Test Equipment Anonymous

tggzzz wrote: ↑Tue Oct 14, 2025 8:50 pm Try the horizontal*10 mode, and see if part of the scan is off the side.
Try horizontal position control to see if you can move part of the trace off the side.
If you can, the deflection voltages are correct and the the issue is is merely the adjusting the sweep width as per the manual.

It's the deflection amplifier, perhaps an output transistor. Assuming that was a genuine square wave, each half of the cycle should be the same width, but it isn't. Nonlinear sweep. Fuggered X amplifier.

Blimus! I've just looked up tggzzz's reference and seen all sorts of ludicrous characteristic impedances. And I thought there were just a few... "For there is nothing either good or bad, but thinking makes it so." Yup, Shakespeare knew about transmission lines too.

Next question: why are there 50, 51, 52 ohm cable specs?

Bonus points: why 61.8ohms for the cable in my lab? I was told the reason it couldn't be anything else, but I've never had the information to verify it.

EC8010 wrote: ↑Tue Oct 14, 2025 9:59 pm

tggzzz wrote: ↑Tue Oct 14, 2025 8:50 pm Try the horizontal*10 mode, and see if part of the scan is off the side.
Try horizontal position control to see if you can move part of the trace off the side.
If you can, the deflection voltages are correct and the the issue is is merely the adjusting the sweep width as per the manual.

It's the deflection amplifier, perhaps an output transistor. Assuming that was a genuine square wave, each half of the cycle should be the same width, but it isn't. Nonlinear sweep. Fuggered X amplifier.

Both high and low look like 2.8 divisions wide.

I don't think this 485 has been working recently, so it could still be simple "x-magnification" tweak correctable as per manual. Or not, of course.

I did once have a faulty Y amplifier that I managed to fix. An internal resistor was open circuit, but I could attach an external SMD resistor in parallel, It worked, but the risetime was 1.25ns rather than 1ns.

I've just held a rule up to the screen; so much for my eyesight! You are correct.

EC8010 wrote: ↑Tue Oct 14, 2025 8:16 pm

tggzzz wrote: ↑Wed Jul 09, 2025 7:43 am I've been known to take one to hamfests, to see whether cables were 50 or 52 ohms

52 ohm? I used 75 for baseband video at the Beeb. 50 ohm is de rigeur for most RF stuff. Nuclear energy spectrometry is 91 ohm. But 52 ohm?

Just different compromises----another was 51.5 ohm!

When we dragged the 1959 Marconi Transmitters at ABW2 moaning & screaming into the brave new world of 1970s PAL colour, one of the changes was to match the 51.5 ohm Sound & Vision Tx into the new Filterplexers & combining units which were made by AWA & were 50 ohm.

To this end, we had quarter wavelength sections of, from memory, 50.7448ohms.----a rather "nice little earner" for AWA, as all they needed to achieve this impedance was the services of a lathe operator.

They were a compromise, of course, as the old analog Ch2 in Oz was 7 MHz wide, with the Vision carrier at
64.25 MHz & the Sound carrier at 69.75MHz.

A compromise implies splitting the difference between two necessities or requirements. So what were those?

The 61.8ohms apparently wasn't a compromise