[Coco] Powering up our retro systems

Dave Philipsen dave at davebiz.com
Mon Jan 27 07:48:40 EST 2020

Yes, I well know that electrolytic caps can degrade and leak over time. 
I have repaired a number of devices over the years whose caps have 
failed. I just think that "reforming" is more of a process reserved for 
big old caps that have failed and would be difficult and/or expensive to 
source replacements. For the kind of caps that we find in our Color 
Computer (low voltage and relatively low capacitance), I say it's easier 
to just see if they fail and replace them if they do. It is doubtful 
that even a shorted cap in the Color Computer will cause any damage to 
the rest of the power supply unless you're the type of person who turns 
on the computer, sees that it's not working, and then decides to just 
leave it powered up for a long time anyway.


On 1/27/2020 6:32 AM, wrcooke at wrcooke.net wrote:
>     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

More information about the Coco mailing list