Re: 7A15A issue or limitation??

Chuck Harris
 

Simple explanation, I think:

"Vertical mode" triggering is only valid when there is
a single vertical signal displayed on the screen, or you
are in ALT (alternate) mode.

It gets more complicated if I try to tell you why:

The only modes where the traces on the screen are exactly
what they appear is when there is only a single channel
displayed on the screen.

Whenever there is more than one channel on the screen,
there is some form of eye trickery going on. This is
because there is only a single beam in the CRT to draw on
the screen. A couple of scope models have multiple beams,
but they are the exception, rather than the rule...

In ALT (alternate) mode, it may appear to the eye that
there are two signals on the screen at the same time, but
in reality, the two signals are being shown singly,
alternating: A, B, A, B... If the sweep time is fast enough,
the eye merges the alternating images so they appear to the
brain to be simultaneous.

At the slower sweeps, the eye can easily see the traces
alternate.

In CHOP (chopped) mode, it may appear to the eye that there
are two signals on the screen at the same time, but in
reality, the two signals are chopped up into alternating
tidbits.... a tidbit of A, a tidbit of B, a tidbit of A, a
tidbit of B...all the while the sweep marches on, making a
sort of square wave with the tops being the tidbits of one
channel, and the bottoms being the tidbits of the other
channel.

Because the chopping of the A and B signals is faster than
the sweep speed, and with no time relationship to the sweep
speed, or the signals, the eye sees the chopped up signal to
have merged into what looks like simultaneous display.

But the trigger isn't a human eye, so it isn't fooled by this
trickery.

When the trigger mode is in "Vertical", the trigger is given
the exact signal the plugin is trying to display on the
screen: Single channel mode, gives the single channel.
Alternate mode, gives an alternation of Channel A and Channel B.
Chop mode gives a composite signal of a little bit of A, B,
and their vertical positions... A real dog's breakfast.

"Vertical mode" triggering is only valid when there is a
single vertical signal displayed on the screen, or you
in ALT (alternate) mode.

-Chuck Harris

tinkera123 wrote:

Hi,

I have been playing around with my 7A15A's, in different 7603 Mainframes, and have a 'jittering' or 'flickering' of the 4V Calibration signal.
The 'jittering' is a movement, always to the left, of the trace along the horizontal axis and return to the correct position.

I can define this issue or limitation as follows:-
Independent of Mainframe - 2 x 7603's used
Independent of Timebases - 7B53 and 7B92A used
Only occurs in TRIG SOURCE/VERT MODE with VERT MODE/ALT - both traces are stable in all other Modes
Only one of the traces becomes unstable
Only occurs when traces from each Vert Amp are 'un-overlapped' - when the two traces are overlapped, both traces are stable

The Manual states that in TRIG SOURCE/VERT MODE .... "each sweep is triggered by the signal being displayed on the CRT"

This suggests to me that when I dive into the Circuit Descriptions, that I should be looking for some 'switching mechanism' which is swapping the Triggering mechanism between the signals from each Vert Amp.
If this 'switching mechanism' is faulty or receiving a weak signal, then this maybe causing poor triggering, hence the 'jittering'.
However, this 'switching' between Vert Amp signals would occur in the Timebase or Mainframe.
But that doesn't make sense because the 'jittering' occurs in both 7A15A's, independently (ie. 7A15A + 7A26 + 7B92A etc), and in 2 different Mainframes, and with different Timebases.
So .... where have I gone wrong here???? ..... or is this a limitation of the 7A15A???

Any guidance is appreciated.



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