Hi Zen,
I've not encountered this particular fault in a 485, but I have seen it in other scopes, where it was caused by a problem with the Schmitt that ultimately feeds the actual trigger circuit. The Schmitt's job is to provide a fast-edged signal, usually through ac-coupling, to the rest of the trigger circuitry. If the edges are lazy, the trigger circuits misbehave.
I just looked at the schematics briefly, and the 485 couples into the trigger TDs through C761 (a small 2.5pF, which won't couple low frequencies at all well). Probe the left side of that cap and see if you are getting good Schmitt-like outputs. I'm betting that you won't.
Good hunting!
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 3/26/2022 12:24, Zentronics42@... wrote:
I am working on doing a restoration on a Tektronix 485 for the lab and I have been arguing with it for a little while. I am curious if anyone has run in to a scope that does not like to trigger at low frequencies? I am currently working on the 50 Ohm calibration and it seems that the scope in 50 Ohm mode REALLY does not like to trigger under 1.2Mhz. So it is struggling with 50 Khz cal signals. Vertical amplitude is good trigger is flakey. I have heard that some of these the TD¡¯s go kind of deaf at the high speed side of things but not the low side. Any ideas what might be causing this. I am continuing to investigate further.
Zen
