After putting up icicle lights at the parents, quite a few of the strings had half or full sections with no light. It is difficult to trace the wiring to see how they are wired. Mini lights are wired in series so that if one bulb goes out, the whole string will be out.
Research indicated that the bulbs have a shunt so if the filament burns out, the bulb should short circuit so the string stays lit. The higher voltage caused by the burnt out bulb should cause the shunt to fuse together. Sometimes, the shunt fails causing the string to stay dark. There is a tool called the Lightkeeper Pro that sends high voltage into your string to force bulbs where the shunts have failed to fuse together.
In my 100 light string, there are two sets of series 50 light strings which are wired in parallel. So even if both strings have a disconnect, power will pass to the next string. To determine how many sections your string has, you need to pull a bulb from a good string. If half the lights go out, you have two sections. A third of the lights going out will indicate you have three sections.
It was difficult to find replacement bulbs at both drug stores and hardware stores. I found some 2.5V mini replacements at an independent dollar store though it looks like Canadian Tire and Rona is also a good source for bulbs. The bulbs don’t include the plastic base so don’t throw those out.
To determine the bulb voltage, you need to count your number of bulbs in each SERIES section. 120V divided by that number is your bulb voltage. So 2.5V is good for 50 bulbs (and multiples wired in parallel), 3.5V for 35 (and multiples), and 6V for 20 bulbs.
I ended up going through the tedious process of testing each suspect bad bulb (by placing each in a lit string) and randomly ended up getting all the strings lit up. The problem was likely a loose connection somewhere and not a bad bulb.
The dollar store and Canadian Tire 2.5V replacement bulbs I bought popped right away so they were bad or mislabelled. 2.5V replacement bulbs I bought at Rona worked fine. It is recommended to replace burnt out bulbs since more current will be flowing into your string and could burn out the rest of your bulbs quicker.
I have a knockoff Fluke contactless AC tester I bought off Ebay and this could be used to check for a broken connection. If your tester is too sensitive, it probably won’t be useful. Here is a guide from Fluke on holiday light testing.