August 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
One of the first things a knowledgeable
restorer of vintage electronic gear does prior to plugging in a newly acquired piece of hardware
is to replace all of the original paper capacitors. Those things notoriously lose the internally
contained smoke that makes them work soon after power is applied. Episodes of conflagration
often ensue. According to Mac McGregor, the typical shelf life of a paper capacitor (and some
mica and ceramics back in the day) is about five years. In that time the insulation resistance
can drop from 5000 MΩ to less than 2 MΩ. Ohm's Law quickly reveals that with
used across a 300 V plate bias supply circuit, the leakage current can be 0.15 mA,
and dissipate 45 mW of power. Considering the number of such connections in products
like the RCA Victor
Model VHR-307 Home Recording Phono-Radio Combination, the current and power can add up
quickly, and the generated thermal noise can get significant. These articles, while apparently
an electronics serviceman saga, is actually meant to be instructive to readers, many of whom
were service shop owners or employees.
There are a couple YouTube videos of people demonstrating the 'cat's eye' tube used to
indicate a peak in the capacitance bridge adjustment.
Shelf Life of Capacitors & Batteries
By John T. Frye, W9EGV, KHD4167
Mac was so busily engaged in what he was doing at the
workbench that he actually jumped when he heard the voice of Barney, the second banana of
the shop, who had entered quietly and was standing behind him.
"So! You sneaked out and bought something new!" Barney was saying accusingly as he leveled
a finger at the impressive little black instrument, bristling with push-buttons, dials, switch
knobs, a magic eye tube, and a large meter sitting on the bench surrounded by all sizes and
shapes of capacitors. "What is it?"
"A Sprague Model TO-6 Tel-Ohmike Capacitor Analyzer," Mac answered. "The other day I replaced
a 0.1-μF coupling capacitor in a tape recorder with one from our stock and found the new
one had less insulation resistance than the one replaced. I tried three more of our capacitors,
and every one showed objectionable leakage, but a fourth was fine. Right then I decided we
needed an accurate method of evaluating capacitors we planned to install in critical locations.
The fact that a capacitor is unused obviously does not mean it is good. Moreover, many surplus
capacitors can be purchased today at such tempting prices that they represent bargains - if
we have a means of separating the sheep from the goats. This little instrument is just the
ticket for telling us all we need to know about surplus capacitors or the ones we get from
"It measures capacitance from 1 pF to 2000 μF, and the applied voltage is low enough
that capacitors rated at 3V can be tested without damage - an important point with capacitors
designed for transistorized equipment. The power factor and leakage current of electrolytics
can be accurately measured at their exact rated working voltage. Finally the insulation resistance
of paper, ceramic, and mica capacitors can be read directly on a meter with two ranges: one
up to 10,000 megohms at 30 V for low-voltage capacitors and the other up to 50,000 megohms
at 150 V for higher voltage types. Incidentally, those leaky paper capacitors of ours have
insulation resistance of less than 2 megohms, while they should have a minimum resistance,
when new, of 5000 megohms. This is according to data given in the TO-6 operating manual as
to what constitutes minimum insulation resistance for all types and values of paper, mica,
silver mica, ceramic, oil-filled, subminiature capacitors, etc."
"We must have got a bummer batch of 0.1-μF capacitors, huh?"
"I doubt it. Those capacitors were probably OK when new but simply deteriorated in the
bin. I have no idea how long they've been there. We don't use many 0.1-μF 600-V units any
more, and I have the bad habit of ordering new paper capacitors when anyone type is running
low and dumping the new ones in on top of the old. Then I use the new ones off the top of
the pile and leave the old ones down at the bottom; and I do this over and over. We're going
to quit that."
"You think paper capacitors go sour on the shelf?"
"'Nothing good nor bad lasts a hundred years,' the Spanish say. All things deteriorate
with time - except service technicians, of course! At any rate, I became curious about the
shelf life of several items we use regularly and dashed off letters to capacitor, battery,
tube, and solid-state manufacturers asking them for information as to the shelf life I could
reasonably expect from their products, what conditions affected shelf life, and what recommendations
they had regarding storage.
Returns are still coming in, but I already have a good response from capacitor and battery
manufacturers. After all, they know that customer satisfaction and confidence comes from installing
components when they are new and fresh. Trying to use an over-age, gone-sour component breeds
dissatisfaction, no matter how unfair that feeling may be."
Shelf Life of Capacitors. "Okay, so what have you learned about capacitors?"
"I've learned the normal shelf life for paper tubular capacitors used in TV/radio is about
five years, as is the normal shelf life for micas (both dipped and molded) and small ceramic
capacitors. The decrease in insulation resistance with time takes place chiefly in the dielectric
material. Heat and moisture are great villains in this regard. Every effort is made to seal
moisture out of the capacitors, and modern techniques do a good job of this; but if the capacitors
are exposed to temperature cycling under conditions of high humidity, some moisture is eventually
bound to penetrate the seals and degrade the insulating quality of the dielectric."
