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Ed,
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My first thoughts after reading the descriptions are switch contact issues in the selector switches and or something in the routing logic that is on the back plane of the mainframe. If I remember correctly there are 3 IC's on the back plane that handle signal selection and routing. It sounds like one of those might not be receiving the correct signals or not outputting correctly for signal switching. Zen -----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of ed breya via groups.io Sent: Tuesday, December 31, 2024 2:42 PM To: TekScopes@groups.io Subject: [TekScopes] 7904 strange problems I have this last 7904 up and running as explained previously. It showed several problems including readout loss, severe vertical compression, A-horizontal failure, and supply crashing when control lighting is turned off. I swapped in another readout board, which got that working about right. I'll have to slog through the rest. The vertical problem is very strange, and depends on the plug-in type, apparently. When I first fired it up, I put in a 7A26, 7A18, and 7B53 - the very same ones I had been using in the 7613 for diagnosing the PS for this unit. The compression was evident immediately, and I blamed the mainframe, but after some swapping around of plugs and holes and other 7A18s and 7A26s, I started to blame the 7A18, thinking it may have just crapped out coincidentally (this kind of thing has happened before). After enough combo trials, I realized the 7A18s were OK and confirmed their proper operation in the 7613. The 7A26s worked in both 7904 verticals, while both of the 7A18s had the same symptom with the line stuck near the bottom of the screen, barely affected by input signal or position controls. After more checkout, it seems the vertical display and triggering are fine in both slots using the 7A26s, and only the B-horizontal works properly. When "A" is selected, it crashes something and the display goes crazy, looking collapsed kind of like in beam-find mode, and missing parts. At this point I know the supplies seem OK, and the display looks good, with full control of intensity and focus. Has anyone seen things like these in a 7904? As I mentioned before, my notes from 2012 and also 2019 indicate that this was kind of a junker, with multiple problems existing or swapped into it, including the bad PS. Now that that's fixed, I get to revisit all of it together. The notes include stuff about the readout, the vertical and horizontal interface boards, suspecting one side of the differential signals missing, and that some plug-ins like 7D01 worked OK, while others didn't. Unfortunately, I didn't make very precise notes, so all I can tell is that these were some of the issues involved, but there's nothing about resolution of any problems. The strangest part I think is why certain plug-in types would work just fine and others not. Ed |
