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 micro-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.