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David Farber

M Philbrook wrote:
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

Hi Jamie,

I did replace the faulty cap. As far using a 5 watt zener diode, the
original one is 3mm long and 1.4mm in diameter. That would seem to be in the
under 1 watt category. Also, please see my corrections to my original post.
The zener diode is paralleled by the filter cap, not the power diode.

Thanks for your reply.
--
David Farber
Los Osos, CA