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| 型號： | MC44007P |
| 廠商： | MOTOROLA INC |
| 元件分類： | 消費(fèi)家電 |
| 英文描述： | CHROMA 4 VIDEO PROCESSOR |
| 中文描述： | SPECIALTY CONSUMER CIRCUIT, PDIP40 |
| 封裝： | PLASTIC, DIP-40 |
| 文件頁數(shù)： | 39/40頁 |
| 文件大?。?/td> | 491K |
| 代理商： | MC44007P |
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MC44002 MC44007
39
MOTOROLA ANALOG IC DEVICE DATA
APPENDIX C – A SUGGESTED METHOD FOR OUTPUT LOOPS ADJUSTMENT
As described in section 4, the MC44002/7 output loops
stage automatically adjust the dc level of the cathode voltage
(cut–off) and the gain of the signal at the cathode (white
balance). These automatic adjustments replace the
conventional manual adjustments. The only adjustment that
must be carried out, either by hand or automatically using an
“intelligent screwdriver”, is for the G2 voltage.
As the G2 voltage is varied, the automatic output loops of
the MC44002/7 will adjust the cathode voltage of the dark
sample level to always obtain the correct dark cathode
current. However, if the G2 voltage is adjusted too high or too
low, one or more of the DAC’s controlling the dc level will
reach the end of their range and the cathode voltage on the
channel will not be correctly adjusted. In order to inform the
operator or machine adjusting the G2 voltage that the control
range has been exceeded, the G2–Up Request or G2–Down
Request flags will be set. These flags are set when any one
of the dc–DAC’s approaches the end of its range. The
threshold for setting the flags lies typically between 15 and
20% of the range from the actual end. Therefore, when a flag
is set, the output loops can still operate correctly. As the gain
of the picture tube varies very little with the G2 voltage, flags
are not provided for the gain–DAC’s.
In order to fix a procedure for setting the G2 voltage it is
necessary to consider several points:
– On a given sample, the output currents from the three
channels corresponding to the dark level are all different. The
range of each DAC is about 2.4 mA and varies little from one
channel to another and from one device to another. For
reasons of stability and control range we recommend that the
feedback resistor of the high–voltage video amplifier be
39 k
. this means that the dark cathode voltage range of
each channel is about 94 V (i.e. 39 k
x 2.4 mA), but the
absolute value of the cathode voltage can vary.
– In a typical application the actual cut–off voltage (i.e.
zero cathode current) lies about 10–15 V higher than the dark
cathode current (10
μ
A).
– When the beam–current in the picture tube increases,
the G2 voltage tends to decrease. With the output loops of
the MC44002/7, the cathode voltage is lowered automatically
to compensate, but this effect would normally cause the
values in the dc–DAC’s to fall, using up their useful control
range. as high beam current is associated to high contrast, in
the MC44002/7 the dc output current (and therefore the
cathode voltage) is reduced directly as the contrast setting is
increased. In this way as contrast is increased, leading to
higher beam current and lower G2 voltage, the dc–DAC’s do
not move much, thus saving range.
– A picture tube can have a difference in cut–off voltage
between guns of up to about 30 V and it is not generally
possible to identify in a particular type and make of tube
which gun has the lowest and which gun has the highest
cut–off voltage. Also, it is generally recommended by the
tube manufacturer to set the cut–off voltage of the highest
gun to a certain value which gives optimum focus
performance.
– As the picture tube ages, the cathode cut–off voltage
falls. It is therefore best to set the G2 voltage when the tube
is new to give the highest possible cathode cut–off voltage.
Taking into account the above points, it is recommended
that the G2 voltage be set up in the following way:
1) Display a black picture with the brightness control to
minimum. (This give minimum beam current and no drop in
G2 voltage.)
2) Set he contrast to maximum. (This causes the dc output
current to be forced to a lower level and the output loops to
compensate by moving towards the top of their range.)
3) Now adjust the G2 voltage so that the G2 Down
Request flag is just turned off. (All the dc–DAC’s are towards
the top of their range and the highest one is just at the level to
switch on the flag. Lowering the contrast setting, increasing
the beam current or aging of the tube will cause the output
loops to reduce the values in the dc–DAC’s, but the available
range will be a maximum.)
4) With a white picture and contrast set to give the
maximum allowable beam current, check that the G2 Up
Request flag is still off. (This is just to check that the G2
voltage is not falling too much at high beam current, but this
step is not absolutely necessary.)
It is not recommended adjusting the G2 voltage to reach a
specific value of cathode cut–off or dark voltage. The reason
for this is that tolerances of the picture tube, high voltage
video amplifier and the MC44002/7 itself will cause the
dc–DACs to be set anywhere in their range and perhaps near
the bottom end, leaving no margin for aging and G2 voltage
drop.
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