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Ok.

which supply is reading near zero?? From what you said, it's the 5 volt supply.

Disconnect the boards at the main power supply.

Does it work?? If so, the fault is on a board somewhere.? If not, fix the power supply first.

If the fault is on a board, plug the main supply back in. Disconnect all the boards, and plug them back in one at a time. The one that kills that supply has a problem.

Disconnect the board.

From your current limited 5 volt supply, and 100 ma is fine IF you remove the TTL chips.? Past a certain voltage they start to draw current, thus holding down the current limited supply, measure voltages only on the 5 volt parts.? Print out the board layout and highlight the 5 volt caps in one color.? If you think you need the other parts, then highlight them in a different color.

Supply only one voltage at a time if you can.? 5 volts will be a heavy current drain because of all the TTL chips.? Removing them lessens the power drain.? With no load on the 5 volt rail other than a few soldered in chips (if any), you should be drawing (certainly less than) 100 ma or so.? Anything that is holding down the 5 volt rail by drawing too much current (thinking only capacitors here) is suspect.

Do not try to diagnose a power supply short by ohmmeter readings if you can avoid it.? You'll get inconsistent readings using 2 wire mode because the leads carry a fairly heavy current on low resistances.? Those connections are fussy, and can be erratic. Depending on how hard you press the probes, you can get different readings.? You are also measuring the resistance of the test leads themselves (test:? measure ohms, short the leads together lightly, press harder).? When you measure voltage, you're looking at a 10 megohm load, and the 1 to 3 ohms resistance of the iffy connection and test leads themselves is irrelevant.

Active loads (circuitry) do not behave like linear resistances. They draw current that's based on the voltage, but not linearly. The reason for the resistance checks is to determine if there's a gross fault on the power rail.? It's not for fine diagnostics.? IF it should measure 20 ohms, is 19 bad?? what about 18?? How much voltage does your ohmmeter put out?? That affects the reading...

Stick to the power supply, constant current, and voltmeter readings.? Measure directly across the part leads.? write the voltage down on the parts layout.? remember the street lamps.? The first one in the line with the lowest voltage directly across it is likely the bad one.? All the other ones past that can only "see" the lowest and bad voltage.


Harvey