Amplifier Card And Inputs - IBM 3279 2A Maintenance Information

3279 MIM
3.3 Amplifier Card and Inputs
3.3.1 Conpergence Circuits
The convergence amplifiers drive the convergence
coils. which provide the necessary correction for all
points on the SCreen to the three beams (see Figure
3-3).
The current in the convergence coils is controlled
by data in a store which is read in step with the
scanning of the screen. The data in this store may
be altered by the operator to compensate for CRT
ageing.
As data in the convergence store in the display is
lost when power is turned off. the convergence
store is supported by battery-maintained storage.
When the 3279 display is switched On. the
convergence logic card B2 generates a 'power on
reset' to the logic. This resets most registers and
sets the status to 'power on reset'. The next poll
from the control unit to which the 3279 is
connected detects the status and then reada the
Chapter 3. Functional Components 3-4
data from the battery-maintained storage and
processes it to set up the correct data in the
convergence store. This is normally complete
before the screen unblanks. The power on
procedure also occurs when the Normal/Test switch
is returned to Normal.
When the operator uses the convergence procedure
in the online tests. the data in the
battery-maintained storage is modified and the
convergence store is updated.
Note that the convergence control circuits are
packaged on two carda (see Figure 3-3):
Logic caril 82. (4Wx3H) carries the volatile and
non-volatile stores and the logic.
• The amplifier card receives four convergence
analog signals from the logic card and amplifies
these to drive the convergence correction coils.
• The amplifier also may contain gain controls that
are factory-preset.