OK, You have observed enough to see that it takes 9v @300mA. with a center positive. Can't you use your multimeter to see if the center is positive with respect to the ring? If you can check batteries you can do this. If the MM is digital, and you put the positive lead in the center and the negative lead on the ring you should see a plus voltage. If the meter has a minus sign in front of the reading the polarity is wrong. With the power off, make sure the plug fits the unit also. Regards, Tom
If you know that the original power supply was un-regulated there is no reason to get one now. An unregulated adapter with the same data will work fine.
You can measure the voltage of a battery, you say.
When you do that, can you see a polarity indicator on the display? Can you see a difference if you shift the probe leads, does it show a minus sign if you put the probes on backward?
If you can see this, you know how to find the plus contact on a battery, and that is marked on the battery too, so you can check that your DMM works as it should. You will know after trying this that the red probe lead is the one you should use on the plus side. If you shift the leads you get negative voltage values.
Now you should be confident enough to measure the voltage that comes from the new adaptor, and find out what polarity it has.
In practical terms you may have to find a piece of wire you can insert into the contact, because the probe will not get into the contact.
Anyway, you fix the mechanical contact with both the inner and outer metal in the contact, and measure the voltage and polarity.
If it needs changing you can cut the cable and connect it again, with the other polarity.
When you measure the voltage of an unregulated adaptor without a load you will get a significantly higher voltage, like 16-19 Volt, and this is nothing to worry about as long as both the voltage and the current is the same as on the original adapter.
If this chess computer is very important to you you could, of course get a regulated supply, and test it before connecting it. That would remove this possible source of error: If the adapter is made for much higher current it will give higher voltage when loaded too light. If also the power supply circuits of the computer are really badly designed this could cause some problems. Very unlikely but not completely impossible.
You're going to need a power resistor to load the supply. You have E=I*R so rearrange the equation for R since that is what you want to find. R=E/I You are going to be playing with R so you don't have to be exact. R=14V/.350A R is about 40 ohms, lower values will increase the current. The power dissipated by the resistor will be P=I*E P=.350A*14V P=4.9W use a 10W resistor and don't burn yourself!
You can build a regulated 9 volt supply most easily by obtaining an LM7809 three terminal regulator. A couple of small capacitors wouldn't hurt either. Page 6 from this pdf shows you how to connect the regulator:
You will need to bolt the regulator onto a heat sink which you can scavenge from a broken power supply or simply cut up an aluminum can.
You test the current by placing the meter leads in series with the load resistor. Don't forget to plug the test leads into the correct socket and set the meter to its highest current range. The power supply + output connects to the load resistor, the other side of the load resistor connects to the meter - lead. The meter
lead then returns to the supply -. + to - to + to - in series....for current. To measure the output voltage, the meter, reset for volts is placed across the supply output + to + an - to - in parallel.
Another option is to get an unregulated supply with voltage switch, so it can be set to positions marked 3V 6V 7.2V 9V 12V. With such a supply you can safely start at a low voltage, connect the computer with correct polarity, step up one step at a time, until the machine works as it should. I have used that several times. Then I fix the switch with tape and use it for that device.
These stepped but unregulated adapters are much cheaper than regulated supplies.
Another idea is to build a regulated power supply yourself. You need an npn power transistor which can handle 300mA and a few watts power.
Connect the collector to the output of an unregulated supply, the emitter becomes your new regulated output, the base we lock at 9.6 Volt, because the emitter will always be 0.6 Volt lower than the base, so we get 9V out if the base is kept at 9.6V.
We can use a resistor and a 9.6V zener diode to fix the base at the right voltage.
That all made sense and I tried it, finding that: I have one adapter that is centre negative (not suitable for this application) and one adapter that is centre positive (suitable for this application).
I did find however that this 9v adaptor is putting out 13.3v.
I'm not sure if the circuitry of the chess computer will have some for of protection to allow for that fluctuation.
Do you think I should get a regulated adaptor? (When the chess computer arrives.)
Wait until the thing arrives and there may be some information about voltage tolerances?
Not be concerned about this extra voltage.
(The adaptor's 50hz too if that info. is of any use.)