7002 does nothing

I recently got a bunch of 7002’s and 7002T’s. Problem is, only half of them work properly.

There’s one with a bad strobe, and another with a nearly dead horn and a bad strobe. I don’t think there’s anything I can do about them.

But two of them (a 7002 and a 7002T) do absolutely nothing when powered up. I’m guessing that means there’s an open circuit somewhere, but I still have no idea what to do. Any advice would be greatly appreciated.

My best guess is that the alarm is so well used that the contacts that make the horn work are extremely carbonated. How a mechanical horn works is, There is a set of contacts, a rod, and an electromagnet. As power to the horn is applied, the electromagnet energizes, pulling a rod backwards into the contacts, making them lose connection, an deenergizing the magnet, causing the rod to hit the diaphragm, making a noise and making the contacts make contact again. Every time this happens, the contacts create a spark, and that spark makes carbon (or whatever it is) collect on the contacts. Over time, the carbon prevents electricity from flowing, and thus the horn stops working. There is a pretty easy solution to this though, as model railways suffer from the exact same issue. You can go to a model railway or hobby shop and get something called a Bright Boy. Its like an eraser but it is a little more abrasive, and designed to remove carbonation from model train tracks, but it works extremely well on horn contacts, so if you were to try anything, that would be your best bet. If you need any more help, please message me.

Test across the wiring terminals with an ohmmeter. A good 7002 should read somewhere between 1.5-2.5 megaohms. A reading of close to 0 or infinite ohms indicates that you have a dead short or an open circuit, respectively.

Also be on the look out for very high resistance readings. This likely indicates corrosion on the terminals or contacts, as suggested by HappyJigg.

Let us know the resistance values if you are able to read them.

I’ll try both. Thank you so much!

Got two of them working! The problem was that the contacts were bent. I just needed to get in there with pliers.