybe i just need to change R7 for 10k?
*********************still not working*********************
However, with 10k instead of R7, the CD74 doesnt work at all.......just get a flat 5v5 output from it. ...So in fact now i am wondering if the UCC28070A is damaged.
Any ideas why the CD74HC123M isnt working?
*********************still not working*********************
Now disconnected R7 from ucc28070a RDM pin, and it (CD74HC123M) still doesnt work.....though every now and again, it does work, for about a second or two after power up....then the CD74 output goes flat at 5V.
*********************
Now just changed the CD74HC123M timing capacitor to fresh one, same value....now it works but only when not connected to the UCC8070A.