This document introduces the principle and maintenance of the switch power supply for Changhong Digital HD (CHD) series color TVs. The switch power supply uses a thick film integrated circuit from SanKen as the core component, which has the advantages of high output power, simple peripheral circuit, perfect protection circuit, and easy maintenance.
The document provides a detailed analysis of the CHD series television switch‑mode power supply, describing the input interference, automatic demagnetization, rectification, filtering, startup, secondary supply, regulation loop and protection circuits. It also outlines the improvements made for 50 Hz HDTV compatibility.
Practical troubleshooting procedures are included, with step‑by‑step tests for the thick‑film block NQ821, common failure points such as capacitors and diodes, and guidance for adjusting the B+ voltage and standby control circuits. This manual is written in Chinese.
| Input voltage | 220 V AC |
|---|---|
| Rectified DC voltage | ≈210 V DC |
| Pulse voltage after transformer | ≈308 V DC |
| Secondary supply voltage | ≈42 V DC |
| Regulated B+ voltage (50 Hz mode) | 128 V DC |
| Secondary output – 8 V | 8 V DC |
| Secondary output – 25 V | 25 V DC |
| Secondary output – 12 V | 12 V DC |
| Secondary output – 9 V | 9 V DC |
Measure the resistance of NQ821 (pin 3) to ground; if it is not shorted, the fault is likely in the 220 V input filter, bridge rectifier, or automatic demagnetization circuit.
Capacitor C810, transformer T830, diode VD821 and capacitor C822 are frequently found failed and should be inspected or replaced first.
When power is applied, resistor R808 increases its resistance, causing a larger current through the CRT demagnetizing coil, which creates a magnetic field that erases stray fields on the CRT shadow mask.
The control circuit uses transistors VQ853 and VQ852 to modify the error‑sampling voltage, which reduces the regulated B+ voltage from 148 V to about 128 V for 50 Hz operation.