These modules (8557-8559) tend to have issues related the mechanical switches.? The finger contacts like to fall off.? They can be replaced with fingerstock.? Specifically, fingerstock from Tech-Etch part # 75RE or from Laird Tech part # 97-134.? The problem is the manufacturers of these fingerstocks only sell in large quantities.? But perhaps you can reach out to them for sample.? Or get lucky like I did and find some used on eBay.? These part #s are almost a perfect match to the original HP contacts.? But you can also make your own some other way.??
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Unless I'm misunderstanding your post, and you're saying the horizontal line appears without plug-in installed.? Then of course the mainframe is where to start.? Troubleshoot the issue without the plug-in installed.? The HP 853A service manual is very thorough and can be found online for free. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.keysight.com/us/en/assets/9018-03045/user-manuals/9018-03045.pdf?success=true
Frank Mashockie