Re: 494P problems - ROM and 1st LO

John Miles

Some years ago I purchased a 494P needing repair and now finally have some
time this weekend to investigate the faults. On power up it returns ROM 1 test
failure. Pushing a button then completes the boot up process and the
instrument appears to function in most modes, however decreasing the
frequency span below 5MHz produces a 1st LO tuning failure, which is then
replaced by Frequency control failure after a minute or so or when the
frequency span is further reduced.

Unfortunately I don't have the extender cards (or manual), and before investing
in these just wondered if anyone has any thoughts as to the best places to
investigate for these errors (I'm not sure if these are separate problem, or a
single problem which is associated with both the ROM and LO failures.)
I'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models had mask programmed ROMs, all of them soldered in place from what I've seen, and I don't think they have ever been archived.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, 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 (not
SMD!) on the first few boards I pulled for inspection.
All of those caps showed severe leakage and many had open leads due to
corrosion. If you own a 497P it might be worth inspecting
the first few plug in boards (LO driver, Preselector driver, etc.) to see if your
version has these caps (photo posted in library.)
Serial number of unit with bad caps is B020199 version 9.7--late production run.
To 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

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