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| Riccardo Omage |
Model: 28WX65ES chassis ICC19
Schematic: download from www.eserviceinfo.com
This new client was sent to me by a friend of a friend, he had previously taken the TV to another Technician for repair and was told that it was the "flyback transformer" at fault and it was impossible to find the spare part. I have no faith in a diagnoses done by others (I think it was probably an excuse). As we know even if Thomson is no longer on the market, HR Diemen always have EHT available, the same for other producers of HT transformers. My doubt was that the TV had been ulteriorly damaged. Apart from the fact that it had been inspected by others the original fault seemed to be not one of the easiest.
The TV is blocked on standby, when switched on the red led lights up but with no attempt to switch on.
All voltages exiting the secondary supply section are regular, but the pilot line signal is absent.
As a consequence I doubt that the microprocessor (or "microcontroller" as Jestine says) is at fault because I have no response from either frontal keys or remote control.
On observation I notice a not original "jumper".
On removal at least the TV makes an attempt to come to life.
The oscilloscope shows me that for a few seconds the pilot line appears on the base of transistor TL001, then also on pilot transformer pin 1 LL001, but nothing on the base of the final transistor TL030; why is this so?
All voltages exiting the secondary supply section are regular, but the pilot line signal is absent.
As a consequence I doubt that the microprocessor (or "microcontroller" as Jestine says) is at fault because I have no response from either frontal keys or remote control.
On observation I notice a not original "jumper".
On removal at least the TV makes an attempt to come to life.
The oscilloscope shows me that for a few seconds the pilot line appears on the base of transistor TL001, then also on pilot transformer pin 1 LL001, but nothing on the base of the final transistor TL030; why is this so?
Explanation:
If cap CL005 is totally open and is not smoothing, nothing passes on the main of the small transformer because the two terminals of the primary spool always have the same potential, there is no signal on the secondary coil.
Needless to say that the replacement of capacitor CL005 cap (470mF-25V) finally resolved the problem, in fact the television restarts to operate normally.
If cap CL005 is totally open and is not smoothing, nothing passes on the main of the small transformer because the two terminals of the primary spool always have the same potential, there is no signal on the secondary coil.
Needless to say that the replacement of capacitor CL005 cap (470mF-25V) finally resolved the problem, in fact the television restarts to operate normally.
Why the jumper?
My considerations are the following…
Perhaps to put in greater difficulty who would eventually have had to repair the TV? Certain colleagues use the oscilloscope, since going digital it seems to be the most useful instrument for fault finding even if it is more time consuming, others maybe hope for a bit of luck?
My considerations are the following…
Perhaps to put in greater difficulty who would eventually have had to repair the TV? Certain colleagues use the oscilloscope, since going digital it seems to be the most useful instrument for fault finding even if it is more time consuming, others maybe hope for a bit of luck?
In any case, the jumper had been inserted between pins 5 and 6 of the chip IP140 (TDA8139) which operates as a double voltage stabilizer and is found on the secondary supply. Pin 6 is related to the reset function (UP_RES) and goes to the integrated Microcontroller (section R). That is why all functions were blocked. Once the jumper was eliminated the TV was no longer in standby and the pilot line appeared even if only for a few seconds. With the remote control I managed to switch off the front red led, but it then re-activated, at least after this operation I saw that the television gave signs of functioning.
This was followed by a normal repair.
This was followed by a normal repair.
