Repair D-Link ADSL Modem, Router, Ethernet switch, Wireless AP

Date: September 2013

Problem:

This D-Link has a strange problem, any computers connected to the ethernet switch cannot connect to the Internet, but they can by connecting to the wireless (WiFi). Resetting and power cycling the unit did not solve the problem. The computers connected to the router by ethernet cable could ping the web site, but not connect. Eventually, the unit does not even detect the ethernet cable plugged in.

Diagnosis:

This D-Link has the opposite problem of a Linksys ADSL modem, router, ethernet switch and wireless access point that I repaired before.That Linksys router had the problem of computers not being able to connect to the internet through its wireless connection. I replaced some capacitors and the wireless function is restored.

More than a year ago, I already replaced two bulging 25V 1000uf capacitors in this D-Link unit. This time, there are no obvious bad capacitors from visual inspetion. Since capacitor often fail over time, it is also possible the electrolytic solution dry up in some capacitors with no physical signs of damage. I suspect it could be the smaller capacitors, they tend to perform high frequency and more specific filtering functions, although they generally last much longer than the bigger power filtering capactiors of higher capacitance like 16V 1000uf, 16V 470uf, 25V 1000uf, etc.

I thought the most likely cause of this problem are either the 25V 100uf capacitors, generally used for de-coupling, or the 25V 47uf capacitors.

 

Repair:

I decided to replace all four 25V 100uf capacitors with Rubycon brand ones.

Result:

My first try fixed the problem. By replacing all the 25V 100uf capacitors, the ethernet switch function is back to normal and all LAN computers can connect to each other and the internet again.

Here are some common capacitors and their values used in computer networking equipment you might want to check first when something goes wrong:

General instability and erratic symptoms: Power filter capacitors, 16V 1000uf, 25V 1000uf, 16V 470uf.

Wireless (WiFi): 35V 220uf

Ethernet: 25V 100uf

ADSL modulation Signal-to-Noise Ratio (SNR) unstable that causes frequently dropped connection that you are 99% certain is not caused by the line itself: Replace 25V 47uf capacitors could help to give more stability. (Not guaranteed, when it comes to the line condition issue, the only real way to know is to take another modem known to be good condition to test the line and see if the same SNR problem persists.)

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