Power Supply; Analog Card - IBM 3279 2A Maintenance Information

3279 MIM
3.1 Power Supply
The power supply switches the line voltage at high
frequency using pulse width modulation.
The modulated voltagc is fed to the primary
windings of a power transformer. The outputs from
the secondary windings are rectified to produce the
following voltages:
Voltage
Tolerance
Max. Ripple
(V de)
(±%) (V pk-pk)
+103 6 0.7
+12 10 0.4
-12 10 0.4
+8.5 10
0.26
+5 10
0.15
-5 10 0.15
The transformer incorporates a feedback winding to
control the pulse width and hence the output
voltages.
A signal from the analog card (LOPT sense) locks
the power supply oscillator to lhe line timebase
frequency, to eliminate interference on the display
caused by the high frequency modulation. This
signal also indicates to the power supply an
overvoltage condition on the analog card.
If either the overvoltage or the overcurrent
protection mechanism shuts down the power supply,
it retries up to seven times, approximately once per
second and the 'POWER GOO~' indicator light 1
flashes at this rate, provided that the failure is not
on the +5 V line. An audible click may be heard
from the supply at each retry by listening near the
ventilation slots above the power supply card.
A fuse is located on the power supply card. If it
fails, the power supply card must be exchanged.
Chapter 3. Functional Components 3-2
3.2 Analog Card
The analog card generates horizontal and vertical
drives for the CRT scan coils and includes the
extra-high-voltage
(EHT)
power "'pply. When the
logic is supplying horizontal and vertical
synchronizing signals, the time bases on the analog
card lock to the imposed frequency.
The line output transformer (LOPT). in the
horizontal drive circuit. provides the following
supplies:
Voltage
(V de)
+25
-25
+12
+70
-150
+280 to +450
+6 kV
+6.3
+25 kV
Where Used
- On-card use, deflection
amplifiers
- On-card use, deflection
amplifiers
- On-card use and card B2
- To video card
- Through brightness control,
amplifier card, and color
balance controls to CRT grid
- Focus circuits and CRT anode
- Focus circuits
- CRT filament (see Figure 6-7)
- EHT supply to CRT
The analog card receives a further signal ('skip'),
which forces the raster to move vertically a defined
amount (see Figure 2-1).
To start up, the analog card requires ±S V dc,
+8.5 V dc, and + 103 V dc.When the power is
turned on the horizontal drive circuit starts and
produces the additional supplies. These enable the
vertical drive circuit to start and this initiates a
delay. After 45 seconds DISPLAY READY
(indicator 2) turns on and a change in CRT grid
voltage unblanks the screen.