[Coco] Powering up our retro systems

[wrcooke at wrcooke.net]

   On January 27, 2020 at 6:05 AM Dave Philipsen < [1]dave at davebiz.com>
   wrote:

   The links you provided for “reforming” electrolytic capacitors imply
   that the cap should be removed from its original circuit in order to
   carry out the process properly. That being said, I have three CoCo 3s
   that we’re purchased around 1990 (so about 30 yrs old). They had been
   stored in a garage where temperatures likely varied from around 32F to
   at least 90F and they all powered up without problem after all those
   years.

   Dae

   It's true that the best way is to remove the cap from the circuit.  But
   it isn't always necessary.  If removing the cap I recommend simply
   replacing it.

   The nature of the power supplies in most of the older computers is a
   transformer to lower the voltage followed by a rectifier to convert it
   to pulsating DC and the input caps to filter that DC (smooth it) before
   a regulator.  If the caps are beginning to degrade, a lower voltage
   limited in the amount of current will begin to reform them.  The
   process is nearly identical to having the cap out of the circuit.
   Removing the caps is mostly useful for protecting the rest of the
   circuit.  The resistance of the other parts, transformer and rectifier,
   helps to limit the current.  It works quite well.  Not as good as
   removing it, but almost always good enough.

   I have powered up old equipment without reforming the caps.  But its
   always a gamble.  You have no way of knowing the condition of the
   caps.  There is no real way of determining when or if one or more will
   degrade.  It might work fine, as yours have, or it might blow the power
   supply.

   In my original post I also recommended powering the computer or other
   device every year for several hours.  Doing that keeps the caps formed
   properly and avoids the issue altogether.

   There are other electrolytic caps in the computer besides just in the
   power supply.  They are scattered around the board(s) on the power
   supply rails for filtering.  They are usually fed with the regulated
   voltage.  They are subject to the same problem.  Using a low input
   voltage allows them to reform as well.

   I'm glad you had no problems.  I wouldn't take that as the final word.
   Your luck may not hold.  This is a well known problem and has caused
   many people a lot of grief.

   Will

References

   1. mailto:dave at davebiz.com