Some ideas if your lights don't work.
First unplug lead-in wire from house. Plug transformer into wall.
Test for power at plug end of lead-in wire. Place test probe pins between the prongs on the lead-in plug. Rotate probe until pins touch both of the prongs on the plug. Light in test probe should light. If light doesn't light, turn on the switch on the lead-in.
If probe still doesn't light, check wall outlet where transformer is plugged in. (Use a lamp or radio that you know works). If outlet is ok, then either your transformer, lead-in or test probe is not working. Take them to your local miniature store and have them checked. If the test probe comes on very dim, you probably have a 3 volt or 6 volt transformer. Most dollhouses are wired for 12 volts. If you have 12 volt lights and a lower voltage transformer, you will need to replace the transformer.
If test probe lights, then plug it into the juction splice on your house.
transformer hum loudly and no lights? Very dim lights?
If no lights come on , and the transformer hums loudly and/or heats up to where it is uncomfortable to touch, you have a short circuit. Unplug transformer.
If they come on dimly you may have a transformer that is too small. Count the number of light bulbs you have connected (that's light bulb, not light fixtures - a 5 light chandelier counts as 5 - a flourette or spot light count as 3). Look at the front of your transformer, and find the OUTPUT watts (not input). A 5 watt transformer will handle 8-10 bulbs. A 10 watt will handle 17 bulbs. 20 watt, 34 bulbs and 40 watt will take 65 bulbs. If you have too many bulbs, you will need to get a larger transformer.
Additional indications of a transformer that is too small: Transformer overheats, hums loudly, lights are on for 10 to 15 minutes (until transformer gets too hot) then off for 10 to 30 minutes (while transformer cools down) then on again, etc.
If none of the lights work, you may have an open circuit. Test the tapewire right next to the junction splice. If it works, then you have an open circuit (see section below). If it doesn't work then place the pins of the test probe between the prongs of the lead-in plug (as I showed in the beginning of this), making sure the test probe lights. Then (this is the tricky part) push the test probe pins into the tape wire just as if you were going to test the wire. (Some of your dollhouse house may come on when you try this test - that means the junction splice is bad) If the probe light goes out, then you have a short. It the probe light stays on, then you have an open circuit caused by a bad junction splice. You can try relocating the junction splice into new hole, or it may be necessary to replace it.
If you don't have too many lights, then you most likely have a short. Typically this is caused by the last thing that you did. Like make a new connection, add and outlet, plug in a lamp, connect a light. Lets look at these one at a time.
NOTE: When working on a house that you suspect has a short, turn off the power whenever you are not performing a test. Before each test, unplug the lead-in and test it as above, to make sure the transformer hasn't overheated and shut off.
Plugged in a lamp or other electrical device:
Sometimes, luckily not often, a lamp will have an internal short that happened at the factory. So, if you suspect you have a short, the first thing to do is to unplug all the lamps, etc. that you have plugged into your dollhouse. If this does fix the problem, then plug in the lamps, one at a time, until it quits working. The last one you did is the one that is causing the problem. You will probably need to replace this device.
Connect a light:
If this doesn't correct the problem, remove all the ceiling/wall fixtures that you installed using canopies/adapters. If this fixes the problem, re-install them until you find the bad one, then replace it. If this doesn't correct the short, you will need to remove the last light that you connected. If the last light you connected used an adaptor or canopy, the problem may be as shown below under "add an outlet".
Did you insert the brads/eyelets incorrectly? On the right is a good connection. Notice that the eyelet/brad goes through both copper wires, and the other one goes through both blue wires.
On the left are some bad connections. Always fasten copper to copper, and blue to blue. The correct connection will always be diagonal, not vertical or horizontal.
If you made a bad connection, remove the brad/eyelet by shoving a knive blade under the brad/eyelet head and prying it out. It is also a good idea at this point to pull the top piece of tape slightly away from the bottom piece, and put a sliver, or small piece of scotch tape between them for better insulation.
Now insert the brads/eyelets correctly.
Add an Outlet:
Notice on the right how you can accidently place the outlet so that it makes a connection between the copper and blue wires. Again the solution is to remove the outlet and position it slightly away from the connection. I show using large eyelets for the outlet, but the same effect can happen with other types of outlets and even with the adapters or canopies for ceiling lights. On the slot and tab type houses (1/8" luan) the large eyelets are longer than the thickness of the wall. In one of these houses, I installed an outlet only to discover that the eyelet went through the copper strip on one side, and touched the blue strip on the other side.
If none of these things corrected the problem, you will need to isolate sections of your wiring to find the problem. Remove the brads/eyelets from a connection near the center of your wiring run. For example, if you have a riser section of wire running up the staircase cutout to connect the floors, then remove the brads/eyelets at the first floor (ground floor to you Brits) riser intersection. Now test and see if the first floor works. If it doesn't work, then the problem is on the first floor. I am assuming that the junction splice connects to the first/ground floor. If it connects to a different floor, allowances in the floor designations will have to be made.
If it works, then the problem is on the higher floors. In that case, reconnect the first floor to riser and disconnect the second floor (first floor in Europe) from the riser. If it works, the problem is on the second (first) floor. If it doesn't work, continue with each of the higher floors until the problem is found.
Once you have isolated which floor is the problem; remove the eyelets from each connection on that floor, testing after each removal, until you have it working. The last run you removed is causing the problem. Follow that wire and see what the trouble is. If all else fails, replace that wire.
If the lead-in plug tests OK but some of the lights/lamps don't work, you either have an open circuit or a burned out light bulb. Test the bulb or replace it with a known good bulb.
Bulbs burning out rapidly: The light bulbs in your dollhouse should last for months to years. The bi-pin bulbs from Cir-Kit should last for many years. The screw-in bulbs will probably have a shorter life time. Sometimes the bulbs have a problem in manufacturing. In this case the bulb will burn out within the first 3 or 4 times of turning it on. If you continually burn out bulbs rapidly, it is probably because you have too high a voltage for the bulbs. Make sure your transformer is 12Volt.
When you first turn on a transformer, it will have a voltage spike. The smaller the load on the transformer, the larger the spike. Therefore, if you run a 40watt transformer with only 5 or 6 bulbs on it, you will probably shorten the life of the bulbs significantly. Either use a properly sized transformer, or add a voltage regulator. Cir-Kit offers a device which connects between the transformer and the lead-in wire which prevents the voltage to the dollhouse from exceeding 12 volts. I recommend you use this if you transformer is too large (like when you are starting wiring but plan to add lots of lights in the future). You can leave this regulator on, even when you add lots of lights.
To find when the open circuit is caused, start at the junction splice and test the tapewire every couple of inches until you find the spot where it quits working. If it is at a connection, then push a straight pin through the tapewire where the copper crosses the copper. If this fixes it, then you have found the bad connection - tighten the brad/eyelet or if that doesn't work, install an additional brad/eyelet along side the first one. If it still doesn't work, try the straight pin where the blue crosses the blue. This should fix the problem. Redo the connection as above.
One of the most common causes of an open circuit is caused by a knife when trimming ceiling paper or wallpaper in the dollhouse. The correction to this problem is to splice in a short section. Note that I used a slightly different color for the splice so it would show up better in the drawing.