v836 wrote: ↑Wed Apr 20, 2022 9:44 pm
I found as well on my 520ST that my WD1772-PH 00-02 failed sometime during a ~250hr burn in with 10k pullup resistors on DAL0 through 7. Possibly it was heat maybe this is just too much current. I'm a little nervous about applying a pullup again, if I do so I might add a heatsink. It's a shame to break parts that are getting increasingly rare.
A word of warning, if I seem to be a bit blunt here, sorry, I've had a bit of a day.
My first question would be "Why on earth are are you doing a 250 hour burn in on an ST?" For what reason would you ever
need one to run for that long?
These machines are home computers and 30 odd years old, and even back in the day, they would probably never run continuously for much more than a 10th of that. I expect the Atari burn in was "does it switch on and get to the desktop and switch off again afterwards".
The ICs are going to get hot when run for that long without cooling of any sort, and they were never designed to do that, they are a home computer, not an industrial machine, built to a budget and now suffering deterioration due to age.
I sincerely doubt that the 10K resistors are the reason for the WD1772 failing, with a 10K pullup and 5v the current of .5ma is negligible, I think you'd have a very long wait for those to get warm, and if they are, it is more likely thermal coupling from the WD1772.
I'd suggest the WD1772 failure was entirely thermal due to the length of time in operation and the age of the IC, which may have been marginal in the first place.
I have several machines with this mod installed and don't have any issues, but then I don't do 250 hour burn ins either...
If you want to run for that long, then you'll need some sort of active cooling, a passive heatsink just won't cut it.