Testing g4 powerbook barrel style ac adapter with multi-meter

Question

I have an old G4 powerbook with a dead battery but it doesn't have an ac adapter.
I bought a cheap replacement one on Amazon. When I plugged it into the G4 nothing happens. It doesn't charge the battery and I can't power on the powerbook.
I wanted to make sure that the adapter I bought was providing the 24v that it should.
The problem is I can't figure out how to test it. It has the barrel style connector with the outside sleeve and a pin on the inside that looks like a male headphone jack.
Does anyone know what is the positive and what is the negative?
I tried testing with the probes on the outside sleeve and the tip of the pin but I get no reading. The pin in the center looks like it's a tip,ring,sleeve type of connector. I tried to test the tip and the ring but also got no reading. If I bridge the ring and the sleeve (that's the sleeve of the tip,ring,sleeve center pin, not the outside sleeve) there is a spark.
I've never seen a power adapter with this type of connector before. I'd like to find out if it is working or if I should return it. If it doesn't work then there may still be a chance that the powerbook is salvageable with a good adapter.
Thanks.

Answer 1

There were several adapters made for the PowerBook G4 and iBook G3/G4 series
and they varied in power output per the devices they were intended to function with.
You could use a higher power output (a 65W for a 45W replacement) though, and
those cheaper clones that have no illuminated DC-out plug provide adequate power.
OWC macsales & others offer generic replacement that did not look like the original.
(Also, they usually have exact replacements for the newer magsafe design, only.)
http://eshop.macsales.com/item/Newer+Technology/APLPBI65W/  - 65W output
For the correct utilitarian purpose, the larger display model computer would need
to have the higher output AC Adapter, so it can power the Mac & charge it too.
An internal optical drive and other items, in addition to a 14" display, use more.
•PowerPC-Based Mac notebooks: Identifying the right power adapter and power cord--US - Apple Support
For instances where an original jack fails, or adapt a different power supply; note pin-outs:
http://faqintosh.com/risorse/en/guides/hw/ibook/pjack/
http://pinoutsguide.com/Power/apple_tibookpower_pinout.shtml
http://repair4laptop.org/wiki/Mac_iBook_G4_Power_Adapter_DC_Plug_Repair_Howto_Gu ide
You may note from reading these pages where someone took apart the power
supply DC-Out end to replace the jack, there is something odd about it. The
larger outer metal part is more of a guide to keep the item in the socket, to
illuminate the plug, and protect the small actual power contacts inside... A
third-party AC adapter without illuminated plug has one less active contact.
{And may not need as complex a chip inside the plug, either.}
How to repair (& understand how it works) a portable Mac G4 power cord:
http://www.instructables.com/id/EYVY1AXU2FEQZJIQHC/
A good multi-meter can tell you if you have Negative or Positive (reversed
power leads to meter) so that in itself should not be a problem...
In the pictured images within the linked sites, see how in concept of using a
replacement 2.5mm jack instead of failed OEM AC Adapter plug; the small
metal pin carries main current, not necessarily the outer barrel. So any voltage
(negative or positive) of the rated output that registers on a good multimeter
should be a clue, if anything is getting past a micro-chipped DC-out end...
In the original Apple AC adapter for PowerBook/iBook G4 this is so, not so
certain that any generic one not looking like the original would have this chip.
But the plug connectors you see, inside the barrel, would be the same power.
And the DC-in board is a minor chore into itself, a guide at iFixit.com shows methods
based on which PowerPC PowerBook model you have (iBook is also troublesome)
https://www.ifixit.com/Device/PowerBook_G4_Aluminum_Series
So, it looks like the two main connections are further down the center plug-pin
and the outer barrel is not a connection, so much, as it is a guide or protector.
Usually the three things that can be in need of replacement in the PPC portable
as well as the newer intel-based portable, are the DC-in board (magsafe board)
the battery, and the power adapter. To have known-good items including another
similar model Mac portable, on hand, helps immensely to troubleshoot these.
Not sure if what appears here will help all that much; the power adapter would
work on any 24v PowerPC iBook (dual-usb white) or PowerBook G4, so that
may be something if you know someone who has a working model of either.
Some of the older site URLs for information no longer work, so I looked online
in hopes of finding similar; but to no avail. So this is a patchwork from that effort.
One of the diagrams of how someone repaired the original dc- out plug by
replacement with a 2.5mm stereo jack, may yield which one(s) have power
and what amount. A generic without an illuminated plug, should be easier.
In any event...
Good luck & happy computing!