[time-nuts] Thunderbolt monitor

Mark Sims holrum at hotmail.com
Thu Jul 17 06:38:41 EDT 2008

All VFD displays (and EL and CCFL LCD backlights) have finite lifetimes.  Very few will last over 40,000 hours (5 years).  Many start to noticeably degrade after one year.  Many EL backlights have lifetimes of less than 5000 hours (ever see a Tektronix 1502/1503B/C TDR with a good backlight?)

VFD displays have two main wearout mechanisms...  phosphor degradation and cathode (filament) degradation.  These displays are essentially the same as CRTs and old fashioned vacuum tubes.  They rely on a stream of electrons from a heated filament to hit a phosphor screen.  Over time the filament and the phosphor degrade.  

VFD displays (and EL backlights) typically have a rapid initial drop in brightness (25 percent over the first 100 hours is not uncommon) followed by a slow, fairly linear, drop  in brightness.  Most spec the lifetime of the device at 50% drop in brightness.

I repair quite a few old digital scales from Sartorius and Mettler that use VFD displays.  Many of these units are discarded because the displays have become so dim that they are unreadable (and the only source of replacements is another balance).  Most of these balances keep the electronics on continuously to keep the units thermally stabilized.  The power switch just turns the display on or off.  Unfortunately some of the balances just blank the display.  They leave the VFD filament energized and this wears out the display about as fast as keeping it lit.  

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