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M Philbrook

In article ,
says...

I have a Netgear cable modem. It is powered by a Jentec Technology AF1205-B
power adapter. The adapter failed and I replaced it with another adapter but
of course I was curious what the fault was. I opened up the case and found a
bad (bulging) 1,000uF 10V filter cap. I did some other circuit tests to make
sure there were no shorts because I was getting extremely low ESR readings
(less than 0.1 ohms) around the bad cap, Eventually I found an SMD zener
diode that was shorted. There are no identifying marks on the diode other
than its color which is the standard looking orange. I am fairly sure it's a
zener because the pc board has it marked as ZD-2. Its location in the
circuit is in parallel with the 5 volt power supply diode that comes off of
the secondary of the switching transformer. The power diode has two sets of
numbers. The top row is marked 540 (perhaps an SB540?) on the bottom row is
marked 849. That diode is ok. Only the zener is shorted. I was wondering if
there was some ballpark zener voltage that I could use to replace the old
one. It's always good to have a spare 5V supply.

Thanks for your replies.

THat is a protection device.. could be a MOV but it sure is a protection
device design to clamp if voltage exceeds, that is, if it is truely
across the output..
The caps are bad and caused higher than desired voltage from the
switching signal, thus shorted the protection device. Basically, it did
its job.

You may be able to get by using a 5 watt zener but you still need to
replace the caps.

btw, it seems a very common problem for the switching wallwarts to fail
in this manner for routers, I replaced my 3 times, now I have a
transformer 60hz type and works great...

Jamie