Electronics > Repair
Yet another Fluke 8050A LED display conversion
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gcwills:
Another update!

Further tests on my 8050A to determine the non-functional "minus" display issue revealed that U11 was not faulty.

Rather, the issue was traced to an error in the PNP transistor configuration that drives the minus LED. The schematic I linked earlier shows the transistor emitter connected to U11 pin 5 (the + output) and the base connected to pin 4 (the - output) via a 3K9.
Those connections should be reversed so that pin 4 connects to the emitter and pin 5 connects to the base via the 3K9. Making this change now provides a working "minus" LED.

Some observations: there is some confusion created with other online 8050A LED display mods as to how this interface is wired. Mr Modemhead has the emitter connected to pin 4 (-) and base to pin 5 (+). However the https://lous.home.xs4all.nl/fluke/Fluke8050Asite.html mod shows the BS170 source connected to pin 5 rather than pin 4 and the Rich Williams schematic follows this arrangement. Better brains than mine could explain the differences?
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