Is it a Concertone unit and look like this?
Does it have a scan mode or auto-program mode and do they latch onto any stations but not deliver sound? Or does it just scan through and never stop on a station? Does static play at all when using the radio function?
I can't help you much as I have a standard automotive style stereo in mine. I can just point you to the manual below. Unfortunately, the ZX25 is a discontinued model at Concertone but you could contact them for additional help.
PDF of manualI would agree that after you go through whatever other troubleshootings steps there are, you may just have to replace it. Unfortunately, if it is the model I found, you'll have to get the large front footprint units or use a filler panel with a standard automotive DIN size.
It looks like the current model in that size, the ZX85, is quite a bargain at $399. Wow!