06-06-2022, 02:54 AM
(05-06-2022, 11:39 PM)Witchy Wrote: Hi Ernest,
I did think of that but all my RAM boards have tantalums and they check out at ~230nF. I swapped C21 from good board to bad and it's still failing. You're right though, this is probably RESET based but for a different reason. This is where my lack of formal electronics training lets me down. On the good board at pin 3 of the inverter I get ~4.8V but on the bad board pin 3 is receiving ~0.4V. I've also noticed that the good board has /KBRST at ~4.8V but the bad board it's only at ~2.8V. After lunch I'll pull the resistors to check them.
Cheers
Witchy
Yep Witchy,
You have your point of problem with the 2.8V at /KBRST this needs to be higher. I would start by lifting one end of D1 to rule out the Power on RESET circuit. The input pin 3 of the inverter may also be pulling down the voltage. There have been a number of engineering changes around the RESET circuit during the life of the Microbee. Check out the resistor value markings are matching that of the working board.
I assume none of these boards have a backup battery fitted, if so remove that until you get normal function then look at re-establishing battery backup as a secondary action. Just incase you have some dry soldier joint issues or even partial shorts a good visual with a magnifying glass and good light source wouldn't go astray.
Ernest