Author Topic: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination  (Read 163 times)

Offline Ed Morris

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I've been playing around replacing radio dial lights with LED's, so I decided to see how that would work with a Princess phone.  These phones came with a light circuit that illuminated the dial when the receiver was lifted. There was also a switch to allow the light to be used as a night light. In night light mode, the circuit passed through a 27 ohm resistor to reduce the light intensity.

Princess phones were powered by a 6-8 volt transformer through a special modular adapter to prevent current from passing to the phone lines. I made one from a surface mount phone jack. The 6 volt wall wart I used was connected to the black and yellow phone jack wires, and tip and ring from the line to red and green. It all works fine.  I connected the negative wall wart lead to the black terminal and the positive to the yellow.  LED's must have correct polarity or they won't illuminate.

I used warm white 3 volt LED's in series to replace the incandescent lamp.  However, the LED's draw so little current that the 6 volt wall wart I was using was putting out 9 volts, so instead of two LED's, I used three. For the night light circuit, the 27 ohm resistor didn't cut down the intensity of the LED's enough, so I added a 470 ohm resistor in series with it, which helped a lot.  The first photo below shows the 470 ohm resistor added in series with the 27 ohm resistor.

The lamps used in the Princess phones were a screw base GE #46 in early models, and a wedge base #259 in later models. All the Princess lamp holders I had were for the #259 bulb, so I made a wedge base and soldered the LED's to the wedge base, so it just slips into the lamp holder like the original bulb.  The lamp holder, original #259 lamp, and LED replacement are shown in the second photo.  The third photo shows the LED's in the lamp holder in night light mode (hook switch depressed).  One advantage of mounting the LED's to a base that can be removed from the lamp holder is that the LED's can be easily reversed in the holder if polarity is incorrect.

The last two images show the phone in night light mode and off-hook.  Neither, unfortunately, is well exposed, but obviously the LED's are much brighter when the receiver is off-hook, which is normal.  I think the LED's provide somewhat more even illumination than the incandescent bulbs.  Light spill from the base is still a problem, though.






« Last Edit: January 09, 2018, 08:40:16 PM by Ed Morris »
Ed

Offline jsowers

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #1 on: January 09, 2018, 08:57:26 PM »
Great instructions and great pictures too. It was very informative and clearly stated. One thing you didn't mention that needs to be said--the 6-volt wall wart you used was DC and not AC, wasn't it? So a regular WE Princess transformer can't be used in this situation. You need 6VDC.

If my electronics from over 30 years ago is correct in my mind, LEDs operate off DC voltage and not AC, which is also why they have polarity, since AC has no positive and negative. I may not have said that exactly the right way--it's been a while. But I did work with LEDs some in my distant past.
Jonathan

Offline Ed Morris

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #2 on: January 09, 2018, 09:12:59 PM »
Jonathan, thanks for raising that point. My wall wart is a DC 6 volt, 300 mA adapter I had laying around.  I was not aware that the original WE transformers were AC, in which case I believe you are correct, they would not work with LED's.



« Last Edit: January 11, 2018, 12:55:51 PM by Dan/Panther »
Ed

Offline compubit

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #3 on: January 09, 2018, 10:47:51 PM »
Any thoughts on a USB power supply? I have a number of USB to RJ45 cables (they came with UPS's at the office - we don't use the integrate functions, but I know they have wires conducive to a RJ-class jack). It should provide 5 volts, so 2 LEDs with a small resistor in series should be plenty of light.

Thanks for the inspiration!

Jim
A phone phanatic since I was less than 2 (thanks to Fisher Price); collector since a teenager; now able to afford to play!
Favorite Phone: Western Electric Trimline - it just feels right holding it up to my face!

Online twocvbloke

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #4 on: January 10, 2018, 01:57:37 AM »
A lot of LED lights you can get from cheap places run off USB power with appropriate resistors in place, so you could indeed use a USP power supply with your USB to RJ45 cables with a USB power supply... :)

As for LED functions, they will work on AC, but they will only light for one half of the sine wave, as they are a diode so will block power going the other way, one problem with this is LEDs don't like reverse power going through them (particularly when it's enough current to light it up), and can fail quite quickly if they're not running on a rectified power source, however, a "cheat" for this is to solder two LEDs back-to-back (so anode on one is connected to the cathode of the other), so one will light on one half of the sine, and the other will light on the other half, effectively making a half-wave rectifier, so you get an alternating light that shunts power through one LED while the other is protected from reverse current... :)

Of course, it is ultimately better to run LEDs on a DC source, either through a full-wave rectified AC to DC circuit (as found in a typical wallwart), or off battery power, as they're sensitive little things... :)

Offline Ed Morris

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #5 on: January 10, 2018, 10:14:48 AM »
My experience with solid state electronics is very limited.  However, you can get a bag of 100 LED's online very cheap, so experimenting with them is very inexpensive.

I prefer the warm white LED's for vintage applications.
Ed

Online twocvbloke

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #6 on: January 10, 2018, 12:27:55 PM »
Yeah, LEDs these days are dirt cheap, even the white ones, so plenty heap to make all kinds of projects that either work or don't, but that's all part of the fun of it... :)

Offline AL_as_needed

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #7 on: January 11, 2018, 08:27:25 AM »
I have tried converting one of these to LED before and had several issues, discussed here:

http://www.classicrotaryphones.com/forum/index.php?topic=17984.msg185589#msg185589

Long story short as I did not want to rework the phones internals that much, i ended up staying with the original incondecent bulb.
TWinbrook7

Offline ThePillenwerfer

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #8 on: January 11, 2018, 12:53:25 PM »
I'd try something like the attached.  That way anything 5V or above (within reason!) would work.
« Last Edit: January 11, 2018, 12:59:57 PM by ThePillenwerfer »

Offline Dan/Panther

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Re: Princess 702B Incandescent Bulb Changed to LED (Bulb) Illumination
« Reply #9 on: January 11, 2018, 12:56:27 PM »
Jonathan, thanks for raising that point. My wall wart is a DC 6 volt, 300 mA adapter I had laying around.  I was not aware that the original WE transformers were AC, in which case I believe you are correct, they would not work with LED's.




Wouldn't they work by adding a diode ? I converted the lights on my Seeburg Jukebox, with an LED, Resistor, and a diode.
D/P

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