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

Ed Breya

For 7K plug-ins, you can do it on the bench, just the plug-in alone, opened up for access. Probe around with an ohmmeter and check across each Ta cap for short or very low R, and mark any that show this. Also check for signs of overheating of series Rs and Ls in the power supply buses. Don't worry about which supplies and caps are which, or that you can't necessarily see or access every Ta in there. Go for the easy stuff first. After you've marked any bad ones, lift one lead - un-solder or clip close to the board (leaving some lead to tack it back) - and check the part itself, and at the board pads. Often, more than one part spot will show bad because there may be several caps on the same bus, and any one bad will cause all of the spots to show the same. Then you just figure out which ones are actually shorted and remove. Don't replace any until the plug-in can fire up and operate without crashing the mainframe. If you can't find any easy ones to blame, you'll have to dig deeper and look for more. In my experience, shorted Ta caps in plug-ins are almost always on the +/- 15 V supplies - that's where they're used the most. Also, don't be surprised if another cap shorts after fixing. Sometimes they chain react, where eliminating the shorted one allows the supplies to return, and the next weakest one pops, and so on. I remember once in an EIP counter, I had about half a dozen or more Ta caps on the +5 V supply light up and burn, one after another - after clipping the burnt one, the next would go within a minute or so.


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