Samsung Plano CZ21K 30G1 Flat TV - TDA9381 (System control_Chroma_Oscillator Jungle IC)
Set got for repair in dead condition today.
The owner said that its LOT is damaged, as somebody told him so; during a home call repair attempt. As I asked some more details, I did understand that the owner is professionally a Doctor. He knows only what the service technician said to him. That’ll be so. Because all doctors might not be aware in electronics.
Suppose that, I’m before him with any physical disorder. What can I do is just to follow his advice; take the medicines he prescribes and do what else he advice. Because, I’m not aware in medicine.
Like ways is here. The TV technician told him that the LOT is damaged; and the doctor recited it to me. That’s all.
Whatsoever it may be. Let me go with my work.
I did open the back cover of the set. There were only two seating screws at the top. The two others at the bottom is lost.
As somebody has already worked with the TV, I should be careful before plug it into AC power.
I pulled out the PWB, and checked all its print side to know whether anybody have worked with it; or not.
No solder work done.
Yet; I've double checked to make sure about it.
After confirming that no work has done with the PWB, I plugged it to AC mains, and switched it ON.
The standby/power LED at the front panel lighted up, stayed for about 2 seconds, and gone OFF.
Yes. Clear evidence that its SMPS power regulator section circuit is working fine. One major check-point is over.
Checked the +B(110VDC) at the cathode of D605.
Found OK. It shows +111.2VDC
Here one thing I’ve to specify is the worst printing quality of Samsung. They have printed all the component numbers with black ink and the PWB is masked with green lacquer. We can’t note the part number without taking some strain to eyes. If it were printed in white color, might be great.
Here the 110VDC supply line is OK.
There are two other important voltages too should be. The 12VDC & 8VDC.
12VDC is used to the horizontal drive stage and the 8VDC used to supply the main system chroma IC TDA9381.
I've noted that the heat-sink to the STR at the SMPS stage heats up slightly. It is normal, as that stage works properly.
Measured the voltage at the collector terminal of the Horizontal Drive transistor.
Found abnormal. It showed only (0.87VDC). It should be around 11.3VDC normally.
With this set, an IC is used as DC voltage regulator to derive 12VDC & 8VDC. [IC802 – KA7632]; a controlled dual voltage output IC.
Checked the input voltage to this IC at its pins 1&2. It was OK. At about 14.5VDC – from the SMPS.
Further checking revealed that the solder terminals to this IC (10 pins) appeared somewhat bad. As this IC will heat up somewhat 40 degrees centigrade, during normal working, the solder terminals to it too will heat up to that temperature. Repeated heating and cooling can make the solder terminals to expand and to shrink, and thus (Dry-solder) joints can be.
I've de-soldered out all the solder from these 10 terminals, and re-soldered by applying fresh solder to all.
Plugged the set to AC mains, and switched it ON
OK. The raster appeared
Here I've to do some more work with this set. Its focus is out. Somebody might have touched the focus and screen voltage pot-meters at the LOT.
Entered the Service Mode to Samsung and adjusted all the data parameters as specified in the service manual. Adjusted the Focus to get picture at maximum clarity. That’s all.
It took around 3 hours to finish this work. It isn’t a matter. Some work may be easy; the others will be time taking.
Voltages at the terminals of horizontal drive transistor.
Collector > 12.29V
Base > 0.531V
Emitter > Ground.
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To check the reset circuit, the reset output is low to discharge the delay capacitor (=Cd). If it’s less than Vo1-0.25V. And the reset output is high when the delay capacitor voltage linearly increased by the internal current source(10µA) if it’s more than Vo1- 0.2V. The equations of delay time are same as below. Trd = (Cd × 2.5) / 10µA. These parameters, although guaranteed, are not 100% tested in production.
The KA7632/KA7633 [ FAIRCHILD] is a multi-output positive voltage regulator designed to provide fixed precision output voltages of 3.3V, 8V (KA7632) / 9V (KA7633) at current up to 0.5A and 5.1V at current up to 1A with external PNP transistor. An internal reset circuit generates a reset pulse when the output 1 decrease below the regulated value. Output2 & 3 can be disabled by TTL input. Protection features include over voltage protection, short circuit protection and thermal shutdown.
> Output Currents up to 0.5A (output1 & 2)
> Output Current up to 1A with External Transistor (output3)
> Fixed Precision Output 1 voltage 3.3V ± 2%
> Fixed Precision Output 2 voltage 8V ± 2% (KA7632)
> Fixed Precision Output 2 voltage 9V ± 2% (KA7633)
> Control Signal Generator for Output 3 voltage (5.1V ± 2%)
> Reset Facility for Output Voltage1
> Output 2,3 with Disable by TTL Input • Current Limit Protection at Each Output
> Thermal Shut Down.
To check the reset circuit, the reset output is low to discharge the delay capacitor (=Cd). If it’s less than Vo1-0.25V. And the reset output is high when the delay capacitor voltage linearly increased by the internal current source(10µA) if it’s more than Vo1- 0.2V. The equations of delay time are same as below. Trd = (Cd × 2.5) / 10µA. These parameters, although guaranteed, are not 100% tested in production.