I'm working on a remodel. The homeowners replaced their old doorbell with a heath zenith LE75 musical chime. The old doorbell (a nutone) had an extender hardwired into it so they could hear the doorbell downstairs. Is there a way to hardwire the extender downstairs into the new Heath Zenith LE75 doorbell? If not what is the best solution?

https://www.homedepot.com/p/questions/Heath-Zenith-Wireless-Battery-Operated-Doorbell-Push-Button-Black-and-Satin-Nickel-SL-7303-02/100626792/2

I found this in the Questions

Q:Hello, I need a wireless doorbell button replacement for the Doorbell model: "desa sl-6180-rx-a". Please, help. Thanks byTim'May 31, 2020

1 Answer A: Hi Tim, Any of our Heath Zenith or Hampton Bay branded buttons will work with that model. You will need to put it in backwards compatible mode. Please contact our technical service team and we'd be happy to help.
byHeathCo'Jun 2, 2020

Also sold by Heath Zenith and where there Product Support is.

https://heath-zenith.com/products/wireless-push-button-sl-7797

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The doorbell and the car wireless are on the same frequency, post doorbell model number, I will see if the doorbel frequency can be changed.

See if there is sufficient voltage between the two wires using a voltmeter. Also check that there is voltage coming out of the doorbell transformer to the wires, and power from the regular AC line going into the transformer. Sometimes the transformer dies and it's time for a new doorbell, although in the old setups, the transformer was separate from the doorbell.

Heath/Zenith SL-6166-C Wireless Plug-In Door Chime Kit ...

www.amazon.com > ... > Door Chimes & Bells > Kits
Amazon.com, Inc.Heath Zenith SL-6198-B Wireless Doorbell Replacement Battery. Total price: ... Plug-in doorbell chime, doorbell button/transmitter with battery, hardware pack ...

Heath-Zenith (I beleive they bought DESA) makes a battery remote push button bell that says its compatible with all Heath Zenith chimes. I bought one at Home Depot and it worked for my SL-6195-RX. (an on line doorbell seller and the Home Depot assoc all said it wouldn't work.)
UPC is 16963961975.
Item also shows a code "11G43" on a sticker on the back of the package.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/steven_7f54ab9063554f66

The unit is receiving electricity so low-voltage transformer and button are working correctly.
Only suspect left is door chime itself.
Check that voltage of transformer matches voltage of chime.
Use cheap multimeter from home center and check voltage at chime, or check transformer.
Doorbell transformers are usually located in basement or garage or attic, and most are hardwired into octagon box.
There are no replacement parts for door chimes.
Buy new one, or contact seller for refund if this is new chime.

The chimes have jumper plugs to select the preset songs.

You CANNOT load your own like cell phones allow.

it sounds like you have a bad push button . if the wires where crossed you would most likely have this problem as soon as you turned on power . 

Yes, there is a way to change the security code. See FAQ here:
http://www.heath-zenith.com/servsup_disp_faq.cgi?cat=5

click on "how can I change the security code."

Change pin configuration on plugin bell and the remote.
The pin configuration should match