Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ed Breya

Yes Reinhard, if the bad caps aren't among the "easy" (to access) ones, you have to dig deeper, and possibly disassemble some things to get to them. Then having an idea of which supply and which sub-assembly is involved definitely helps to narrow it down, especially for the bigger, more complicated plug-ins. Murphy's Law would dictate that the bad caps will be the most inaccessible, which happens sometimes, but on single-wide ones, especially verticals, there aren't all that many Ta caps to begin with, and not many that are too hard to access. Horizontals tend to have more layers of stuff, but often can be quick-tested with simple disassembly if needed, or accessing from the back (board layout info from the manual helps here too) or other accessible points on the same nodes.

The quick test can actually be even quicker than I described, in cases where you know that all the caps are simply bypassing to ground, and not between supplies. I think this is almost always the case with 7K plug-ins. Instead of probing caps with two leads for the ohmmeter, one lead can be grounded, and the other one used to make two measurements - one from each cap lead. One will show ground continuity, and the other should be a reasonably high resistance, unless the cap or the line it's on is dumped to ground. You don't care which is which, only that one lead is ground, and the other far from it.


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