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Inactive
Inactive, Master Electrician
Category: Electrical
Satisfied Customers: 4772
Experience:  Electrical Contractor
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Im troubleshooting my c7 bulb christmas lights. its a parallel

Customer Question

I'm troubleshooting my c7 bulb christmas lights. it's a parallel circuit. none of the bulbs light but if I plug a test light in the female end of the strand, it lights up suggesting that there's normal current flowing through the strand. What should I check next?
Submitted: 6 years ago.
Category: Electrical
Expert:  Inactive replied 6 years ago.

The Electrical Doctor :

Are you wanting to know the best way to trouble shoot and fix these.....

JACUSTOMER-120icf3x- :

yes please

The Electrical Doctor :

What you do is go to your local home improvement store and purchase a non contact tester, or some call them stick testers - and even at Christmas time they refer to them as Christmas lights tester. It will cost about $10. Then what you do is plug in the lights, then start at the beginning of the string and place the tip next to each bulb. If it beeps its a good bulb. If it does not beep, that is your bad bulb. Replace it and you should be good to go. If not keep repeating this process until the lights come on.

JACUSTOMER-120icf3x- :

these aren't mini lights

JACUSTOMER-120icf3x- :

do those testers work with "old school" C7 bulbs with E12 bases?

JACUSTOMER-120icf3x- :

and even though the system is a parallel circuit and there's current from the male end to the female end (I plugged in a trouble light) a bad bulb could still be causing the problem?

The Electrical Doctor :

If they operate on 120v this tester will work just fine.

JACUSTOMER-120icf3x- :

okay and I'll find a bad bulb and that will fix it?

The Electrical Doctor :

That will help you find the bad bulb - then you can fix it.....

JACUSTOMER-120icf3x- :

okay I'll give it a shot. so it's not likely that the string itself needs to be replaced (recognizing that there are no guarantees)

The Electrical Doctor :

Im sure its just going to be one or two bulbs... .happens all the time.

JACUSTOMER-120icf3x- :

cool thanks very much!

Inactive and other Electrical Specialists are ready to help you
Expert:  Inactive replied 6 years ago.
To help you out, here is a quick one minute video to show you how to use the non contact tester to find the bad bulb.

Customer: replied 6 years ago.
here's what I did. since I didn't have the non-contact tester (and it's snowing like crazy today so I didn't want to go out) i systematically replaced all of the light bulbs on the string with light bulbs known to work on another string. we found one bad bulb, but the string still doesn't light :-(
Expert:  Inactive replied 6 years ago.
Did you use the tester to check each and every bulb?
Customer: replied 6 years ago.
no, i don't have one and wasn't able to go out and get one, so I replaced all of the bulbs with one's known to be good (they were working on another string)
Expert:  Inactive replied 6 years ago.
That tells me there may be another problem. If you look inside the plug - they sometimes will have a small fuse in them. If yours does - check it to see if its blown
Customer: replied 6 years ago.
checked the fuses first with an electrical meter. they're all good.
Expert:  Inactive replied 6 years ago.
Ok then at this point your only alternative is the non contact tester. Once you get it, let me know and I will guide you step by step on what to do and what to look for.
Customer: replied 6 years ago.
I don't mean to be contradictory, I'm just intellectually curious. What will the tester tell us that replacing the bulbs doesn't and why would the strand produce current at the female end, but not light any of the good bulbs?
Expert:  Inactive replied 6 years ago.
The string of lights operates becasue the lights are wired in series with eachother. You need to have a complete circuit in order for the entire string to light up. If one of the light bulbs is bad, then the entire string will be out. Say for instance one of the wires going into the light bulbs is loose or bad, then you will have the same effect - nothing will work.

Using the tester will allow you to pinpoint whether the bulb is bad or if you have a loose wire. Additionally, it will pinpoint the location of the bad bulb or connection. Does this make more sense?
Customer: replied 6 years ago.
yes, but I'm certain that these bulbs are wired in a parallel circuit. On the other strings, I can remove one bad bulb and the rest remain lit.
Expert:  Inactive replied 6 years ago.
If you pull that one light - do any other lights go out at all?
Customer: replied 6 years ago.
nope
Expert:  Inactive replied 6 years ago.
Ok then its possible that the one socket on the string is simply bad, or you could be using a bulb style that is not quite the same as what was originally in there.
Customer: replied 6 years ago.
all of the bulbs are original to this string. they're screw in c7 bulbs. if a socket is bad that could take out the whole string?
Expert:  Inactive replied 6 years ago.
Not in your case if the lights are in parallel with eachother. So if the socket was bad, it would only take out that one light. Do you have a digital volt meter handy?
Customer: replied 6 years ago.
i've got a volt meter with a sweep needle.
Expert:  Inactive replied 6 years ago.
Ok that may work. What you will want to do is take your meter and put one lead on the side of the socket, and the other in the middle bottom of the socket and see what if any voltage your getting from it.
Customer: replied 6 years ago.
i get no voltage in a socket on the bad string. on a good string I get voltage similar to a standard wall outlet
Expert:  Inactive replied 6 years ago.
Ok and do the light bulbs work on either side of the socket that is not showing any voltage?
Customer: replied 6 years ago.
no the whole string is out
Expert:  Inactive replied 6 years ago.
Oh ok sorry - Then use the volt meter and check the very first socket that is first in line after the plug.
Customer: replied 6 years ago.
okay i did that. no voltage
Expert:  Inactive replied 6 years ago.
Your only other option at this point is to change the plug on the end of this string. If your not getting any voltage at all, then its the only place the problem could be.
Customer: replied 6 years ago.
yeah, it's weird, cause I get full voltage at the other end of the string--at the female plug into which you might plug the next string
Expert:  Inactive replied 6 years ago.
Ok you have three or four wires coming off of the plug?
Customer: replied 6 years ago.
three
Expert:  Inactive replied 6 years ago.
Ok - if you wiggle the wires where they enter the plug - do any of the lights turn on?
Customer: replied 6 years ago.
no
Expert:  Inactive replied 6 years ago.
Ok heres the deal. with the being wired in parallel, and the entire line being out but you have power at the other end of the string tells me you have a broken connection in the male plug. If you want to use this string, you will need to replace this plug on this line.
Inactive and other Electrical Specialists are ready to help you
Customer: replied 6 years ago.
okay thanks!
Expert:  Inactive replied 6 years ago.
No problem.... if you need anything further please be sure to let me know. Good luck......

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