The (Excelltel) MK-208 PABX (and alike) is a modern
local telephone analog exchange for small offices. "Modern"
because it was built after 2010, which is strange since plain old PSTN
analog phones were already declining in favor of VoIP. "Small offices"
because it features up to 2 incoming lines and 8 internal branches.
There is a whole series of these PABX with different combinations of
incoming vs branches line, and they all fit in the same case.
 |
| MK-208 boards |
Why
did I buy such a thing? Because I want to be able to recreate an analog
telephone line that can ring those old phones, run a BBS, transport a
FAX, all natively without passing through VoIP codecs. This is going to
be used in a public interactive exhibition. With 8 branches of the
MK-208 there can be up to 4 simultaneous calls.
The
PABX has a nice case and its weight might induce you to think it is
metallic for heatinking purposes. As it will turn out, it's not.
The
PABX powers up and the green status LED blinks. This is normal, whereas
you would expect a solid light. I connected two phones, they rang and
communicated but the line was extremely noisy. No, not the kind of noise
caused by a rusty connector. It was a strong hum. Ouch. The hum/noise
increased when I picked up the second phone. Ouch ouch. But there is
hope: it might be a problem in the internal power supply, even if a
poorly designed product would not the THAT bad (and the Internet would
be full of bad reviews, which is not).
Let's
look inside. The nice heatsink-shaped case is made of plastic, thick,
but plastic. The power supply area is easy to locate, with the
transformer and the regulator heatsink in good sight. Needless to say,
there are three bulging electrolytic capacitors. This unit is 10 years
old, it might have been in service non-stop for 8 years. Probably it
gets quite hot as there is no fan and the slots for air cooling are very
small (to my under experienced eye).
 |
| Close-up on failed capacitors. |
I
will replace the failed capacitors and those that were subject to too
much heat. If this fixes the hum, I will also drill holes in the case
right above the power supply area to provide a well needed escape to hot
air. In between the two I should measure the temperature inside to
confirm that extra holes actually help.
