Re: Troubleshooting an old Tek 475

Tom Lee

Minor nit (sorry to be picky): It's double-h, double-f in Kirchhoff.

(can't help it; it's an occupational hazard)

-- Cheers,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/7/2020 01:57, Jeff Dutky wrote:
Okay, I'm not nearly qualified to understand what C1059 was doing in the sweep generation circuit, I mean I can trace out what it's connected to, but my EE fluency ends with "which way to Kirchoff Cafe" and "My hovercraft is full of eels." The failed part still appears to act like a capacitor (in the 200nF range), at least from what my multimeter can tell (I don't have an ECR meter or a working comparator bridge to check it any real way). Whatever the case, it was clearly the culprit in disabling the horizontal sweep. I suppose it also may have been distorting the timebase, but even before this failure the timebase wasn't off by more than a few percent (if that, as I was judging the timebase by comparison to the built-in calibrator signal, which appears to be off by 5% or more), and if C1059 played any significant part in the timebase itself then I would expect a change of an order of magnitude in C1059's value to have more than a 1% effect on the timebase.

As it is, I checked the value of the replacement capacitor to make sure that it was within 10% of the specified value. I even ordered several replacement parts so I could select the one that was closest to 3.6uF, and both the candidates were right about 3.5uF, so I feel pretty good about the transplant not screwing up the host circuit.

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