Do the adjustment procedure for R2214 which can drift over time. The
procedure is at the end of page 5-53 in the service manual and can be
done independently of other adjustments except maybe the power supply
voltages. It basically comes down to installing a bunch of plug-ins
and adjusting R2214 to the middle of the range where the character
scan is correct. R2183 is adjusted in the same way.
If either cannot be adjusted such that all characters are correct,
then something else is wrong.
My 7854 has a similar problem with my 7T11A where it converts ps to ns
sometimes but I have not bothered to adjust it.
On Mon, 23 Jul 2012 20:32:39 -0000, "max_3289" <max_3289@...>
wrote:
@David:
if i use a 7A26 only CH1 is affected (upper middle text), CH2 is ok (lower middle text). I prefer using the 7A22 to get the "?" too.
R2214: i did not *adjust* the pots, i did simply remember the position (easy, because of characters on the device), deudjusted for some degrees and then back to the original positions, just to move the contacts a bit.
7493: levels seem to be ok, if you send me your mailadress, i can send back some screenshots of a DSO (obviously its not possible to attach files here?)
@Albert:
not yet, i was not able to play around to get the right connections in a useful way (next step...)
Max
--- In TekScopes@..., David <davidwhess@...> wrote:
Do both channels of the right vertical bay still show up? If one is
missing, then that may be easier to track down and repairing it could
fix the bad character problem.
This pattern of errors indicates a problem between U2159, the right
vertical bay, and U2244 causing a decoding off-by-one error. Leakage
is a definite possibility.
How did you adjust R2214? The same off-by-one error is showing up in
at least two different time slots and two different columns so I would
suspect something in common between them which leaves R2214, the time
slot connections to the center bay, and pin 8 of U2250.
Check the levels of the outputs (pin 8, 9, and 11) from U2250 with an
oscilloscope. Pin 8 only goes high when the right vertical bay is
selected. Maybe it is not switching as completely as it should. If
you have a 7493 available, you could just swap it. A 74LS93 or
74HCT93 should work as a substitute.
David Hess