TDA8374A (Chroma)_STR S6707 (SMPS switching)_LA7830 (Vertical Scan Output IC)_ Sharp PQ 09RE 11 (9V controlled voltage regulator IC) _ 2SD1877 (Horizontal Scan Output Transistor)_NX1733/m (FBT/LOT)
Set received for repair with no picture condition; today.
On power ON, the standby LED blinks 3 times and then shuts off.
As the standby LED blinks, at least once time, it is clear that the SMPS section circuit to this set is working properly.
But, the main system control mcro-computer IC detects something short circuit with any component/s at the main board.
As the set is a very old model, the spare part availability will be somewhat difficult. So; I should take much care with work; not to damage more components.
I opened the back cover. A visual inspection is done for any damaged component/s on-board. Such as bulged/shorted capacitors, damaged IC/s and such.
I found that the vertical scan output IC is cracked. I’ve attached the photo of the same here.
Checked all other main components, which can likely to be faulty; using multi-meter. Checked the Horizontal Output Transistor for any leak with its junction The Horizontal Output Transistor used with this set is D1877. I did notice, it is the original one fitted by the company. Nobody have replaced it. We can detect this by inspecting its solder terminals.
Yet, de-soldered it out from the circuit board, and checked it alone using meter set to KOhms range. No abnormal measurement noticed. De-soldered it into place again. Screwed it on to its heat sink.
De-soldered out the damaged vertical scan output IC from the circuit board.
Switched ON the set without it.
The standby LED lighted up once, then 3 times and OFF, as before.
[With this set’s circuit, it won’t switch ON properly, or produce EHT or even a horizontal line will appear on the screen, as other company’s sets will; except the +B voltage [112VDC] only will be present.
I did check the +B112VDC. It was OK. No fluctuations. Checked the voltage at the primary winding of the line drive transformer, and at the collector terminals of the Line Drive Transistor. All found OK.
Switched OFF the set, and checked all the solder terminals at the solder side of the PWB, once more; using a magnifying glass.
After some time, at about 25 minutes, I detected a minute loose solder terminal at the 9V voltage regulator IC. This IC has 4 solder terminals. It is a controlled voltage regulator, which in turn controlled by the main microcomputer IC. If the microcomputer IC detect any short circuit anywhere at the circuit board, it will shut down the control voltage to this IC; and it will down the 9VDC, which goes to the main chroma oscillator jungle IC [TDA8374A], and that was happening here.
But here there were two faults. The loose solder terminal at the regulator IC said above (IC521 Pin-1). The vertical scan output IC (LA7830) short.
Re-checked all the solder terminals at the main board to make sure that there are no other loose solder terminals.
Set switched on; fed to RF signal [On-Air]. Tuned it. OK.