[Boatanchors] BBOD's are really reliable (sic)

[Jim Wilhite]

There are a couple of things I have noticed about capacitors over the years 
that has served me.

First of all, some have a finite lifetime, especially electrolytic.  I have 
successfully reformed them only to have them fail within a year or two of 
regular usage.  Of all the radios I have resurrected, only one has the 
original filter still in it and it is only 2 years into operation.

Electrolytic have a semi-liquid paste in them which will dry even when not 
in use.  A factor in how long they last will be storage, where and how long. 
Provided they cannot do harm to the transformer or other components, it is 
ok to reform and use them until they begin to fail.  It is best, as stated, 
to check leakage under operating voltage.  It is also good to have a checker 
that will check the power factor.  After a while, undo the wires and check 
them.

With ceramics and mica caps., they are a bit different.  Most will continue 
to give yeoman service for years to come, but occasionally, you will find 
they will go bad, evidenced by slowly changing characteristics of the 
receiver or transmitter in the first 30 minutes of operation.  If you can 
tune it correctly in the beginning and it changes dramatically over then 
next 30 minutes, look for a bad ceramic or mica (not as likely as postage 
stamp micas) .

Tubular (BBOD and the like) are made of paper, which will disintegrate over 
time.  Again this depends on where the unit has been stored and how it has 
been operated as well as how much heat the circuit develops where they are 
installed .  Many people just replace them wholesale and that is ok.  Some 
people with good troubleshooting skills wait until they fail one at a time 
and replace.  That I will leave to you, but have checker such as the Sprague 
that will test capacitance, leakage, power factor and insulation resistance.

Complete replacement is a long process in many of our old boatanchors and I 
have done it just once.  Many of the old caps. were still within specs.  By 
the way, while we are talking of specs, look up the particular cap you are 
worried about.  You may find they have very wide tolerances.  Remember when 
you purchase a digital cap checker, you have something unavailable to 
engineers and technicians of the past when these gems were constructed. 
Don't be too exacting.  10% is a lot to some of these caps.

Off my box!

73  Jim
W5JO


----- Original Message ----- 
From: "Philip Atchley" <beaconeer at sbcglobal.net>
To: <jeffv at op.net>; <Boatanchors at mailman.qth.net>; <k5vkl at juno.com>
Sent: Sunday, June 05, 2005 3:26 PM
Subject: Re: [Boatanchors] BBOD's are really reliable (sic)


> Hi All,
>
> In checking capacitors, there is one slight problem with Meters, whether 
> they're digital or whatever.  They check the "value" or capacity just 
> fine. Some even test ESR (useful if you're troubleshooting switching power 
> supplies).  I have one of the "better" R/S do-all DVM's that "checks 
> capacitors".  It is actually quite accurate (as checked against some 1% 
> capacitors) and results are repeatable.
>
> HOWEVER, these type of meters are TOTALLY USELESS in checking for the 
> usual failures of BBODs or other capacitors in our beloved boatanchors. 
> The reason is quite simple.  The capacitors have excessive electrical 
> leakage and the usual meter isn't sensitive enough to read this leakage 
> resistance.  A meter operating at 9 VOLTS CANNOT simulate conditions of a 
> capacitor that is operating with 400 or so Volts applied.  There ARE ways 
> of checking them with the meter and an external power supply, but that has 
> been covered elsewhere.
>
> Most of the better resistance/capacitance bridges (Heathkit, Eico, Sprague 
> etc) CAN check your capacitor at rated Voltage.  The Heathkit IT-28 I have 
> is very good for this task.  IF YOU DON'T HAVE such a way to test your 
> capacitors in a BA, then it definitely is to your advantage top do a 
> wholesale replacement of said capacitors without even trying to measure 
> them on your "DVM".  Even with the proper test gear I usually do a 
> wholesale "shotgun" replacement of ALL paper and electrolytic capacitors, 
> it saves destroying expensive or hard to find components later if one 
> fails.
>
> And, what to do with your "Capacitance Meter"?  Well, it's good for 
> "matching capacitors" for value if you're making filters etc. Not a whole 
> lot else.
>
> 73 de Phil,  KO6BB