My local hackerspace has equipment that gets treated... roughly.
The first thing I saw was a bench power supply moved away from the electronics bench looking rather forlorn. I knew it had been used to charge some batteries and that someone had turned the voltage up leading to both a very hot power supply and battery. But now it was producing 0V and the current meter was doing odd things.
My first thought was the diode bridge or the poor pass transistors, but they and the internal fuses were all fine.
The penny dropped when I measured the resistance across the output terminals and it was a dead short.
The obvious thing was a dead reverse polarity protection diode, and it turned out to be the case.
The unfortunate news is that it's brother had been disposed of recently having the same fault :-(
The second thing was a capacitance meter. This is a fairly old digital unit capable of reading 1pF to 9999pF, 0.1nf to 999.9nF, and 0.01uF to 99.99uF in three ranges.
The fault as described was "It shows weird and wrong values. They are different for different capacitors, but they're wrong"
OK, this is an old unit. I'd guess it was a 1980 Electronica Australia project -- anyone with EA going back into the dark ages can look it up.
It uses a 74C926 to count clock pulses during the period of oscillation as determined by three switched resistors and the capacitor in question.
The fault turned out to be operator error. The user was trying to read a 220uF capacitor. The count overflows, with the hundreds of uF disappearing out of the carry-out. On the nF and pF ranges the gate is so slow that you don't see the numbers change.
The fix was to take the carry-out, and use a spare inverting schmitt trigger to make a pulse extender and to use this to turn on an otherwise unused decimal point on the display. Along with this, the decimal point is labelled as "over-range". Also some instructions have been attached to the top of the case to tell people how to use it and how to recognize that their capacitor is too large.
The first thing I saw was a bench power supply moved away from the electronics bench looking rather forlorn. I knew it had been used to charge some batteries and that someone had turned the voltage up leading to both a very hot power supply and battery. But now it was producing 0V and the current meter was doing odd things.
My first thought was the diode bridge or the poor pass transistors, but they and the internal fuses were all fine.
The penny dropped when I measured the resistance across the output terminals and it was a dead short.
The obvious thing was a dead reverse polarity protection diode, and it turned out to be the case.
The unfortunate news is that it's brother had been disposed of recently having the same fault :-(
The second thing was a capacitance meter. This is a fairly old digital unit capable of reading 1pF to 9999pF, 0.1nf to 999.9nF, and 0.01uF to 99.99uF in three ranges.
The fault as described was "It shows weird and wrong values. They are different for different capacitors, but they're wrong"
OK, this is an old unit. I'd guess it was a 1980 Electronica Australia project -- anyone with EA going back into the dark ages can look it up.
It uses a 74C926 to count clock pulses during the period of oscillation as determined by three switched resistors and the capacitor in question.
The fault turned out to be operator error. The user was trying to read a 220uF capacitor. The count overflows, with the hundreds of uF disappearing out of the carry-out. On the nF and pF ranges the gate is so slow that you don't see the numbers change.
The fix was to take the carry-out, and use a spare inverting schmitt trigger to make a pulse extender and to use this to turn on an otherwise unused decimal point on the display. Along with this, the decimal point is labelled as "over-range". Also some instructions have been attached to the top of the case to tell people how to use it and how to recognize that their capacitor is too large.
