I recently starting having a problem with my ESG when trying to modulate a digital signal. Rather than a correctly modulated signal, all I can get from the output is gibberish. It passes all of the self tests.
With the rest of the unit working OK I went straight for the UND option card and measured the I/Q signals coming out of it. Result: Gibberish. During testing I noticed that the output is correct for about a second, then instantly goes bad.
Rather than going on a long and complicated adventure trying to work out exactly what might be wrong with it (I could not find any service information for it) instead I just replaced the eight SRAM chips attached to the DSP, because from looking at the symptoms this would appear to be a likely cause.
Boom. Fixed. With the chips only a few bucks a piece it wasn’t worth my time to try and work out exactly which one was bad.
There’s no need to use exactly the same SRAM chips as it originally came with. I just bought the cheapest parts that were at least the speed grade of what was in there, which were CY7C109D-10VXI at the time I did the repair.