[Collins] Restoring a Collins 32V-3

[Gerald]

On Fri, 2005-12-16 at 08:10 -0500, Bruce H McIntosh wrote:
> On Thu, 2005-12-15 at 22:23 -0600, Gerald wrote:
> 
> > Very likely, every paper capacitor (whether waxed or molded) will have a
> > lot of leakage current. They will need to be replaced. Every
> > electrolytic capacitor will be even worse and will need to be replaced. 
> > 
> > Resistors of that age will have drifted so each should be checked
> > (taking into account circuit connections that modify the resistance
> > reading) that its within the original tolerance. Age, humidity, and heat
> > makes carbon composition resistors rise in value.
> > 
> > Depending on storage conditions, transformers can have excessive leakage
> > from moisture absorbed in the paper insulators. They can be dried gently
> > in ovens or by running current at low voltage through the windings. If
> > wet insulation breaks down from applied voltage, it has to be replaced.
> > Drying won't remove the carbon tracks from that arcing.
> > 
> > Then there will be raspy potentiometers and variable capacitor wipers.
> > Cramolin makes products that help them. Some will be too bad to be
> > helped and will have to be replaced.
> >  
> > And the most fragile components, the tubes. Some will be weak, some
> > worse.
> 
> So, from this litany of woe, one can surmise that, unless someone else
> has already done the restoration before one buys it, the purchaser of a
> boatanchor is essentially purchasing a chassis, wiring harness, and if
> lucky a properly-strung dial cord. :-)  Makes this whole "old radio"
> thing just a trifle off-putting, doesn't it? :-)
> 
> (Thomebdy pleeth hand me tht cwobrar; my tongue ith thutck in my cheek!)
> 
Many of the resistors survive such a restoration. Leaky coupling
capacitors will take out tubes by upsetting the grid bias (driving it
positive). Tubes are more expensive that Orange Drops these days.

Electrolytics age whether used or not, but age faster when NOT used. And
when they relieve excess pressure by blowing their guts into the radio,
the cleaning process can be very difficult. All the contents are
conductive, thin aluminum foil, crepe paper saturated with conductive
electrolyte, and that electrolyte. Sometimes electrolytics give warnings
by hissing, sometimes they just blow.

The paper capacitors used in 32V vintage radios were junk when they were
put it. They haven't improved with age. They have gotten worse. Leaky
bypass capacitors lower screen voltage (when they are the screen bypass
capacitors) and lead to low stage gain. And they increase the power
dissipation in the screen dropping resistors when tends to make carbon
composition resistors increase in value dropping the screen voltage and
the stage gain further.

In my experience (been fixing radios since before I was licensed 50
years ago), shotgunning paper capacitors saves much trouble shooting of
marginal interacting failures. Especially when the paper capacitors were
the old color coded black beauties, waxed, or other molded papers
including those that are flat much like large postage stamp mica
capacitors. Mica and ceramic capacitors can fail but I've not
experienced the 95% failure rate of oiled paper capacitors.

The wiring harness can be fragile with the insulation falling off each
wire moved.

Good soldering tools and tools can emulate the factory gal. She had to
learn to solder once upon a time then doing thousand of solder joints a
day, she got good at it. You can too, but practice on something other
than the radio needing restoration.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer