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[SOLVED] Receiver - LNB Short circuit, only receiving Horizontal Transponders.

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ohdev83

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Hello there,
the reason why i want to fix it is just for fun and for learning, to fix, debug and read circuits and know what parts are defect

i have a Dvb receiver, its a chinese brand so its not important, that's why it does not have protection for short circuit on the LNB input.

anyway i accidently short-ciruited the LNB cable, which resulted in the receiver being only able to receive the Horizontal transponders.

anyway how to fix the circuit, what to search for and replace,

if anyone has any simple guide on debugging circuits in general, like how to know if a diode, transistor, etc is defect, and how to read a circuit,
because im a noob, and all i know about electronics is very very basic, and its a hobby afterall.

thanks in advance :p
 

betwixt

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I can't help with specifics without a schematic to work from but it sounds like you have a voltage switched LNB and one of the voltages is incorrect. As you shorted the supply from the receiver box, it's unlikely the fault is in the LNB itself.

There are two control signals sent from the receiver to the LNB, they are both sent up the cable where the LNB makes a decision on frequency band and polarization. As most LNBs can't cover the entire satellite band in one sweep, they split it into two, a high band and a low band although there is usually some overlap between them. The band switching is done by super-imposing a low level 'tone' on the co-axial cable, normally at 22 KHz. The polarization switching is achieved by changing the supply voltage applied to the cable. Typically it changes from about 10V to about 16V, a detector inside the LNB sees which it is and switches antennas accordingly. The LNB receiver circuitry normally works on 5V and is regulated from the incoming voltage so it remains constant regardless of which voltage is on the cable.

So it looks like shorting the supply has damaged the circuit that selects the voltage applied to the cable. That's where to start looking. If you connect a voltmeter across the LNB connector on the box and change channels between one you know to be horizontal and a vertical one, you should see a distinct change in voltage .

Brian.
 

ohdev83

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thank you very much Brian,

I'll start by testing the voltage and let you know the results.

the LNB itself is OK, as i tested others.

the Receiver from inside has two seperated boards, connected through a cable, one is for the power supply, and the other is for the signal, chipset....

i will also test the output of the 3 different voltages the power supply board outputs and see if there is something wrong with it.

Just a question, the curcuit that selects the voltage, is it something i can play with/replace or is it a chipset that's responsible of that?
 

betwixt

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The LNB probably draws around 100mA current which is fairly high so it is unlikely to be produced directly by the chip set but the voltage does have to be controlled per channel so the circuit is probably driven by the main microprocessor. Most likely, there is a small regulator circuit on the main board which has gone short circuit. If it shorts, the voltage would be stuck at high level which won't harm the LNB because it has it's own regulator but it would make it think only one polarity was being requested.

The steps I would take are:
1. remove the power cord so the unit is isolated from the AC supply, then remove the LNB connector ('F' plug) from the rear of the receiver.
2. Open the box and gain access to the pins on the screened module behind the LNB connector.
3. Using a test meter, measure the resistance between the center pin of the connector and each of the module pins. You might have to push a wire into the connector to make good contact. You are looking for the pin with a low resistance connection, probably less than 50 Ohms and maybe even a short circuit. This is the pin where the voltage to the LNB is inserted before going up the cable.
4. Follow the circuit track from that pin along the circuit board, it should go the the damaged regulator circuit. There is a good chance it will lead to a transistor which has shorted out.

Brian.
 

ohdev83

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I think you are right about the transistors, i will start with that,
because i can only see two transistors surrounded by some Diodes, so sure this is the circuit you are talking about.

PS: to be able to test the transistors, should i desolder them first?
and what it the easiest way to test it using a simple Multimeter?

---------- Post added at 18:07 ---------- Previous post was at 17:45 ----------

4. Follow the circuit track from that pin along the circuit board, it should go the the damaged regulator circuit. There is a good chance it will lead to a transistor which has shorted out.

Brian.

Well, i desoldered the two transistors, and using the multimeter, and using the help of a page of how to test the transistor which is a pnp type,

one is responding OK, and the other is totally dead!
so this is the cause? not allowing any power flow?

thank you very much!
 

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It could be the problem but without a schematic it's difficult to predict what each faulty component might do. You should be careful that you are actually testing a transistor, not everything with three legs is a transistor! Can you see any markings on it, or, can you photograph it close-up so I can try to positively identify it. What is the receiver make and model? Sounds like you are on the right track though!

Brian.
 

ohdev83

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It could be the problem but without a schematic it's difficult to predict what each faulty component might do. You should be careful that you are actually testing a transistor, not everything with three legs is a transistor! Can you see any markings on it, or, can you photograph it close-up so I can try to positively identify it. What is the receiver make and model? Sounds like you are on the right track though!

Brian.

Dear Brian, good news, it was the transistor, a ss8550 PNP type.

replacing it fixed the issue :), I'm so happy, not because i need the receiver, i have two other, but because i was able to fix it and of course i would not have done it without you, because I'm a noob.

thank you again, and have a nice day further!

---------- Post added at 23:47 ---------- Previous post was at 22:58 ----------

dear Brian, if you can help with the following thread, i would be gratefull.
https://www.edaboard.com/threads/245578/#post1050866
 

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