Author Topic: Resistor in the light circuit on a 701b  (Read 833 times)

Offline Dave

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Resistor in the light circuit on a 701b
« on: January 04, 2012, 08:26:57 PM »
There is a resistor in the light circuit. I think it might be 27 ohms. Silver, black, purple, red I think. Anyway does this resistor dim the light in night light mode? And then it's bypassed in off hook mode?

Can less resistance be used there if one wanted a brighter night light mode?


Offline Phonesrfun

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Re: Resistor in the light circuit on a 701b
« Reply #1 on: January 04, 2012, 09:01:35 PM »
Not to be picky, but the color code on the resistor starts with the red (2)
then purple (7)
then black (Multiplier of 1)  Actually, times 100
then silver (Tolerance of 10%)

So, you are correct, it is a 27-ohm resistor, and yes, it dims the light when the receiver is on hook and the switch in the back is set to night light.  If you turn the switch on the back to off, then the light goes out entirely when on hook.

I suppose a lower resistor could be used to brighten the light when in the night-light mode, but the resistor would need to be able to handle the wattage.  I am not sure what the watt rating of the resistor is, but it is probably 5 watts.  You could just put a jumper wire across the resistor, and you would get no dimming at all in the night light mode.
-Bill G

Offline Dave

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Re: Resistor in the light circuit on a 701b
« Reply #2 on: January 04, 2012, 10:19:19 PM »
Thanks, Bill

Somewhere back in the 60s I learned to read resistors. I can't even calculate the amount of water that has gone under that bridge since then.

4 color bands X the amount of white in my hair / the water underneath that particular bridge = I was lucky.

If I wanted to double the amount of light coming from the bulb in night light mode (instead of full on with a jumper) would I halve the amount of resistance while keeping the (let's assume) 5 watts the same? In other words a 13 ohm, 5 watt resistor?

Offline Phonesrfun

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Re: Resistor in the light circuit on a 701b
« Reply #3 on: January 04, 2012, 10:41:39 PM »
I just don't know the answer to that one, but your assumptions make sense, but it would depend on the ratio of the resistance of the bulb to the resistor, and how the bulb reacts to various voltages.  The wattage rating of the tra nsformer could impact the whole thing too.

My advice is to try it and see.  You could experiment with it outside the phone, and make any changes you want to make when you have the final configuration.

On another note, however, I will say that a 701B non-ringer model was only made for a couple of years.  If the Princess is in any decent shape, I would be more inclined to leave it alone, just because when phones get modified and sometimes just plain hacked, it lowers its value down stream.  That phone without the internal ringer will usually bring a few more bucks than the later model.

-Bill G