Re: 468 DSO B timebase triggering

Colin Herbert


It seems that I may have not described what I called "intensity cycling"
very well. It is rather like a signal being applied to the Z-axis which is
close in frequency to that applied to the Y-axis, but not identical. The
brightness (or should I say the blanking) is "rolling" along the last full
waveform cycle under the conditions that I mentioned. The other feature
(which I think David has described as being "normal") is that the horizontal
trace doesn't extend over the last two divisions of the screen when the A
and B time-bases are set to the same rate. The waveform doesn't matter -
square, sine and triangle all show the same behaviour.

I appreciate that this isn't a display situation which one might normally
seek to use. It all came about from me checking out the triggering for both
time-bases. I understand how the B time-base can be used as a delayed
time-base but I don't feel insulted by Tom's suggestion that I am unfamiliar
with this.

I don't possess a 465B, though I do have the manual. I have to say that I am
surprised that there is similarity between the 468 and 465B scopes.
Superficially, most of the 400-series look similar, but the 464 and 466,
while looking very much alike, don't even share a CRT (I have 464, 468, 475A
and two non-functioning 466s from this series). The nature of the switching
of the 468, being a DSO, also has some obvious physical differences, largely
due to the digital logic, which I don't understand much at all!


From: TekScopes@... [mailto:TekScopes@...]
Sent: 20 January 2017 17:10
To: TekScopes@...
Subject: Re: [TekScopes] 468 DSO B timebase triggering

On Fri, 20 Jan 2017 16:33:04 -0000, you wrote:


Thanks for this useful info, David. I thought something like this was the
situation. I assume that the x10 attenuator is there because the CT-3
pick-off voltage is 10% of the through signal and so all inputs will get
pretty much the same level of signal.
Exactly. The levels do not need to be matched but if they are roughly
close, it makes things easier.


I don't know what I was up to when I started this thread, because in the
cold light of day everything seems to check out ok according to the manual.
However, the "cycling" in intensity of the last full cycle of the 1kHz
square-wave still happens, but only when the horizontal display is "B
DELY'D", A triggering is "NORM" with the trigger level control fully
clockwise, B triggering is at "NORM" and adjusted to give a (relatively)
stable display with the B-trigger level and the A and B time-bases are set
to the same time (0.5ms is ok, making the time shorter increases the rate
of the intensity "cycling" and vice versa; changing to a faster B time-base
rate results in a display without the intensity "cycling"). Adjusting the
"Delay Time Position" multi-turn control results in the intensity "cycling"
remaining with the last full cycle, but the whole horizontal scan is
shortened from the right side going toward the left. Could this all be
caused by a damaged Q230? (Aargh, I hope not - they are a matched pair of
FETs and probably unobtainium).
The shortening of the B sweep when both timebases are set to the same
time/div is normal. It is caused by the B sweep being reset when the
A sweep completes. It also happens when the B sweep is slightly
faster than the A sweep and the delay time is high enough.

I can create the situation you describe on my 2232 if I adjust the B
trigger level to the very top or bottom of the waveform so that it
sometimes triggers on different cycles. *This makes me suspect that
any problem is with the B trigger itself.*

Is there any adjustment of the B trigger level which produces a stable

One thing which occurs to me is that if the B trigger input circuits
have a problem, then the fast edge of a square wave can make it
through but triggering performance will be poor. If so, testing with
a sine or triangle wave should make the problem worse.

The service manual has a test using sine waves for verifying the
performance of the A and B triggers separately.

I do not know what the access is like but just measuring the waveform
at the output of the B trigger input buffer will reveal if it is
operating correctly up to that point.

I have to say that, apart from this being a complex piece of kit, the
can be confusing, to say the least, in places. Page 4-49 is the beginning
the "Triggering" section and refers to adjustment locations 2 and 3, but
they are, in fact, 4 and 5. If only one function needs checking, it is
necessary to go through all of the other earlier function checks to get the
controls to the right settings. I suppose this saves a lot of extra "Set"
sections for each function, but it does involve a lot of ploughing through
earlier parts of the manual.

The 468 is basically a 465B with the added DSO hardware so it might be
worth studying the 465B manual for additional operating instructions.
There is also a lot of similarity with the 465, 475, and 475A.


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