alexanrs wrote:Yeah, but it could always be that this specific case is about the motherboard, and the others would be able to bring the Voodoos back by leaving them on for a while. The only way to be sure is to have more data.
I would like to know what the ESR is on the caps.
A big lot of NOS boards that don't work sounds a lot like they were stored someplace too hot.
Time (with no DC applied) and heat are both hard on caps and these cards are ~15 years old.
In storage the oxide layer (the actual dielectric) slowly dissolves back into the electrolyte solution.
Heat accelerates that.
DC voltage applied reverses that process and rebuilds the layer. (This is called "Reforming".)
The power-up for 2 hours I suggested is a red-neck ghetto version of reforming but it usually works fine for low voltage (under 50v) caps.
For 15 year old caps 2 hours should be enough.
The one possible problem is if the layer got so thin that the leakage current is too high that could destroy the cap rather than reforming it.
Not usually a problem but it might be with caps that have been improperly stored for 15 years.
The "official" way to Reform caps is to put them in series with a 1k ohm resistor (which prevents the possible problem of excessive leakage current damaging the cap) and apply a low voltage (1 or 2 volts).
Then slowly raise the voltage in steps until the cap has the rated voltage across it.
Then it is held at the rated voltage for a period of time.
- There are as many variations as to how large the voltage steps should be and the times as there are people who have written down the procedure.
An example would be: Start at 2 volts. Go up 2 volts every 5 minutes until the rated voltage. Hold there for 20 minutes + 15 seconds for each month it has been stored.
Some capacitor companies recommend capacitors be reformed if they have been unused for more than 1 year.
Obviously that's a bit excessive for caps used in computer equipment.
Their recommendation is general for ALL caps they make including 400-450 + volt caps and those the size of a trash can.
[Edit][ I just checked at Nichicon's site and they have added the following to one of their tech docs since last I read it.]
When an aluminum electrolytic capacitor is stored under no load conditions for a long period of time, its leakage
current tends to increase slightly. This is due to a drop in the withstand voltage of the dielectric caused by the
reaction of the anode oxide layer with the electrolyte. When the voltage is applied to the capacitor, the leakage
current returns to its initial level because of the re-forming action of the electrolyte (called voltage treatment).
If the storage temperature is high, the leakage current will increase substantially. Therefore, it is desirable to store
capacitors at normal temperature level with no direct sunlight. A voltage treatment is recommended when using
a capacitor stored for a long period of time. The treatment for an individual capacitor is accomplished by charging
up to its rated voltage through a resistance of about 1k Ω and applying the voltage for approximately 30 minutes.
When a capacitor is already built into an appliance, the appliance must undergo aging. If the input voltage is
adjustable or the power supply that supplies power to a module, first set the voltage to a low value (approximately
half the rated voltage) and let it run for about ten minutes. Then, increase the voltage to the appropriate value little by
little while monitoring the working of a device.
If the voltage is not adjustable, turn on the switch and let it run for about thirty minutes while confirming if the device
complies with the specifications. Then turn off the switch before using the capacitor for practical applications.
Generally, if the capacitor has been stored within 2 years in the storage temperature range of 5~35˚C, the
capacitor can be used without voltage treatment.
.
GRUMPY OLD FART - On Hiatus, sort'a
Mann-Made Global Warming. - We should be more concerned about the Intellectual Climate.
You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.