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marik1234
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RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 12:04 am

Hi. I also have a problem with a low TX signal as per the closed topic: viewtopic.php?t=56747

I have about twenty such boards for possible repair (unfortunately no more warranty).
Literally nothing helps, so I recognize that the problem is hardware.
Is it possible to repair these boards? What could be the cause of the malfunction? Replacement of the processor?
I would like to repair them, because they used to work very well.
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 11:13 am

[…] (unfortunately no more warranty) […]
The device have 10 years, is time to change it.
 
marik1234
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 11:53 am

What could be the cause of the malfunction?
 
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 1:44 pm

Generic question, generic reply: anything.

And honestly, if a specialist does not check it, it can really be anything.
 
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 4:47 pm

You can try emailing support if they could give you some details.

But generally if it affects just TX power (and RX sensitivity is fine) it must be caused by PA or possibly antenna switch chip. Because signal goes from antenna to switch and then there are separate RX and TX paths. On RX path there is LNA and on TX is power amplifier.
On the PCB switch is S700 and U702 is power amplifier. The RX path is probably to the right (fat PCB trace) with small SMD speck being LNA.
U702 PA could be faulty or there could be something wrong with supporting circuitry so it can't run at full power (voltage regular issue? caps? hard to say).
If you have spectrum analyzer or power meter you can check if the TX power is still regulated when value is changed (but lower than it should be... but in that case PA chip is probably working) or if there is no regulation at all (PA is faulty or not getting commands to control power).

But all this is just a guesswork, it would help if support could at least tell you what the original problem was (which chip is faulty). These chips can be swapped with hot air and a bit of patience, for example from some other board with dead ethernet... but doing so blindly without knowing the real cause would be just a waste of time.
 
marik1234
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 6:45 pm

@r00t
Thank you very much for your valuable tips. Will try to replace such a system, I have hot air and flux. I would like to repair these boards and share if successful.

I noticed that the Tx signal is exactly the same whether the antenna is connected or not. Rx signal after disconnecting the antenna decreases.
 
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 9:16 pm

I noticed that the Tx signal is exactly the same whether the antenna is connected or not. Rx signal after disconnecting the antenna decreases.

This sounds like a faulty wireless PA to me.
 
marik1234
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Wed Aug 24, 2022 10:38 pm

This sounds like a faulty wireless PA to me.
Yes! :-)

I replaced the U702 chip with another one from the board after the storm and the signal did fine! :)

The culprit was the circuit (amplifier):
MSC
5530
133M
Is it possible to get it somewhere e.g. on aliexpress?
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r00t
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Thu Aug 25, 2022 2:49 pm

Congrats on successful fix!

PA is https://www.microchip.com/en-us/product/lx5535
If you search LX5535 on Aliexpress you will find some, best deal is for 20pcs for $20, but even 10pcs for $15 is OK...
 
marik1234
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Thu Aug 25, 2022 2:55 pm

Thank you :)

But in description is Freq (GHz) 2.4-2.5. Will it work?
 
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Thu Aug 25, 2022 3:34 pm

This one (LX5530) is for 5GHz: https://www.microchip.com/en-us/product/LX5530
 
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Re: RouterBoard RB711-5Hn low TX power how to fix?

Sat Aug 27, 2022 5:52 pm

Choose one depending on your board: LX5530 is for 5GHz board, LX5535 is for 2.4GHz version RB711.

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