"EADGBE" wrote in message
...

I picked up an Ariston Q Deck turntable at a thrift shop recently, but
it did not come with its 12v wall AC adaptor.

The back of the unit simply says "12V DC 350mA" but gives no
indication of the polarity of the AC adaptor. I can find a 12V DC
adaptor that can supply the recommended mA output, but I want to be
sure about the polarity.

The socket for the adaptor is the standard type round hole with a male
connector in the middle.

MY QUESTION: To properly power this turntable, what polarity should I
look for in the AC adaptor's female connector---in other words, what
part of the female connector should be positive (+) and what part
should be negative (-) ?

Inside hole positive and outside sleeve negative, or the other way
round?

Many thanks in advance.........

When these DC connectors were first around, the convention was (oddly) "-"
to centre. In more recent years, the convention is almost invariably "+" to
centre. It is very possible that the turntable signal ground will be shared
with the DC supply ground, so as a first move, just try your meter on its
lowest ohms range, and see if the socket outer is common with the signal
cable screen, or with the separate signal ground wire (usually black) if one
is fitted. Any metal parts on the deck, will also almost certainly be
connected to the DC ground.

Failing this, you wil need to go inside the deck. The motor is usually a
'pattern' cassette-type motor, where the terminals are clearly marked "+"
and "-", ( "A" and "B" might also be there to go out to external speed-set
pots for 33 and 45) so it's just a case of following the "-" marked terminal
back to the socket. The "+" marked one will route back to the socket via the
power control switch(es). There may be an electrolytic cap across the power
leads also, for decoupling purposes. The polarity markings on this, if
fitted, would also tell you the correct way round.

Just as a matter of interest, these motors are pretty robust against
reversed polarity, at least for short periods. All That normally happens, is
that the control electronics mounted inside the motor, go berserk, and just
run it at high speed, backwards.

Arfa