The lamps could be in parallel. The schematic is badly drawn. Two on
line vendors of lamps
http://www.donsbulbs.com/cgi-bin/r/b.pl/1820|28v|0.1a~usa.html
and
http://bulbster.com/lightbulbs/eiko-1820bts-49790-p-2306.html
claim the lamps are 1820 but two 1820 in parallel at 28 volts will draw 200
milliamps and fry that 20 ohm 1/2 watt resistor with 0.8 watt dissipation.
73, Jerry, K0CQ
On 10/2/2010 5:36 PM, Dr. Gerald N. Johnson wrote:
> I've checked the downloaded manual. The lamps appear to be in series and
> are run off the 28 volt control supply through a 20 ohm 1/2 watt
> resistor. The maximum current (dissipating 1/2 watt) through that
> resistor is 158 milliamps which would give a voltage drop of 3.16 volts.
> Its a surge limiting resistor and probably the lamp current is more like
> 80 or 100 ma. And 14 volt rating. But without knowing the size and base
> of the lamp I'd be speculating too much to guess at a number like a 756
> or 1813 if the size of a 47 or 44. There are some other 14 volt lamps
> with more light output but they draw more current than the 20 ohm
> resistor can survive but would likely be found at most truck stops being
> used in marker lamps. If the package is right for these lamps I'd go for
> the 756 because its rated at 15,000 hours while the 1813 is rated for
> 1000 hours. That package is T3-1/4 with a single connection bayonet
> base. That's just over 3/8" diameter for the bulb, actually 3.25
> eighths, really close to 10mm.
>
> With the lamps in series, if one opens both go dark.
>
> Seems to me we've thrashed these lamps on this forum before.
>
> 73, Jerry, K0CQ
>
> On 10/2/2010 4:42 PM, Ken Brown wrote:
>> You could look in the manual at the schematics and see if they are
>> powered by the same transformer secondary as the 3CX800A7 filaments. I
>> think that is 6.3 V which would make them plain old #47 lamps. Don't
>> take my word for it though, confirm before you buy, and when you buy,
>> order enough to get the quantity discount from Mouser, Digikey or other.
>> You could look at the number on the burnt out lamp. Some just have a two
>> digit number, some have a prefix such as CMI (Chicago Miniature). Once
>> upon a time this pilot lamp bulbs used a color code. The glass bead the
>> holds the wires in position to suspend the filament coulb be various
>> colors, which represented the bulb type. Old Amateur Radio Handbooks had
>> that data in the back along with the tube and transistor data.
>>
>> DE N6KB
>>
>> John Oglesby wrote:
>>> Can someone tell me what the lamps are that are used to illuminate the
>>> meters in a Titan 425? I can't find them in the manual and both of mine
>>> have failed. I wanted to make sure I had the right lamp before I
>>> dismantled the station to replace them.
>>>
>>> Thanks& 73
>>>
>>> John
>>> N9RE
>>>
>>>
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