Try tweak thermal balance of the vertical amplifier.
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If you let the trace go outside the display area, many models will have this type effect. I have seen it on 7834, 7904, 7903, 7104.
Someone on internet took me this is about thermal imbalance in the amplifier.
My 7613 or 7633 never had this problem. So, I guess faster is not always better.
On Sat, Jul 18, 2020, at 3:46 PM, FAZ wrote:
I hope this can help somebody. The readout on my 7834 was wobbly. When
a signal was displayed by any of the vertical amplifiers the readout
would sometimes flicker up and down. Gating the readout made for a more
stable display, but the characters were not aligned vertically. The
vertical signal to be displayed was being coupled into the readout.
I found the section in the 7834 service manual that mentioned the x-y
inhibit from the readout system, so I wanted to make double sure that
the x-y inhibit signal was fine. I checked pin 13 of the timer IC
U3433, which on the A13 readout board was marked as just 33. I observed
that signal while the readout was functioning normally, then waited for
the intermittent waviness. No relationship, so I followed that same
signal through the manual to the A8 vertical interface board and it
appeared the same. On the vertical interface board are two transistors,
Q4633 and Q4652 that are responsible for receiving those x-y inhibit
pulses. They tested fine, and even after cleaning the leads the
intermittent waviness would still happen. That pair of transistors just
receive the inhibit pulse. The action really happens downstream in
transistor array U4641. Just wobbling it while the scope was on made
the readout even worse, with about two centimeters between some of the
characters and adjusting the position on the vertical amplifier would
take the entire readout with it. Removing the array, I tested each
transistor. They were all fine and very uniform as expected. Pressing
it back into the socket and spraying contact cleaner didn't help. I
noticed that pressing down one end of the IC would lever the opposite
end out. Doing that, I managed to get an intermittent trace and stable
readout haha. I swapped the socket and steel wooled the pins on the
transistor array. Now all fourteen connections are held firmly in place
and I haven't had problems since.
Hope this helps if anybody has a similar problem. I was worried it
would be something power supply related and didn't want to go down that
rabbit hole again