[R-390] Capacitors (was New Owner...)

[Drew Papanek]

Mikea wrote:

>It is possible -- even probable -- that the power supply filter caps, and 
>many other caps in the radio, are leaky and/or out of tolerance.  The 
>filter caps can be re-formed: bring the rig up _slowly_ on a      Variac, 
>starting off at something like 50VAC input.

Paper and electrolytic capacitors are very common failure items in old 
electronic equipment.   Many recommend wholesale replacement of these 
capacitors.

Modern plastic dielectric caps (orange drop, et al.) used as replacement for 
paper work very well and will likely outlast most of us.

A better but more effort consuming method of reforming electrolyticapacitors 
is connection of cap to be reformed to a DC source of at least rated voltage 
with current limiting resistance of about 100K.   Voltage across cap can be 
monitored with high impedance meter (VTVM or DVM) as it slowly rises over 
minutes or hours.  A few hundred microamps (voltage across current limiting 
resistance divided by resistance) or less at cap's rated voltage denotes 
success.

Beware of R-390A electrolytics made by General Instrument.  These will 
frequently fail even after reforming.  Cleaning corrosive guts of exploded 
electrolytics from inside radios is a royal pain in the scrot.

Ceramic disc capacitors seldom fail even after many years use.

Much information about all of these topics and more can be found in Wei Li's 
compilation of postings over the years from this list.  Goto r-390a.net    
Click on "References", "Pearls of Wisdom".

Drew





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