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Ancient but still useful HP8753C display fault

Question asked by barleybill on Sep 27, 2015
Latest reply on Oct 2, 2015 by barleybill
I have a HP8753C which after many years of faithful service has developed a fault. I have been using different versions of the 8753 for over 25 years, so this is not finger trouble.
When the analyser is first turned on it starts up and the display is normal. I then select a thru calibration and all is normal. After performing the thru all is still OK. On selecting done response: the upper labels for the menu keys become corrupted but the lower ones remain OK. If I then save the set up in register 2 (I can’t read the label because it is corrupted but  I know the menus so well that I do not have to read them) and then recall it from register 2, the display is still corrupted, but on selecting MKR the menu labels sometimes recover and the display is normal. Recently even this little trick does not always work so the condition is deteriorating.
Does anybody have any idea of where to start to look for the cause? I do not have any circuit diagrams and suspect that they have never been published.