Vestel, Sanyo and Mivar and many others CRT TVs with a BU808DFI transistor installed in a horizontal scan are no longer included in the repair.
WHY ?
Unfortunately, these transistors have long been absent from the free market. BU808DFX is nowhere to be found.
It should be noted that the analogue - 2SC5388, also became inaccessible. Counterfeits have appeared on sale, and the prices for this transistor are growing.
It is also not easy to find the original BU808DFH on sale, which has a smaller size and power, but can work fine in these TVs.
You can distinguish the original transistor from the fake one using an ohmmeter by measuring the resistance between terminals B and E. If there is less than 50 Ohm in both directions, it is re-labeled from some cheap one, but similar in body. The original multimeter will show the resistance between the base and the emitter, usually 200-300 ohms.
How to replace a composite transistor in a line scan?
This issue has been repeatedly discussed on forums and is relevant to this day.
I have tried and tested two options:
1. Installation of a functional analogue of two transistors.
2. Installation of 2SD2499 with some changes in the circuit.
Attention! Important Note: In most cases, the BU808 breakdown is caused by the electrolytic decoupling capacitor in the 10uFx63V driver.
It is necessary to replace it with a new one that has minimum active resistance loss (ESR). Capacitor quality is critical.
Sometimes a breakdown occurs due to the formation of an annular crack in the soldering of the OS connector of the line coils.
1. Functional analogue BU808 - in fact it is a composite Darlington transistor.
The internal structure is shown in the figure below:
BU808DFI
When using a line transistor (Q2 in the diagram) with a snubber diode and a resistor, for example 2SD2499, it remains to add three elements to the circuit - transistor Q1, as a current amplifier, as well as a resistor and a diode in its base.
The value of the resistor R1 is 270-330 ohms. You can take any diode D1 with a maximum current of 1 A, preferably a faster one, for example, the HER or UF series. Transistor Q1 is best used in a small package, for example BU1508AX, there is no need to attach it to the radiator.
The entire structure of three elements is mounted with a canopy and attached to the terminals of the second transistor (Q2), according to the connections and according to the scheme.
Q2 is screwed into place on the radiator.
The conclusions of its collector and emitter are soldered into the standard contact pads.
This inclusion somewhat complicates the design, but does not change the circuit design. With proper and careful installation, excessive heating and other negative consequences are unlikely.
2. Installation of a bipolar transistor instead of a composite transistor is possible if a transistor with a sufficient current gain and good dynamic characteristics is used.
The original 2SD2499 was used for the experiments. Its fakes from unknown manufacturers, which are currently in widespread sale, do not work normally in this inclusion, they overheat within a few minutes. It is sometimes easy to distinguish a fake 2SD2499 from the original by the conclusions. The original one has thick tinned leads, while the counterfeit ones have flat ones, made of thin sheet steel.
Attention!
The replacement method was checked , but cannot be recommended to repairmen who do not have sufficient experience and theoretical knowledge.Due to the fact that this replacement option is somewhat unreliable, contradicts the manufacturer's circuit design and can be implemented only with individual types and instances of line transistors, it cannot be considered and recommended in detail, in order to avoid negative consequences when it is used by unqualified craftsmen or TV owners.
The use of a transistor with sufficient current gain and good dynamic characteristics is a must.
HERE the Necessary modifications in case of using the original 2SD2499:
- The quality of the decoupling capacitor in the driver is critical.
The denomination 10uF 63Volt was changed by me to 47uF 63Volt Panasonic/Matshushita.
- Choke in the driver - short-circuit.
- Remove the standard BE 47 ohm resistor installed on the board.
- Install the BE diode, cathode to the base, anode to the body. Anything from 2A will do, preferably fast.
It is imperative to carry out temperature control.
Checked on more than 20 TVs with crappy Vestel 11AK30 chassis........SUCCESS !
The possibility of overheating is not excluded and depends on the properties of a particular transistor instance.
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