Grundig CUC 7303 Color CRT TV SMPS schematic - UC 3842 power control IC - power circuit description – how to enter service mode - adjustments

Power Supply - Basic Circuit
Current mode converters can exhibit sub harmonic oscillations when operating at a duty cycle greater than 50% with continuous inductor current. This instability is independent of the regulators closed loop characteristics and is caused by the simultaneous operating conditions of fixed frequency and peak current detecting.

At t0, switch conduction begins, causing the inductor current to rise at a slope of m1.This slope is a function of the input voltage divided by the inductance. At t1, the Current Sense Input reaches the threshold established by the control voltage. This causes the switch to turn off and the current to decay at a slope of m2, until the next oscillator cycle. The unstable condition can be shown if a perturbation is added to the control voltage, resulting in a small ∆l. With a fixed oscillator period, the current decay time is reduced, and the minimum current at switch turn-on (t2) is increased by ∆l + ∆l m2/m1. The minimum current at the next cycle (t3) decreases to (∆l + ∆l m2/m1) (m2/m1). This perturbation is multiplied by m2/m1 on each succeeding cycle, alternately increasing and decreasing the inductor current at switch turn-on. Several oscillator cycles may be required before the inductor current reaches zero causing the process to commence again. If m2/m1 is greater than 1, the converter will be unstable. By adding an artificial ramp that is synchronized with the PWM clock to the control voltage, the ∆l perturbation will decrease to zero on succeeding cycles. This compensating ramp (m3) must have a slope equal to or slightly greater than m2/2 for stability. With m2/2 slope compensation, the average inductor current follows the control voltage yielding true current mode operation. The compensating ramp can be derived from the oscillator and added to either the Voltage Feedback or Current Sense inputs.

Normal / Controlled Operation 

For the power supply of this TV receiver a blocking oscillator-type converter power supply with a switching frequency of 50kHz approximately is used (at normal operation and a mains voltage of 230V).

The collector contact of the power transistor T665 is connected via the primary winding 3/1 of the blocking oscillator-type transformer TR601 to the rectified mains voltage, D621…D624. At a mains voltage of 230V the voltage level present at the charging electrolytic capacitor C626 is approx. +320V.

The IC630 is responsible for driving, controlling and monitoring the bipolar power transistor T665. The supply for the control-IC is 12V and is present on Pin 7. As soon as the switch-on threshold is reached on Pin 7 via the resistor R633 and the capacitor C667, the IC feeds out a positive start pulse (1µs) of 10V pp at Pin 6. After start-up of the IC, the supply voltage is obtained via the diode D667 from the winding 5/7 of the transformer. During the conducting phase of the transistor, energy is stored in the transformer and this is transferred into the secondary winding when the transistor is switched off. The IC630 controls by the period during which the transistor T665 is switched on, the transfer of energy at Pin 6 so that the secondary voltages are stable and are largely not affected by variations of the mains supply, mains frequency and the load.

The power transistor T665 is driven by a pulse-width modulator which is triggered by an oscillator integrated in the IC. The frequency of the oscillator is determined by the components C652 and R652. For stabilization, the feedback voltage which is rectified by D654 is compared in IC630 with the 5V reference voltage provided at IC630-(8).  If the feedback voltage decreases by a small amount due to a heavier load the drive pulse to the transistor T665 is prolonged. As a result, the conducting period of T665 will be longer so that additional energy transfer will be provided to compensate for the load. Pin 3 of IC630 is a current sense input and will stop the drive to T665 at IC630-(6) in the event of excessive current drain from a heavy secondary load.

UC3842

If there was a short circuit condition at the transistor T665, the circuit UC3842 would be destroyed. Therefore, the diodes D666 and D664 are provided to avoid the voltage at pin 3 exceeding 1.2V. The components D668, C669, and R669 work as a snap stage.  The components CD654, C656, CD656, and CR656 delay the rise of the pulse start duration (soft start).  The adjustment control R654 is used to set the secondary voltages by regulating the +A voltage at minimum brightness and contrast.

Standby Mode

In normal operating mode, a voltage of approx. 10.5V is present on IC676-(1) (LM317). If the TV receiver is to be switched to standby, the µP switches UStandby to "High" level so that the level on IC676-(1) is < 0.7 V. As a result, the voltage +B is switched off and the TV receiver goes to standby.

Secondary Voltages

+A: Supply for the horizontal output stage from the winding 2/10 and D682. The power supply unit is set to this level.

+33V: The upper tuning voltage limit for the tuner is produced at the Z-diode D683 and the resistor R681 from the winding 2/10 via D682.

+M =16.5V Supply for the sound output stage from the winding 6/10 and the diode D671.

+B = 12V Power supply for the Tuner and the horizontal driver T501.

This voltage is supplied from the winding 6/10 via the diode D671 and is stabilized by the adjustment control IC676.

Switching off of the +12V supply, see "Standby Operation".

+E = 8V Power supply for the Video Processor IC150. In Standby mode it is switched off.

+H = 5V Power supply for the µP IC850, the infrared amplifier IR810, Tuner, and CIC105.

This voltage is also present in Standby mode.

Additionally necessary voltages
+D: +25V Power supply for the vertical output stage from the line transformer winding B/H via D444.
+C: 125V The power supply for the picture tube panel is obtained 190V from the line transformer winding G/H via R543 and the diode D543. 125V/14" CRT; 190V/15…21" CRT.

Service mode

Settings via the Service Menu
Calling up the Service Menu
Press and hold button "i" on the remote control and switch on with the mains button.
AGC Alignment
Select "AGC ALIGN" in the Service Menu. Alignment is possible in range 0...62 with the "V - / + C" buttons.
OSD Position
Press and hold button "i" on the remote control and switch on with the mains button. Select "OSD" (V or H) in the Service Menu and with the "V - / + C" buttons position the menu table in the center of the screen.

Activating the Hotel Mode
Select "Hotel ON" in the Service Menu. When the Hotel mode is activated:
it is no longer possible to call up the programme setting menu with the "i" –> "OK" buttons.  The currently set volume level is stored as the maximum level possible in this mode.
Decoder
Via the Service Menu switch the decoder "ON" or "OFF".
Decoder "ON":
Automatic identification of the switching voltage at Pin 8 of the EUROAV socket (e.g. descrambler operation with TVs in French version, or external RGB mode for Italy).
Continuous Station Ident Indication
So that the program name is displayed continuously on the screen press the "i" button. After about 8 seconds the program is displayed in reduced size.

Settings via the AUX Menu

AUX Overview

The AUX menu is called up by pressing the "PC/AUX" remote control button quickly.

Calling up the Contrast Setting Option

Call up the AUX menu and adjust the contrast with " Bright - / Bright +" button.

Calling up the Sleep timer

Call up the AUX menu and activate the timer with the "TXT" button.  Enter the desired stop time with the numbered buttons on the remote control and leave the menu with button "i".

Optimum Values for Analog Functions

Call up the AUX menu and press "OK". The optimum values are now called up.

ATS

Call up the AUX menu and press "PC/AUX" for approximately 4s.  Press the "OK" button to start the system.

Tint with NTSC

Call up the AUX menu and adjust with the button " Color - / Color + ".