Electronics > Beginners
Troubleshooting Microprocessors
james_s:
It's safe for the CPU chip, reset is an input. Whether it's safe for whatever drives the reset pin depends on what that is, but there is certainly a way you can force the reset. A safer method is to use something like a 10 ohm resistor to ground, this will limit the current to something reasonable in case a pin is being driven by a low impedance source.
bostonman:
When I pull reset low, it causes the address lines to be a continuous 1.xMHz clock pulse, 0 to approx. 3.5v. Although they are not switching states, they do look close to a good unit.
james_s:
The address lines are a pulse but they're not switching states? That's contradictory, are they pulsing or are they just sitting at a fixed voltage?
bostonman:
See the two pictures I included for a better idea of what I mean.
On a working one, the signals are jumping around (I assume because the scope isn't synchronized to the board). On the broken C64, only half the signals are present (see picture), and with reset held low, the full screen is filled with signals; but they don't jump around.
Shock:
--- Quote from: techman-001 on December 28, 2019, 08:42:31 am ---We didn't have a vacuum de-soldering gun to share between 20 technicians who produced 90 complete commercial games a day including enclosures (stand up or sit down) 5 days a week. Spending more than 15 minutes on a board was not worth it.
My point is that there are two ways to fix this old technology, the fast way with a few simple tools and lots of experience, or the *really* slow way with a workshop full of hitech gear, schematics and every possible aid.
--- End quote ---
Vacuum desoldering is actually the fast way but you still need experience as it's easy to butcher PCBs if you are impatient. If parts are scarce, expensive or need further testing they also may need to be removed intact.
Perhaps not on company time but I still think it's best to learn proper manual desoldering without ripping out traces. You will a hard time convincing most people that though that schematics and oscilloscopes aren't useful especially on complex repairs.
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