Tuesday, June 23, 2015

TDA8873 SYSCON-CHORMA IC – CIRCUIT TROUBLESHOOTING

Some Faults that can occur due to defects to this IC are:
Power does not switch ON.
TV/AV selection is not possible.
Picture lacks black level.
Tuner does not work.
No EHT.
Horizontal Output Transistor fails frequently.
Audio problems.
Out of synchronized picture.
Displaced picture [horizontally]
Before proceed to troubleshoot, make sure that the SMPS section circuit is woking properly and all the secondary voltages generated are present.  Check the +5VDC & +8VDC.  These two voltages are the supply voltages to TDA8873.  If all these voltages are present, and still the IC do not work, we can assume that the IC is defective.  Check the main supply voltage; 8VDC at pin-17.  The 8VDC is supplied to this pin only.  No other pins receives this voltage.  Check this voltage first.
If it is OK, proceed.
If not, check V507-2SC8080 transistor and associated components at the SMPS section circuit. 
Assuming that this voltage is OK;
Check the voltages at these pins; 3,9,36 & 55.  At all these pins, the voltage should be 5VDC.  If absent, check the 5V voltage regulator IC used at SMPS section, IC607-78L05 & transistor V505-C8050 and related components.
Here one point to note is, with the SMPS circuit used here with this circuit; if there is no 8VDC; the 5VDC too will be absent. 
Assuming that all the voltages stated above are Ok; but the IC do not function:
If so; the first check should be concentrated toward the soldering terminals of the IC.  There are 64 pins.  If any of these pins have loose solder, erratic symptoms will be the result.  So; first make sure that these solder terminals are OK; and have no dry solder points.  Normally it will be too difficult to detect a dry solder terminal, as it will be too microscopic for normal vision.  So it will be the best practice to re-solder all these 64 pins; by applying a little more fresh solder to each terminals, without making any solder-bridge short in between adjacent terminals.  Be sure not to apply too much heat for long time, at these termilas, as it might damage the IC.  The soldering should be done as fast as possible; and should be clean.
Now check all the other pin voltages. 
Measure the voltages at SCL & SDA [Serial Clock & Serial Data] pins; 58 & 57 respectively.  Both of this should be around 2.8VDC.  if any of these voltages are either too low or high; the IC is defective.
No repair.  Replacement is the only remedy.
Next check the Power-ON signal voltage present at pin 64.  When you press the Power On button on the remote control, this voltage should be vary to High/Low [4.8VDC.0.5VDc.]
If not; the IC is defective. The system control section inside it does not work properly.
Check the voltage at pin-5.  This is the Reset voltage to IC and should come around 3.2VDC.  if absent; check Q203-A1015 and Zener diode VD201, connected to its base terminal.
Assuming that all the above voltages are normal:
Measure the voltage at pin-13 [horizontal out signal].  It should be about 0.4 to 0.7VDC.  will have an oscillating frequency of ~ 15.625KHz.  if your meter is capable of measuring frequency; you can measure this frequency directly from this point. If this frequency and the voltage at these terminals is abnormal, the IC is defective.
In most cases; measuring the voltages said above will reveal the condition of this IC; whether defective or good.
Click on the schematic to zoom in