Asus RT-N66U Dark Knight Router [SOLVED]

[Armadillo]

Dear Community of Experts;

I am attempting to repair the ASUS Router. The 1.8V step down IC appeared to be dead, all other voltages from other step down ICs are OK.

This 1.8V rail is step down by the FR9706 IC Chip. There is no pulse on any of the pins. I have replaced the chip with a new one but same result. I have isolated the circuit by removing the smaller inductor just above the capacitor but still same, no pulse.  The B240 schottky, 15uH inductor, the capacitor at the output all checked OK. There is no service manual from ASUS for any of its router to aid in the troubleshooting.

Unfortunately, it seems like there is no datasheet for FR9706. All other chip from FITPower can be downloaded except this chip. It appear that the chip disappeared from this world. The pinout of this chip appeared to be very much different from FITpower series of bulk converter.

Are you familiar with ASUS router. Would appreciate your help. Thanks in advance.

[Ton]

Hi

try this link http://www.datasheetarchive.com/FR9706-datasheet.html#

or the attached datasheet :-)

dont know anything about your router - sorry

BR Ton

[45Overload]

Oh, please!  Surely we are talking about a much simpler LDO regulator....aren't we?
http://www.intersil.com/en/products/power-management/linear-regulation/linear-regulators/ISL80510.html?utm_source=google&utm_medium=cpc&utm_campaign=New-Linear-Regulators-NA-Search&gclid=CJehg-rjqdECFQNqfgodiQsFOA

Armadillo, you say there are no pulses on any of the pins.  What about d.c. volts?  What makes you believe this 8-pin chip is something more than a simple buck switching regulator?

-45O

[wraper]

Oh, please!  Surely we are talking about a much simpler LDO regulator....aren't we?
http://www.intersil.com/en/products/power-management/linear-regulation/linear-regulators/ISL80510.html?utm_source=google&utm_medium=cpc&utm_campaign=New-Linear-Regulators-NA-Search&gclid=CJehg-rjqdECFQNqfgodiQsFOA

Armadillo, you say there are no pulses on any of the pins.  What about d.c. volts?  What makes you believe this 8-pin chip is something more than a simple buck switching regulator?

-45O

Big inductor besides, diode,  it and the datasheet in the post above yours.

[tru]

According to the datasheet on 2nd post, the IC is a step down taking input voltage of 4.75V to 23V.  But you say it steps down from 1.8V?  Not possible.
There is a typical application circuit that is likely the exact circuit.

Things I would check:
1. whether there is voltage in >=5V (probe pins 3 and 5)
2. whether the resistors are ok
3. the output diode
4. the inductor
5. the capacitors

You may have to desolder some of them, but in circuit at least you can check for open on resistance (use a good meter for this, e.g. fluke, brymen, etc), do not use a UNI-T!!

[Armadillo]

According to the datasheet on 2nd post, the IC is a step down taking input voltage of 4.75V to 23V.  But you say it steps down from 1.8V?  Not possible.
There is a typical application circuit that is likely the exact circuit.

thanks for your input.

The input is 19VDC bulk convert to various voltages. A description of it can be found in http://www.natthapol89.com/2014/03/asus-rt-n66u-b1.html [in Thai]
Specifically it is from 19VDC to 1.8VDC as used by various of the processors and memory chips.

[wraper]

Check the voltage on EN pin. As you changed this IC and it still does not work, most likely, there is just no enable signal. And therefore, an actual fault is somewhere else.

[Armadillo]

Check the voltage on EN pin. As you changed this IC and it still does not work, most likely, there is just no enable signal. And therefore, an actual fault is somewhere else.

Thanks wrapper, the Enable pin is tied to a Delay start circuit formed by a resistor and capacitor in series over the 19V lines. The pin read 19Vdc.

However,

There is one thing I noticed. As Ton provided the datasheet, I am able to trace the circuit and noticed that a 0402 high side boast capacitor is missing [left of IC and hugging it]. I must have braided it off during the post pad desoldering process of the FR9706. But I doubt one small tiny 0402 capacitor will make me detect zero burst/pulse at any one of the pins since the OSC is a independent circuit.

Here are the Pins voltages;
Pin1 = 0.03;
Pin 2 = 1.65
Pin 3 = 19
Pin 4 = 0.086
Pin 5 = 0
Pin 6 = 0.048
Pin 7 = 0.27
Pin 8 = 19

In case you are wondering, Pin 1 voltage is correct, as other similar IC [SS = PIN 8], like the FR 9888 and the FR 9886 measured same voltage at the pin. I reckoned that my fluke meter over-loaded the 6uA, somewhat?

Ton provided datasheet traced almost identical to the circuit. According to the datasheet, the capacitor is 10nF. I have no intention of stocking 0402 capacitor and have absolutely no thrill in working with tiny creature. Will update once I can land my hand onto the 0402 cap.

[eevbstedt]

Update?

[Armadillo]

Update?

Repaired and problem solved. The 1.8v supply need to start up otherwise the whole unit will hang. It appears that the power supply voltages start up according to sequence are monitored and managed. Once the 1.8v rail is repaired, the unit starter up properly without any problems. Replaced the IC and the missing cap.

[eevbstedt]

Thanks for the quick response!

You replaced U3? ... and which cap? 

[Armadillo]

Thanks for the quick response!

You replaced U3? ... and which cap?

I would suggest you change the SS cap first, actually don't even need to change U3. I changed U3 because I already bought it.

Edit: The attached show the cap location, is one of the 2, can't remember which one. You can trace it yourself.

[eevbstedt]

Is that the lower (smaller) cap just below the U3 in the pic?


Edit:  Thanks for the update, I'll figure it out if needed.

Cheers!

[Armadillo]

Is that the lower (smaller) cap just below the U3 in the pic?

See the attached, Its one of the 2. You can trace continuity to pin 1 yourself.

Edit: The 0402 is really small. I think you better use a very fine tip soldering iron.

[SoundTech-LG]

(0201) is really small...

[Armadillo]

(0201) is really small...

I did used a microscope and a soldering pencil. Those smd stuffs are necessary.

[texaspyro]

(0201) is really small...

A friend of mine bought some SMD caps, but did not read the fine     print.  They were 01005 caps.  I think that he had them on a sheet of paper on his desk and sneezed.  They are sill floating in the air.  Ever try to hand solder a grain of salt?

[SoundTech-LG]

The background PCBA has 0.100" header mounting holes for comparison to the foreground 1005 components. Sheesh! they'll be asking for removal, and swapping these any day...

[Armadillo]

0402 = 1005
Yes all these are small but fortunately they are widely spaced.

[texaspyro]

0402 = 1005
Yes all these are small but fortunately they are widely spaced.

These were definitely imperial 01005... 1/4 the size of imperial 0402's.   I think he thought he was buying metric 1005's.

[boobai]

Hi all.  I really want to get my RT-N66U working.  It just died. No lights.  Power supply fine. Power switch fine.  I get 3v 6v and 19v at the capacitors below. Any other ideas?  

(When I put my finger across the back of the first and last LED, it does softly glow blue). 

(I have a 2nd same router, but I want to get the settings off of the first one.)

Thanks