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8590A sweep display issue

Question asked by bobm0rhe on Mar 4, 2016
Latest reply on Apr 3, 2016 by PA0ALV
I am in the fortunate position of having two 8590A's, one fine, the other with a display issue.
The RF sections are fine, when connecting the analogue outputs to a 'scope the waveforms are as expected.
All digital aspects of the display are perfect,but the sweep from the ADC is broken up (see picture).
I strongly suspected the ADC chip had a stuck bit, so after some research chose a Analogue Devices AD774BBD part which seems to work OK, but the same display fault occurs.
I have compared the 12bit bus information between the two analysers set to the same sweep etc, and that made me think bit 9 was not correct, it did switch but only in blocks. However I have concluded that is a red herring.
The instrument occasionally lights LED 4 on A16 which is marked 88230. The handbook implies a ROM fault when this light comes on.
At present I am contemplating trying the ROMs in my working instrument in the defective one, I cannot see that would cause a problem. The instruments are a year apart according to the label on the ROM, one is 88, the faulty instrument 89.
Any thoughts gratefully received!
One waveform is 299MHz centre, 10MHz sweep. Other is from reset.