derkom wrote: ↑Tue Jul 26, 2022 5:34 pm
Oh, heh, when looking through the service manual myself, I'm reminded that serial does not work on an ST if you don't have +/- 12V. So powering 5V via the floppy power plug is fine for checking screen output, but not sufficient for looking for diag output on serial.
Have you looked for diag cart output on serial with the DC-DC PSU installed, or just with the floppy power connection? And while you're at it, have you verified that the DC-DC PSU is producing both +12 V (pin 4) and -12 V (pin 5)?
Hi derkom, I tried to swap the EPROMs (HI, LO with LO HI, of course always respecting the reference notch), but still white screen. So I really think the problem doesn't lie with those chips. I then measured, as per your advice, the voltage in the DC / DC module and I got this:
+ 12.25v in the pin marked with the red arrow in the photo;
-11.77v in the pin marked with the purple arrow in the photo.
I took the ground from one of the three pins marked with the black arrow.
I think the values are in the norm with this difference (correct me if I'm wrong, I also post a photo of this measurement because I would not like to have the wrong pins to carry out the voltage checks),
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I performed the test with the serial and the diagnostic cartridge, both using the power supply in the floppy pins, and using the DC / DC module, but in both cases I had no results in the serial data terminal.
I had already downloaded the PDF of the service manual, and I think the time has come to put into practice the procedures for identifying a dead unit.
To do this I created an extension to connect the DC / DC module (see photo), so that you can also perform measurements in the CPU, as was suggested to me by stephen_usher (I hope you are reading me right now). I would like to take those measurements you advised me to do. Currently with a multimeter, but if you give me advice on which oscilloscope to buy, that would be appreciated, as I don't have a clue which one might work for me in this case.
Thank you!!!!
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