Re: long term reliablity computer boards

FWIW, reforming is a technique that's performed to aged electrolytic capacitors to help reduce the amount of leakage caused by age (leakage in terms of internal current leakage, as related to insulation breakdown).

Reforming doesn't improve or change any other electrolytic capacitor test parmeters other than internal leakage.

A lot of folks that restore vintage electronic equipment will slowly bring up the line input voltage with a variac, the first time they power up the equipment, to reduce the possibility of damage caused by excess leakage in the capacitors.
High current leakage paths in capacitors will often destroy other ciruit components, compounding the restoration costs and effort.

The capacitors in vintage gear are often the size of the paper tubes in the center of a roll of toilet paper, not typically anything to be found in modern low voltage circuits today.

The reforming process for (aged old stock) out of circuit electrolytic capacitors is to apply a very low voltage from a low current source while monitoring the current with a meter.
It's very important that the current is limited to a low value, even a current limited power supply of only 10mA will be adequate.

The current limited applied voltage is gradually increased when it's determined that the current has decreased to near zero mA from the previous voltage setting.
No electrolytic capacitors have zero leakage at their working voltage, but nearly zero current should be seen at low applied voltages (uA if the display is sensitive enough to indicate below mA current levels).

When a capacitor's current leakage doesn't achieve near zero current before reaching the rated working voltage, it's discarded because reforming won't correct the excessive current leakge condition present in the capacitor.

The applied voltage is only increased to the capacitor's rated working voltage. If the level of internal leakage is determined to be within acceptable limits, the capacitor can be considered safe to be installed for use in a circuit application appropriate for the capacitor's rated working voltage.

Reforming can take up lots of time, and it's not a practical procedure for most repair technicians.

All of Sencore's test equipment that will test components for leakage are clearly labeled: Do not hold the capacitor (device being tested) in your hand while testing.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"Wes" <clutch@xxxxxxxxx> wrote in message news:PE5Kl.250604$UQ6.162076@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
cavelamb <cavelamb@xxxxxxxxxxxxx> wrote:

One of the long term storage problems you'll face is that electrolytic
capacitors will eventually dry out and fail.

A few episodes back, Steve Gibson was talking about his collection of PDP's. One of the
issues with firing them up was the capacitors. Some caps can be reformed.

http://www.vmars.org.uk/capacitor_reforming.htm


Wes

.

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