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Hi all,

Much has been written here about the epoxy potted HV transformer thermal runaway problem in later 500 series scopes including the 545B, 547, 549, 556 etc. I recently encountered a similar problem in a 535A, a scope with a beeswax potted transformer that is not known for thermal runaway.The symptoms were almost identical. The scope would start normally and then after a minute or two the display would dim and bloom. A voltmeter on the screen of the 6AU5 oscillator tube showed a steady increase from 75 to 150 as the feedback loop tried to keep the output constant and then oscillation would cease. The speed of the increase suggested thermal effects in a small component. Everything looked normal and the tubes tested OK by substitution. The fault was no different with the CRT unplugged. It turned out the culprit was the 0.001 mfd resonating capacitor across the HV transformer primary. This was a black moulded Sprague cap with red printing, not an oil filled or bumble bee type. After it was removed there was a bulge visible on the underneath that was not easy to see when it was in situ. Replacing it with a modern 630V polyester cap cured the problem. I wasn't game to put a finger on it while it was going to see how hot it got!

Crappy old capacitors - the gift that keeps on giving!

Morris

Had same fault many years ago,replaced the resonator cap with, from memory, a 0.001/1000V
Styroseal (white) cap...should outlast the scope.
John