开云体育

I was just going to use a 7B80 in a 7904 setup, but found that the horizontal sweep width is short by about 10-15 percent. At first I thought it was a mainframe problem, but swapping another timebase proved both slots are OK.

The 7B80 seems to work otherwise, except that the external trigger attenuator switch detent doesn't hold the in position - it's always out, so divide 10. This shouldn't affect the sweep though, and it's not needed anyway. This is a standard (non-option 2 X-Y) version, so I assume that shouldn't be an issue either.

I've cranked the front panel "SWP CAL" pot to the max, and worked over all the controls lots of times, and rocked the ICs and transistors in their sockets, hoping to find something a little loose, but that all had zero effect. I looked at the manual, and there doesn't seem to be any other H adjustment inside, or anything else obvious to blame, so there must be something that's slightly off in the amplifier or the sweep gate comparator - I haven't made any measurements yet to narrow it down between those two.

I hate it when something works but is just a little bit off - it could take a lot of diagnosis to find something slightly drifted out - I'd rather have it be a gross error that can be narrowed down quickly. I'm also tempted to just leave it as-is for now, since it's not critical, but it would be nice if I can find a quick fix.

Has anyone seen this behavior on a 7B80, or other TB plugs, that may be a common problem? Am I missing anything?

Ed

I have not seen this happen yet with a 7000 timebase but I would start by
checking the horizontal calibration which is trivial. This will reveal if the
sweep is short because the horizontal gain is low or because the sweep is being
reset early.

You could put a Vert amp in a horizontal slot & see if the deflection is the same as the Vert axis.If not, the Horiz amp may need a tweak.
?HankC, Boston
WA1HOS

So now it seems to be just fine. Last night I fired it up and it still had the problem. Then I took it out and poked around in the circuitry looking for anything suspicious, jiggling stuff around some more, and pondering how to measure some of the resistors.

Then I put it back in to make some measurements, and the trace was nearly the right width. I also learned that the "SWP CAL" pot has nothing to do with setting the width, only the actual sweep ramp velocity. I initially assumed it was a gain control, but no. The specs just say that the width should be at least 10.2 divisions, so that must be the minimum that it gets to considering tolerances and drift - there's no 1X gain adjustment or comparator setting. So the trace was just about 10 divs wide, and stayed good for quite some time. I could adjust the sweep cal and set it right on to match the calibrator signal, so close enough to call it done.

This morning I fired it up again, and the width was about 10.5 divs when cold, then warmed up to where it is now 10.2 divs and holding. So, it's a mystery exactly what caused it, but I think the combination of poking around in there, working the controls more, and running time has restored it to normal. So, I'll have to call this one a done deal - I'm sure it will be just fine until I need to use it.

Ed