Daewoo DTR-14D3VG, Service Manual

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• Continuous Cathode Calibration (CCC)
The Continuous Cathode Calibration (CCC) loop (or two point stabilisation loop) is an auto-tuning loop which
stabilises th black level (offset) as well as the cathode drive level (gain) of each gun of the CRT sequentially and
independently on alternating fields.
In case of no stabilisation, the transfer characteristic changes as function of temperature and ageing of the tube.
The CCC-loop can be divided into two loops :
- a black level stabilisation loop. (offset compensation)
- a cathode drive stabilisation loop. (gain compensation)
• Beam current limiter / vertical guard
The beam current limiter circuit functions as an average beam current limiter(ABL) as well as peak white limiter
(PWL). The average beam current limiter needs external circuitry to function it a relative slow function. The peak
white limiter is an internal detection circuit, which has a fast function;
Both functions reduce the contrast and brightness of RGB signals.
The peak white limiter comes into action if the difference between measurement DC level and white level exceeds
approximately 2.6V (at nominal white point settings). Then an internal (peak white limiter) discharge current (I
PWL
) of
200µA will flow, then V
BCL
decreases; if no capacitor is present at the BCL (no integration), then V
BCL
will fall
immediately after peak signal detection.
For both average beam current limiting and peak white limiting :
- contrast reduction begins when V
BCL
< 3.0V;
- brightness reduction begins when V
BCL
< 2.0V.
V
BCL
is normally 3.3V when average beam current limiting and peak white limiting are not active.
The contrast and / or brightness reduction of the RGB
OUT
is proportional to the voltage decrease on the BCL pin.
Besides the beam current limiter input the vertical guard input is added to the same pin of the TDA884X, both
functions are independent of each other.
The vertical guard blanks the RGB
OUT
when the vertical deflection fails. The vertical deflection works correctly when
the vertical gurad circuitry in the TDA884X detects a pulse level above 3.7V during the vertical retrace.
The vertical guard function indicates vertical deflection as “failure” when there are no pulses during vertical retrace
and / or the DC level is continuosusly above or below the verticl guard detection level of 3.7V.