"Then these capacitors should be stored away from heat and moisture."
"Right. As one capacitor manufacturer wrote me, 'I suppose that one way of looking at it
would be for you to keep the capacitors under conditions similar to that under which your
wife would keep spices - not near any heat, such as a stove or radiator, and as dry as possible.'
You and I can believe this because we both know that when we get a radio in for service that
has been stored in an attic or basement, we are certain to find several leaky capacitors in
"How about dry electrolytics? Should they be stored under the same conditions?"
"As far as keeping them away from high temperature, yes; but here we are not so much concerned
with keeping the moisture out as keeping it in. Let me explain. A dry electrolytic really
might be called a 'damp' electrolytic because the electrolyte inside it is in the form of
a moist paste. The actual dielectric is a very thin oxide film that normally forms through
the combined action of an applied voltage and the chemical action of the electrolyte. The
anode constitutes one plate of the capacitor, the electrolyte forms the other. Therefore the
drying out of the electrolyte destroys the capacitor, and the presence of heat speeds up this
"Manufacturers seem to agree that the normal shelf life for dc electrolytics is one to
two years. However the drying up of the electrolyte is not the limiting factor here as much
as is the gradual deterioration of the dielectric film under the eroding action of the electrolyte
when no polarizing voltage is present to maintain that film. The life of an electrolytic capacitor
on the shelf can be materially extended if a polarizing voltage is applied to it through a
current-limiting resistor every few months. Heat increases the chemical action of the electrolyte
on the oxide film under storage conditions and shortens the normal shelf life."
"What happens if the capacitor is stored at extreme low temperatures?"
"The series resistance goes up and the capacitance goes down due to ionic immobility because
of the freezing of the ionizing agent. Capacitors that have been out of service at extreme
low temperatures react as though open circuited at first but start returning to normal with
the temperature rise of the equipment."
What About Batteries? "Okay, now tell me about the shelf life of batteries. Transistorized
equipment has made this an important subject."
"I'm indebted to Union Carbide, maker of Eveready Batteries, and to RCA for the information
I have on this subject. Union Carbide defines the shelf life as the period of time, at a storage
temperature of 70° F, after which a given battery retains 90% of its original energy content.
Shelf life is reduced by high temperatures because of wasteful zinc corrosion and side chemical
reactions within the cells and because of moisture loss from the cells through evaporation.
The shelf life of a battery stored at 90° F may be 1/3 that of one stored at 70° F.
"RCA has conducted some interesting tests on the effect of temperature on shelf life of
carbon-zinc cells. For example, an A-size carbon-zinc cell stored at 70° for 24 months
retained only 50% of the rated capacity, but cells stored at 45° F and 0°F retained
70% and 90%, respectively, of their rated capacity. Other tests showed carbon-zinc cells stored
at 48°F were in better condition at the end of five years than those stored at 104°F
at the end of one year.
"UC says the shelf life, as defined previously, of silver-oxide, mercury, or alkaline batteries
is one year. The shelf life of carbon-zinc batteries is slightly less than that. Other types
of batteries mentioned do not benefit as much from cold storage as do the carbon-zinc cells.
"RCA, on the other hand, while agreeing on the shelf life of silver-oxide cells, finds
mercury cells have a shelf life of two years and that alkaline cells have a shelf life almost
as good. RCA further states that the shelf life of mercury cells can be extended by storing
them at lower than room temperature, provided suitable precautions are followed. Since both
companies concur in these precautions, let me list them: (1) Don't handle frozen batteries
any more than necessary and be gentle with them to avoid cracking the internal and external
seals which may become brittle at low temperatures. (2) Allow the cells to reach room temperatures
in the containers in which they are stored to avoid excessive condensed moisture, which will
generally destroy the jackets and increase electrical leakage. (3) Do not put the cells into
service until they have reached room temperature."
"How about recharged primary cells? Is their shelf life as good as it was originally?"
"Definitely not. RCA says such recharged cells have a very poor shelf life and should be
put into service immediately after recharging."
"Well," Barney said, "this certainly has convinced me that capacitors and batteries should
be purchased from a source that moves these items rapidly and keeps a close tab on how long
they have been on the shelf. By the same token, we should buy in small enough quantities that
they will not be long on our shelves. And we should work on a first-in-first-out basis. But
now tell me about the tubes and transistors."
"I'm afraid that is going to have to wait," Mac said, starting to put the capacitors he
had finished testing back into their bins. "I haven't heard from all the people I wrote to
in those fields yet, and it is high time we got to work. But I promise that someday soon we'll
talk about the shelf life of tubes, transistors, IC s, etc."
Posted July 24, 2017
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of none other than John T.
Frye, creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine
(which itself started as simply Radio News), and then changed its name to Mac's
Service Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his
eager, if not somewhat naive, technician assistant. 'Lessons' are taught in story format
with dialogs between Mac and Barney.