If the jack is not some proprietary abomination, any decently capable techie should be able to replace it - if the device is "comeapartable" to get to anything inside. That you can buy a new earphone that plugs in sort of implies it's a standard kind of jack.

Since it's a discontinued model, you probably don't have a warranty that would be voided by making the attempt.

If there are screws holding the thing together, it can be opened. (Quite commonly the screws are "sunken" and covered by plugs to make it look like they're not there in devices of this sort.) If it's "welded shut" it's a little more difficult. If it can be built, it can be rebuilt (but the cost of exotically skilled labor may be "exotic").

Jensen was quite popular in the "high hifi" days of the fifties, but I haven't seen much from them here since about then. (But then I haven't been much into "media" stuff since then either.) It's nice to hear that they're still around.

John