Posted By
TLC
on 2012-05-15
14:59:48
|  Re: Faulty Plus 4
Hmmm, the 9V is not critical, the machine should work even in the complete absence of that circuit (it's only used to power the datassette and User Port cartridges). Anyway, this is 9V AC, maybe that's why you could measure 5.8 volts here (standard multimeters are rarely capable of measuring small AC voltages correctly). As you said, you have perfect 5V, well, it'll still worth checking around the chips when you start inspecting things using the 'scope (so that you can make sure there aren't any spikes and similar interferences on the supply). Needless to say, this is certainly the point where things usually start becoming difficult.
You might probably give a try to crock's system test suite. This comes in an eprom replacing the system roms (you can burn your own chip if you have some means to program an eprom or eeprom yourself; the binary dumps should be available to download somewhere, I just couldn't find them right away). It runs tests for most parts of the computer so it may or may not give you useful hints.
Edit: looked up and found the document ie. Diag264.
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