[TenTec] Titan III screen current question.....

[Dr. Gerald N. Johnson]

The TT manual that I downloaded only identifies the lamps on the 
schematic and shows them separate from the meter and the meter wiring. 
The lamps are hard wired to the 12 volt supply but the meters plug into 
the meter switch board A12.

Tiny 12 volt lamps can be had in hobby shops for train lighting or for 
scale doll house lighting as well as sometimes at Radio Shack. Many 
different 12 volt lamps in tiny packages can be bought from Mouser and 
Digi-Key.

Its worth selecting a lamp with a long rated lifetime, though that will 
reduce its illumination efficiency.

It can also be worth dropping the supply voltage about 10% with a series 
resistor which will also lengthen the life by lowering the operating 
temperature and limiting the lamp surge current. A typical incandescent 
lamp's cold resistance is in the range of 15 to 16 times lower than its 
operating resistance. So a lamp rated to draw 100 milliamps will draw at 
least 1.5 amps the instant its connected, unless the external circuit 
limits that current, and nearly all incandescent lamp failures seem to 
be at power on.

So if that 100 mil rated lamp at 12 volts had a 12 ohm series resistor 
the peak surge current would be limited to about 0.6 amp. And the 10% 
drop in operating voltage would quadruple the lamp life. Rule of thumb 
from a GE lamp book years ago is that a 5% change in lamp voltage causes 
a 10% change in light output and if going down in voltage a doubling of 
life or if going up, halfing the life.

73, Jerry, K0CQ

On 3/11/2010 2:33 PM, Jim Brown K9YC wrote:
> On Thu, 11 Mar 2010 12:09:08 -0600, John wrote:
>
>> TT does not stock the original incandescent meter lamps.
>
> What's the mfr/part number of the lamp?  Is it a standard industry type?
> Older Ten Tec gear (Titan 425, Herc I, Herc II, 229 and 238 tuners) all use
> standard bayonet lamps that are easy to source from industrial vendors. I
> simply buy boxes (a dozen or so lamps per box) of 14V long life bulbs. I
> replaced all the bulbs in one 229B with LEDs and series resistors, but am not
> satisfied with the result, so I've stuck with incandcescents in the other
> four. YMMV.
>
> 73,
>
> Jim K9YC
>
>
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