Re: Type 519 -- Why 125 ohms?

Tom Lee

Hi Bob,

The schematics I have for the 475 seem to show a differential termination of 184 ohms for the vertical deflection. But in any case, I don't think that it's good out to 1GHz. That said, I would be very interested in seeing the TDR results to see where the impedance starts to look screwy, if by some chance you and your colleagues happen to have taken a photo. TDR'ing CRT deflection structures is one of the many things I have on my never-to-be-completed to-do list.

Higher impedances are of course possible (by increasing the L/C ratio), but there is a complicated tradeoff among bandwidth, deflection sensitivity and physical size. Higher impedances are nice for sensitivity, but it's hard to get that in a small size without trading off bandwidth.

The following values are differential impedances for the vertical deflection lines of a few scopes:

475, 475A 184 ohms (from service manual)
7904, 7904A 385 ohms (from John Addis's article, Fast Vertical Amplifiers); the 7704 is (or should be) the same
7104 200 ohms (also from John)

-- Cheers,

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

On 5/13/2021 08:51, wrote:
The distributed deflection structures in CRT's used in instruments such as the 475 have impedances of over 300 ohms, as evidenced by the termination (or load) resistors in the vertical outputs. We made a measurement to confirm this using a TDR at the vintageTEK Museum.

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