Craftsman 1/2hp garage door opener not recognizing remote
My house came with 2 139.53970SRT model Craftsman garage door openers (roughly 10 years old) with no remotes. My wife and I replaced one of them which was broken with a 139.53753 model which included remotes, and I was hoping to be able to program one of the remotes to use the remaining older model unit. However following the programming procedure the only thing that happens when I hold the learn button on the old unit is the light will go off...never flashes like it should, nor does it appear to respond when pressing the remotes. I tried all 3 remotes that came with the newer model and none of them provoke a response with the programming procedure, though they work fine with the new unit.
Any tips on what I'm missing here? I do have the other broken unit still if it would be necessary to pull a board out of it.