I recently acquired a broken Fluke 45 multimeter in an attempt to repair it. The meter had a bad power supply - four times out of five, it would fail to achieve regulation and the rails would only hit half of their specified voltage. I traced the problem to the switching transistors, replaced them, and now the power supply works perfectly. Unfortunately I seem to have caused more problems (before I did anything, on the one time of five that the power supply started up properly, the whole meter worked just fine). I kind of suspect some digital chip is fried, which does not make me happy, but I can't really imagine how it happened.
When the meter powers on, the system is inactive. The display does not initialize, no relay clicks, and the keypad scan lines are silent. All voltages are fine. The microprocessor clock oscillates briefly (about 400 ms) at the correct frequency. During this time, there is activity on the address and data bus. However, the "E" pin on the microprocessor, which the service manual says is the first thing to check, is dead. It should have a clock signal on it, and it simply pulls high as soon as the meter is powered on and stays there. It doesn't appear to be shorted to anything, and since the only thing it connects to is the GPIB option card, I guess lack of this signal is just a symptom of whatever the problem is, not the cause. I really don't know anything about traditional microprocessor architectures, so I have absolutely no clue what the pin does. The service manual just says to check it (and what to expect on it), it says nothing about what it means if it is dead.
Clearly something is preventing the processor from starting. The power-on reset works fine, as does the watchdog (it fires in about four seconds and the process repeats). As mentioned above, the oscillator works, as does whatever circuit shuts it down when the processor goes to "sleep". The behavior is identical if I unplug the socketed ROM, so I'm pretty sure the processor isn't running any code at all - not sure what the bus activity is. (I'd guess it's trying to read code out of the ROM, but I was under the impression that it would just jump to the reset vector and start trying to run, not screw around with the bus for 400ms.)
Am I correct in guessing that the processor has died? Is there anything else I may have missed? (And would anybody be willing to dump the ROM out of the same meter so I can compare to my own and check for corruption?)
Service manual is
here.
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