28-01-2013, 10:42 PM
Its a VFD display. it normally has a grid per digit and an overall common cathode (the heater).
If a digit lights and goes off OK then the grid for that digit is ok and the overall grid supply is OK. If any digit lights then the cathode is OK and the overall cathode supply (and heater supply) is OK.
Normally these are multiplexed. i.e. the anodes which are coated in blue-green phosphorous are in parallel so you only need n + 7 control lines where n is the number of digits.
Here it looks like the digits are direct drive. very odd. and that one anode driver has gone AWOL. Or there is a possibility that the faulty anode has somehow broken free off the chain and is floating.
So could be a display or anode driver fault but not a PSU fail.
Note that due to their construction VFDs make reasonable audio triodes... they have a heater/cathode, a grid and an anode (even if the anode glows, it's still an anode!).
EDIT: AHA! Found the display schematic - its not multiplexed - its direct drive, single grid, individually switched anodes:
It has an AC grid with LDR control so its brighter in the day and dimmer at night (these are apparently prone to failure too). Although the schematic is difficult to read, I suspect that the chip is a National Semiconductor MM5316N - datasheet included below - they are on eBay for a few quid NOS, e.g. item #110998188239.
Looks like an output driver problem in the MM5316N... these are MOS devices and rather sensitive to static and prone to breakdown...
HTH
If a digit lights and goes off OK then the grid for that digit is ok and the overall grid supply is OK. If any digit lights then the cathode is OK and the overall cathode supply (and heater supply) is OK.
Normally these are multiplexed. i.e. the anodes which are coated in blue-green phosphorous are in parallel so you only need n + 7 control lines where n is the number of digits.
Here it looks like the digits are direct drive. very odd. and that one anode driver has gone AWOL. Or there is a possibility that the faulty anode has somehow broken free off the chain and is floating.
So could be a display or anode driver fault but not a PSU fail.
Note that due to their construction VFDs make reasonable audio triodes... they have a heater/cathode, a grid and an anode (even if the anode glows, it's still an anode!).
EDIT: AHA! Found the display schematic - its not multiplexed - its direct drive, single grid, individually switched anodes:
It has an AC grid with LDR control so its brighter in the day and dimmer at night (these are apparently prone to failure too). Although the schematic is difficult to read, I suspect that the chip is a National Semiconductor MM5316N - datasheet included below - they are on eBay for a few quid NOS, e.g. item #110998188239.
Looks like an output driver problem in the MM5316N... these are MOS devices and rather sensitive to static and prone to breakdown...
HTH
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