Wreck
Experienced Member
Just measured R12, the resistance is OK but there is a voltage drop across which can't be right, unless I miss something.
Joe
Joe
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Dear Hugo,There is something you could do, perhaps not a textbook recommended method, but I will explain how it works by analogy.
Many years ago fixing AM transistor radios, it was noticed that sometimes some sort of conductive material would get between the aluminium vanes of a tuning capacitor. It could be nearly impossible to see where it was , especially if the fragment was between extremely closely spaced vanes, and at times shorting them out.
Somebody came up with the idea of connecting the capacitor across a power supply with a low internal resistance, when the particle shorted the vanes out, it got blown to pieces, much like an insect in a bug zapper, I am sure you can see where this is going.....
Any IC's that are normal, connected to the /INIT line, won't draw excessive currents on any of their input pins, and won't mind being temporarily pulled to hard 5V supply. But the failed IC will, it will likely draw more current and probably the short or leakage pathway in it might get vaporized, or at least it might heat up and you could spot the defective IC that way. If connecting the /INIT line to +5V directly (ie short out the pull up resistor) cleared the problem, at least then, most of the computer would likely start working and you could fault find it better to locate the defective IC. If it doesn't clear the abnormal leakage pathway, nothing is lost with the experiment as no normal chips connected to that line will be harmed.
Now one thing to consider before doing anything is to make the theoretical statement and then test it: "All chips connected to the /INIT line are perfectly normal" . Now if this is true, what could be wrong that could cause all/many of the chips to draw current from the /INIT line ? The answer to that is power supply failure, if one or more chips is not getting its +5V supply. So before doing anything else, I would recommend identifying every chip on the /INIT line and checking on the actual power pin entering its package, with the meter, that it is definitely being powered. If they all definitely powered, only then move to the bug zapper method. Of course an internal failure in an IC, if the die loses its power, will cause the IC to start sinking significant current at its input too. Especially check any IC in a socket, because if the IC's power pin failed to connect that would do it.
(If a chip loses it + 5V power, it can get powered indirectly from its input pins via substrate diodes or diodes intended to prevent the input pins going significantly above the +5v rail)
.....don't forget to check all the IC's power supplies before trying the bug zapper method, there is still the possibility that no IC's are defective and one has simply lost power.Dear Hugo,
Thanks for your input!!
I can very well visualize the idea of a bug zapper
Funnily I just today took my schematics out again and am trying to formulate a plan regarding further troubleshooting of the (beasty) PET.
I will beg my wife today to allow me an hour or two in the basement with the PET and will definitely try your idea; the alternative would probably be to de-solder each IC eventually, but this would change the PET's originality considerably, so I will try other options first.
As I consider myself an absolute beginner with PETs and electronics, it gives me also an excellent opportunity to read schematics & visualize connections between the various ICs.
I will definitely report back with an update....
Joe