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531/535/541/545 are the first generation of 500-series scope. 533 is second-gen, 533A is third. Black Beauty and Good-All from this era use plastic film insulation just like modern caps. In general they are no more likely to be bad than a modern cap.
That said, some also contain a layer of paper (Sprague trademark ¡°DiFilm¡±), which by now has caused the cap¡¯s Dissipation Factor to increase. In most applications, it doesn¡¯t matter. The 1nF cap that resonates the transformer primary is sensitive to DF however, and a lossy one can prevent oscillation. Something to watch for but not in the ¡°front row¡± of suspects. If you have a bad cap in the HV section, it¡¯s most likely to be one of the 10kV-20kV ¡°doorknobs¡±. Now and then they short-circuit. The ceramic disks Tek used are very high quality, better in some ways than modern caps unless you really shell out the dollars. My 500-series mainframes are still using their original electrolytic caps. If it was my scope, I¡¯d just power the thing up. If you test the electrolytics in-circuit, remove all the tubes first. (Note where each one goes, some are aged/screened/matched.) Otherwise you will ruin some by puncturing the heater-cathode insulation. That¡¯s why Tek uses a time-delay relay. It¡¯s good you¡¯re cleaning the ceramic strips; there are a couple of ¡°sore spots¡± where high potential between adjacent terminals causes silver migration which will eventually arc over and (at the least) ruin the strip. The SNAP! made me jump a foot. Rather than replace the strip (lower left corner of the B sweep chassis, that¡¯s not present in your model), I started an experiment by grinding off the flashed-on silver and painting the spot with corona dope. Not to suppress corona but to forestall further migration. I don¡¯t know if it helps, ask in ten years. Be sure you¡¯re not leaving any conductive residue behind, it will just hurry the strip to its doom sooner. HTH, Dave Wise 535, 535A, 545, 547 From: [email protected] [mailto:[email protected]] On Behalf Of Mark Vincent via groups.io Sent: Monday, July 19, 2021 7:32 PM To: [email protected] Subject: Re: [TekScopes] Working on the HV Section of a 533A Jeff, Brenda is right about the corona dope. Leave those strips in. Clean them to remove dust. I have not seen any of the black beauties any good. These are a wax condenser in a plastic case. I replace them on sight. The Good-all ones I also replace on sight. Replacements I use are Sprague Orange Drops or other high quality ones at 630V, e.g. Panasonic. Radial types work fine to replace axial. Mark |
