Is it safe?

My DSL modems 9V adaptor burnt so I'm using another adaptor (output 12V) for
the modem and its working fine!
Just wanted to know if this may damage my DSL modem?

 

Depends on its circuitry and what sort of filtering and regulation it
has. It MAY be okay, but then again it may not. Consider that
accelerated life tests sometimes stress unit with higher applied voltage
and higher temp.

 

In all likelihood, what's happening internally is that there is a voltage
regulator which is running noticeably hotter than before, and is coping
OK for now. Probably the whole thing will age faster than before, or it
might just go pop one day.

OTOH I've had some cheap far eastern network gear which was sometiumes
delivered with 9V and sometimes with 12V supplies, and the 12V versions
ran far more stably for a long time.

If it works, and it's a cheap router, use it. If it goes pop, buy another
one.

 

Pop it open, trace the power circuit, see what the parts in it are
rated for. You didn't mention whether either adapter was regulated or
not, and what their current ratings were vs the rated power
consumption of the modem instead of just peak requirement, which if
you're lucky will be in it's manual and somewhat accurate.

 

I Don't know - is it safe? Hindu

 

It will be OK until it stops working.