Some years ago I purchased a 494P needing repair and now finally have someI'd want to straighten out the ROM test failure before even thinking about any further troubleshooting. Trouble is, the original 494 models had mask programmed ROMs, all of them soldered in place from what I've seen, and I don't think they have ever been archived.
I would suggest checking ko4bb.com, but the ROM images no longer seem to be stored alongside the manual files. All I see for the 494 is a note (ROM_images.txt) that says "Firmware version info and ROM images are in 3_ROM_Images." There is no "3_ROM_Images" directory on the site, and the "Tek roms" folder under "01_Rom_images_and_drivers" is empty.
Didier, any input on how to navigate this apparent change in the directory structure?
Meantime, it's never a bad idea to have a look at the usual suspects, supply rail voltages and capacitors. Service volume 1 contains some troubleshooting guidance on the first LO failure, so it's a logical place to continue looking once the firmware has been ruled out.
Re: the caps ...
During trouble shooting, I've found several bad 50 uF at 25 VDC radial lead (notTo be precise, those aren't radial-lead capacitors, they are axial-lead parts (think of an 'axle' with wheels mounted on the ends of the component leads.) An ESR meter is helpful for checking them, but often bad ones can simply be spotted visually as in your case.
Radial tantalums are the epoxy-encapsulated spheroidal ones that are famous for failing shorted, especially in circuits with a lot of inrush current and/or insufficient voltage derating. Both the radial and axial parts are common trouble sources, but they are constructed very differently as Chuck says and they fail in very different ways. I've rarely seen shorted axial caps or open radial ones.
-- john, KE5FX