sa44b passes self test, but has bad (no) output data

We have several SA44Bs. One now has no output. Using Spike in swept mode, the display is a solid color. In zero span mode, there is no line, data shows not a number. Using LabVIEW API, the device can open and return API ver, serial number, etc., but when retrieving zero span IQ data, the data is NaN. No error is returned. I hooked a cable from the Self test output to the input, and ran the Self Test in Spike, and it passed.