"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

"EADGBE" wrote in message
...

Arfa:

For what it is worth, there is continuity between
the contact that touches the SLEEVE of the
female connector and one of the turntable's
motor mounting screws. There is also very
low resistance.

I couldn't get any kind of reading from any
part of the signal wires or the external
ground wire....strange!

So based on these observations, I'm
leaning toward the assumption that the
centre of the female connector has to be
positive (+), and the sleeve is negative (-).

That would be my thought also. Sometimes with turntables employing DC
motors, the signal ground is not commoned with the DC ground, to avoid
introducing motor brush-gear noise into the following amplifier. A motor
mounting screw going to the sleeve connection on the socket is pretty
conclusive though, as the case of the motor usually has a tab down by the
connector board, which solders straight to the "-" connection.

Arfa