I'm doing battle with a 7T11 with a few problems. The one that is puzzling me is that in Real Time mode the trace is a full 10 divisions wide, but in Equivalent Time mode (Time Pos Rng < 50uS), the trace is chopped short at about 5 divisions. This isn't a compressed trace but rather it is truncated as the signal is correctly displayed.

I've checked the slow ramp sweep and it has the correct amplitude.

I've been re-reading the Theory of Operation section to try to get my head round this one, but so far my little grey cells are being defeated!

Any help will gratefully received.

Dave

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Hello David,

I repaired a set of 7S11/7T11 some time ago. What I do recall is that it was unusually difficult to understand how these units work.

You can easily lose track if the controls are set in some wrong state. If you have a second 7T11 you can adjust the controls for both

Into the same positions and then compare.

?

G?ran

?

Hi Goran,

Yes they are quite the devil to calibrate, however in this case the front panel position control is maxed to the right and the sweep cal is almost all the way clockwise, and the slewing ramp adjustment (R310) is fully clockwise (into the end stop) as well.

Clearly something is amiss here. The annoying thing is this morning it behaved almost right, and then about half way through the afternoon, it failed back to the short sweep (8 divisions, getting shorter as the Time/Div control is roatated clockwise).

In Real Time mode the sweep is full length.

Regards,
David Partridge

On Fri, 25 Jan 2013 18:16:19 -0000, "David C. Partridge"
<david.partridge@...> wrote:

Hi Goran,

Yes they are quite the devil to calibrate, however in this case the front panel position control is maxed to the right and the sweep cal is almost all the way clockwise, and the slewing ramp adjustment (R310) is fully clockwise (into the end stop) as well.

You are not kidding about the calibration but from what I have read on
the list in the past, they are easier to deal with than the older
sampling sweep plug-ins. I am only up to having recalibrated the
trigger circuits on my 7T11A which had just enough DC offset to cause
problems. I have gotten better results when triggering off of pulse
outputs that do not cross zero by adding a DC block inline.

Although it is not strictly necessary, I want to build another SMB to
SMB cable so I can diagnose my 7T11A or 7S11 outside of the mainframe
with both the strobe and internal trigger connected. I only just
recently picked up a working S-1 which should make fixing my broken
S-1 and 7S11/7T11 calibration easier.

Clearly something is amiss here. The annoying thing is this morning it behaved almost right, and then about half way through the afternoon, it failed back to the short sweep (8 divisions, getting shorter as the Time/Div control is roatated clockwise).

I am going to assume that when the sweep is short, the time/div is
still calibrated. If the time/div is proportionally low, then I would
look for a problem in the horizontal output circuit path.

Do you get different results when SCAN is set to MANual or EXTternal
INPUT instead of REPetitive? You might want to try using a function
or ramp generator as a signal source for the external scan input
signal.

I would take a close look at TP636 on schematic 7 to see if the slow
ramp is being reset early. The text says that the ramp (in repetitive
mode) should make it to 10 volts but graph 30 shows only 1 volt. I
think the graph was mislabeled 500mV/div instead of 5V/div when
someone forgot to factor in a x10 probe.

Wow, what a clever current source that is with C620 differentiating
the ramp. Doing it that way removes all non-ideal and non-linear
operation of Q636 although I am not sure doing so is really
neccessary. I do not see how the integrating capacitor C631 is
discharged. Somehow Q626 must be latching on.

In Real Time mode the sweep is full length.

This makes me think that the horizontal output circuit path is ok.

The fault was in the Dot Position Memory circuit on the logic board, or more exactly the lack of a gate signal arriving at the base of Q650. Once I'd got that determined, removing and re-seating the timing and logic boards solved the problem.

So the sweep wasn't truncated, it was partly off screen.

Dave

"David C. Partridge" <david.partridge@...> wrote:

The fault was in the Dot Position Memory circuit on the logic board,
or more exactly the lack of a gate signal arriving at the base of
Q650. Once I'd got that determined, removing and re-seating the
timing and logic boards solved the problem.

So the sweep wasn't truncated, it was partly off screen.

Nice shooting!

-ls-

Well, at least I started diagnosing on the correct page this time. :)
You could grow old thinking about all of the things that could go
wrong with a 7T11.

I have had problems with the board to board pin connectors Tektronix
used before that were solved by going through and bending the little
leaf springs to tighten them.

Tektronix used that dual transistor transmission gate design in a
couple of different places in the 7T11. I wonder how its performance
compares to using diodes or a single JFET.

On Sat, 26 Jan 2013 18:31:43 -0000, "David C. Partridge"
<david.partridge@...> wrote:

The fault was in the Dot Position Memory circuit on the logic board, or more exactly the lack of a gate signal arriving at the base of Q650. Once I'd got that determined, removing and re-seating the timing and logic boards solved the problem.

So the sweep wasn't truncated, it was partly off screen.

Dave

You could grow old thinking about all of the things that could go wrong with a 7T11.

As they say: Ain't that the truth!!!

Dave