CRT AUTO BIAS AND AUTO BRIGHT CIRCUIT DESCRIPTION.
The beam current feedback circuit uses a PNP video
transistor 954 to direct most of the beam current to the auto bias circuit while passing the voltage waveform, from the video amplifiers to the CRT cathodes. Diode 958 and capacitor 956 insure that no video waveform
distortion occurs. An additional benefit of this circuit is that it protects the video amplifiers from the destructive
arc energy. Resistors | 948 | and | 955 |
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to CRT arcing, between | the | video amplifier transistors | ||||||
and the beam current feedback transistor |
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beam current is filtered by capacitor |
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C10 and is buffered by an operational amplifier, which translates the beam current into a low impedance voltage. This voltage is applied to a coupling capacitor
| 921 | through a 200 ohm resistor | C8 | . | ||||
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program value which sets the black level |
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action of the program pulse. |
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| Capacitor | 922 | is used to stabilize the |
transconductance amplifier which is used at the channel input of the auto bias IC 927 . The auto bias IC stores the bias voltage of this channel in capacitor 895 at pin
21.This voltage is buffered by an internal amplifier, with output at pin 20, which is connected to the Blue video amplifier bias control input.
Resistor 908 , 910 , and 911 are part of the auto bright circuit. They are used to sum the bias voltage of each of the three channels via a voltage node at the auto
bright amplifier, | 920 |
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voltage then controls | the screen grid via transistor |
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Resistor |
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881 |
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current | during | arcing. | Capacitor |
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878 |
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AC impedance to GND to insure that the CRT gain is | ||||||||||||
constant during each horizontal line. | Resistor |
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858 |
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914defines the current gain of, and stabilizes, the auto bright control loop.
Load resistor | 909 | eliminates crossover distortion from | ||||
the OP Amp. |
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920 | 852 | 856 |
transistor and OP Amp. from damage due to CRT arcing.
PNP transistor 928 is used as a voltage translator to
direct the grid pulse from the auto bias IC to G1. The voltage on G1 is normally
low, the current from resistor | 933 | is conducted to | ||||||||||||
resistor |
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874 | ||||||||||||||
G1 line. | Capacitor |
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| 855 | 873 | ||||||||||
protect transistor |
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| 928 |
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The auto bias IC | (CA3224E) is designed for a supply |
voltage of +10V and since the video amplifier requires
+12V, three diodes | 903 | , |
| 905 | , and | 906 | are used to | |
supply this IC. Resistors |
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C4 |
| C7 |
divider which supplies the, auto bright, bias voltage to the LM324 920 . The green and blue channel circuits are identical to the red channel and are controlled by the timing logic in the same way.
Refer to the waveforms at the bottom left of page 34 for the timing relationship. The vertical retrace boost pulse, from the LA7838, (15KHz models) or the delayed vertical sync pulse from the sync delay circuit (25 & 31KHz models) starts the 21 count auto bias state counter. This pulse is applied to the auto bias IC through resistor 891 . The negative going flyback pulse which is used to heat the filament also supplies the horizontal sync to the auto bias IC via diode 884
and resistor 888 . The grid pulse becomes active between the 15 and 18 horizontal cycle and the program pulse is active between the 18 and 21 horizontal cycle. These two pulses in conjunction with the internal control of the transconductance amplifier output switch are what establish the timing for the measurement and setting of the video bias.
CRT AUTO BIAS, VERTICAL SYNC CIRCUIT DESCRIPTION.
The auto bias vertical sync comes from from a buffer circuit for 15KHz operation. For 31KHz operation this signal is generated by a delay counter. For both cases, the vertical boost pulse is "and" connected with the Vertical Osc. O/S to provide flicker free operation and laser beam protection. In the case of vertical deflection failure, the loss of the boost signal causes the auto bias vertical sync to stop, which stops the auto bias function, and blanks the screen via the vertical blanking circuit, thus providing for laser beam protection.
The vertical oscillator one shot (LA7851 pin 16) supplies the start timing for the auto bias vertical sync. This signal is conducted to the emitter of
379by jumper 089 . The base of 379 is connected to the retrace boost pulse by resistor divider 383 and 384 . Combining these signals in this way produces a collector waveform which has the vertical oscillator one shot timing and is dependent on the retrace boost pulse.
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| 099 |
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PN2222 |
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100 |
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| 098 |
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0Ω | PN2222 |
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100A |
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200K | 379 |
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383 |
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| 201 | 101 | Delay |
| 220pF | 381 | |
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Retrace Boost |
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Vertical Osc. O/S, LA7851 pin 16
For the 15KHz case, transistor |
| 100 | inverts the | ||
vertical oscillator one shot signal to produce the | |||||
CRT auto bias vertical sync signal. | Resistor | 099 |
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is the pullup and resistor 006 reduces the chance of arc damage to the transistor.
For the 31KHz case, the vertical oscillator one shot signal is directed to the clear of the 7 bit counter 100 . This O/S time out must occur after the autobias delay time out and come before the bias active pulse from the CA3224E. When the clear is low, the counter counts horizontal pulses, by the clock connected voltage divider 099 and
201 . When the counter outputs ones at each diode
connected output, further counts are inhibited by diode 006 and pullup 381 . This diode "or" signal is also used for the CRT auto bias vertical sync. The delay is set to locate the grid pulse generated 3 faint lines at the top of the screen with full vertical deflection. Capacitor 091 produces a
delay to avoid a race condition between the counter clock and the auto bias horizontal sync.
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