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 Post subject: What substitute for miniature 40 volt DC incandescent bulb
PostPosted: Nov Wed 24, 2021 4:03 am 
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Joined: Nov Wed 24, 2021 3:44 am
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The bulb in question is the pilot or "on" light for an early 1970's Realistic 4 channel integrated amp....a QA-680.

Attached is a photo of said bulb.

Power to the bulb is the 40 volt DC rail voltage. My experience is not vast and I have not encountered a miniature panel lamp bulb needing a voltage that high. 6.3 volts, 8 volts and 12 volts comprise all of the spares I've managed to accumulate.

Browsing part vendor's websites makes me think prospects of finding a direct replacement are slim and none.

Can an LED be adapted to work with 40 volts DC? And if so, how?

Thanks in a advance to any who choose to respond.


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 Post subject: Re: What substitute for miniature 40 volt DC incandescent bu
PostPosted: Nov Wed 24, 2021 5:06 am 
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Joined: Jul Fri 18, 2008 10:02 am
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Location: near ST Louis Mo 62002
This lamp would work with a dropping resistor , so would an led with a large enough dropping resistor .

https://www.galco.com/buy/Chicago-Miniature/327

About 300 ohm 1 watt resistor

https://www.goldmine-elec-products.com/ ... ber=G25735

1 k ohm 2 watt resistor

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 Post subject: Re: What substitute for miniature 40 volt DC incandescent bu
PostPosted: Nov Wed 24, 2021 5:32 am 
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Location: Northeast Illinois 60188
62vauxhall wrote:


Power to the bulb is the 40 volt DC rail voltage.


Are you certain that the lamp is connected directly across the 40V power rail without a series resistor? That would be very unusual.
If the lamp is open, and there is a series resistor, and you are making the voltage measurement with a high impedance meter such as a DVM, there will be no voltage drop for you to measure so your meter will indicate the full supply voltage.


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 Post subject: Re: What substitute for miniature 40 volt DC incandescent bu
PostPosted: Nov Wed 24, 2021 6:21 am 
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Joined: Nov Wed 24, 2021 3:44 am
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jimtech wrote:
62vauxhall wrote:


Power to the bulb is the 40 volt DC rail voltage.


Are you certain that the lamp is connected directly across the 40V power rail without a series resistor? That would be very unusual.
If the lamp is open, and there is a series resistor, and you are making the voltage measurement with a high impedance meter such as a DVM, there will be no voltage drop for you to measure so your meter will indicate the full supply voltage.


That had me (try) taking a closer look. I cannot manipulate the power supply much at present.

There is a resistor present on that board and looks to be associated with the pins to which the lamp bulb is connected. It is 470 ohms and I believe 1 watt.


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 Post subject: Re: What substitute for miniature 40 volt DC incandescent bu
PostPosted: Nov Wed 24, 2021 1:20 pm 
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If that 470 ohm resistor is indeed in series with the lamp, it was probably a 9-12v lamp, depending on the lamps' current rating.


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 Post subject: Re: What substitute for miniature 40 volt DC incandescent bu
PostPosted: Nov Thu 25, 2021 5:22 am 
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Since I have on hand a couple of different 12 volt bulbs with leads but of different milliamperes, I measured voltages with bulbs connected. Voltages differed and neither produced a 12 volt reading.

But a 5mm green LED I have with a 4.7K ohm resistor looks like it will do nicely. Not overly bright but bright enough.

My sincerest thanks to the members who took the time to reply.


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