Repair CentreCOM MX26F 10Base-FL Ethernet MAU

CentreCOM MX26F


My unit behaves completely dead. With cables attached the "LINK MONITOR" LED stays off.
The AUI-Interface supplies power to the MAU on Pin 6 (GND) and Pin 13 (+12V).
Attached to a power supply the measured idle current to the MAU was zero. Likely a blown fuse.

This page contains some pictures and hints how to disassemble the unit (some screws are under the labels and not visible).



There is one visible Philips type screw between the optical connectors. Two other screas are hidden under the label on the top of the unit.
The pictures below show the position. It is possible to pull the very thick top label from the case with minimal damage.

Partly remove top label

Use a knife and a screwdriver to lift the top label on the side with the AUI connector.

Remove screws

First remove the screw between optical connectors.

Then turn out the two screws under the label:
Screws under toplabel
(Click to enlarge)

Screw under toplabel
(Click to enlarge)

Now pull on the bottom cover (below the AUI connector) to open the unit.
Identification plate
(Click to enlarge)

Top view:
Open, Top view
(Click to enlarge)

Open, Top view
(Click to enlarge)

Side view:
Open, Side view
(Click to enlarge)

Find and test fuse

The fuse is easy to find, simply follow the copper trace from Pin 13 (5th pin of the second row) of the AUI connector:
Blown fuse
(Click to enlarge)

Supply 12V power to Pin 13 (+) and Pin 6 (-) to verify that the fuse is the problem.
Measure the voltage at the capacitor C11 (left/bottom on the picture above):
If you don't measure the supply voltage, the white 1A SMD fuse is blown (this was the case on my unit).

Replace fuse

To test whether the PCB is still working, I connected an external fuse:
Glass tube fuse
(Click to enlarge)

Now the MAU is working again as expected.
It is unclear why the original fuse was overloaded. Maybe the reason was an unallowed hot-plug event (and the current to load the capacitor C11 and the capacitance of the intermediate circuit of the DC/DC- converter has blown the fuse with very fast-acting characteristics).

The SMD fuse has red glue on the bottom side. To remove it, heat one side with a soldering iron and use a slotted screedriver to break off the fuse from the PCB.
Note: Because of the J-Lead type of the case, the PCB pad on the opposite side will not be damaged (see left pin on picture below).
Removed fuse
(Bottom view of blown fuse, Click to enlarge)

Now a new SMD fuse is required.
You need Littlefuse Series 459 "PICO SMF" (1A very fast-acting).

Mounted replacement fuse:
Replacement fuse
(Click to enlarge)


Case parts, sleeves and screws:
(Click to enlarge)

Pull up the top label again and insert the two long screws:
Case without screws
(Click to enlarge)

Screws inserted
(Click to enlarge)

Now push back the two long screws as shown on the pictures below:
Screws pushed back
(Click to enlarge)

Screws pushed back
(Click to enlarge)

Now insert the bottom part of the case on the side with the optical interface first:
Insert bottom case

Now turn in the two long screws.
Then turn in the small screw.

The top label is very thick and the glue is still sticky.
The unit still looks good with only minimal damage top the top label:
Insert bottom case

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