Don't let the title fool you. This is not a "bees-birds-and-flowers
routine" being provided to Barney by his boss, Mac. It turns
out to be a brief introduction into the fine art of troubleshooting
intermittent problems in radio and television circuits. As is
usually the case, while the specifics of the scenarios Mac describes
might not apply to your challenge at hand, the general philosophy
always does. It is basically the old process of elimination
where after rapping components mechanically and/or heating or
cooling them in hopes of observing a tell-tale change in performance,
the next step is to divide the suspected circuit portion in
half (electrically, but sometimes also physically) and look
in one direction. If the problem isn't there, then divide the
circuit in the other direction in half and go there. Repeat
until the problem is found.
One of my personal favorite first steps is to verify all
mechanical connector interfaces (if any) are contacting properly.
Clean with alcohol if possible, and burnish with sandpaper if
appropriate, then plug and unplug the connections a few times,
just to make sure proper seating is occurring. It is no exaggeration
that up to 50% of the problems I have encountered with everything
from wall-mounted light switches to computer hard drives have
been remedied by cleaning connector interfaces.
Mac's Radio Service Shop: Barney Is a Big Boy Now
By John T. Frye
A sudden January thaw had melted the snow into a slushy mess;
and as Barney returned to work at Mac's Radio Service Shop after
his lunch hour, he had to pick his way carefully around the
soot-covered puddles standing on the sidewalks. The dazzling
sunshine and the bright blue sky overhead, though, were the
things that struck a responsive chord in Barney's naturally
cheerful nature; and he was doing his whistling best to recall
last summer's hit of "Woody Woodpecker" as he stepped inside
Mac was sitting on Miss Perkins' desk talking to her, and
Barney had the uncomfortable feeling that they had been talking
about him. What led him to this intuitive conclusion was the
fact that they were still doing it. Mac was saying:
"But he has to know sometime. He can't go on being just a
big, innocent, trusting lummox all his life. He is a big boy
now, and it is better that he should hear things from me and
get them straight than to pick them up goodness knows where."
Miss Perkins looked wistfully over her glasses at Barney
standing pop-eyed in the door and said in her best soap-opera
imitation" "Yes, I suppose you are right, but I can't bear to
see that innocent, carefree expression go out of those lovely,
lovely blue eyes. But go ahead. Take him into the back room
and tell him all, but please, please close the door behind you!"
"Hey, look, you two -" Barney began to protest as Mac took
him gently by the hand and led him into the service department.
"I'm not so - I mean I know -"
"Yes?" Mac encouraged as he shut the service room door and
leaned against it.
"Well, doggone it," Barney spluttered, "why are you giving
me this bees-birds-and-flowers routine?"
"Whatever are you talking about, Barney my boy? I was just
telling Miss Perkins that I thought it was high time you learned
something about the seamier side of servicing, or intermittent
radios," Mac explained blandly.
"Is that all!" Barney said with a sigh of relief as he stretched
his lanky frame out on the service bench and waited with a stubby
pencil poised over his dog-eared notebook.
"Don't dismiss the subject of 'intermittents' so lightly,"
Mac told him. "Radio men spend more time 'cussing' and discussing
such sets than they do on any other subject. The discussing
is fine, but I think the 'cussing' is rather shortsighted and
"Why 'shortsighted'?" Barney wanted to know.
"Well, if the servicemen would stop and think a bit, they
would be grateful for the 'intermittents' Since these sets are
the most difficult to repair and take the most experience and
good equipment to solve, they separate the men from the boys
in the service game. If the only troubles that radios ever had
were burned out tubes and blown bypass condensers, anyone could
learn to be a serviceman in a month, and all the equipment he
would need would be an ohmmeter. It is the tough sets, such
as the intermittents, that serve as a challenge and give a fellow
a chance to prove just how good a serviceman he really is- or
"What do you call an 'intermittent', Boss ?"
"That depends upon how long I have been working on it," Mac
said with a slow grin; "but seriously, an intermittent is any
set that suffers abrupt changes in the quality of reception
at irregular and unpredictable time intervals. That phrase 'change
in the quality of reception' may cover everything from slight
changes in volume or tone-quality to the set's going completely
dead; or it may be a noise that comes and goes along with normal
reception. The time interval may be every few seconds or once
or twice a week. The significant point is that the objectionable
feature is only present part of the time."
"What's the most common type?" Barney asked.
"I'd say that the set that changes volume abruptly shows
up the most often. This is the radio that the customer usually
brings in with the remark, 'This set will be playing along just
fine until someone flips on a light in the house or the refrigerator
starts up, and then it will blare out and scare you half to
death. You turn it down, and then a few minutes later the volume
drops so low you can't hear it.'''
"I can't see what makes such sets so hard to fix."
"You will, my boy, you will!" Mac promised. "For one thing,
almost any part that is used in a radio can cause just such
a symptom: a bad tube, a defective coil winding, a poor resistor,
or almost any condenser in the circuit can be the culprit. It
is like having an oleomargarine salesman sandbagged at a dairymen's
convention; there are lots of suspects!"
"Another thing that complicates matters," Mac went on, "is
that the intermittent is touchier than a sunburned rattlesnake.
Often the slightest jar, the smallest voltage surge, or the
least circuit-loading, such as is caused by using a voltmeter
or a signal tracer, will cause the low-volume condition to disappear
for the rest of the day."
"What is the best instrument with which to tackle such a
set?" Barney wanted to know as he scribbled away at his notebook.
"I use every instrument in the shop on some of them," Mac
replied, "but I honestly believe that the best service instrument
of all is good old-fashioned horse sense. The best way to find
the cause of an intermittent condition is to corner it. By that
I mean that you must use a process of elimination. Let's take
one of those sets that hop up and down in volume as an example:
"The first thing to do is to determine whether the trouble lies
in the r.f. or the audio section. A signal tracer, a v.t.v.m.,
a scope, or even a pair of headphones in series with a condenser
across the detector output will tell us this. If the signal
drops at this point when it does in the speaker, our trouble
lies in the r.f., oscillator, or i.f. circuits; if not, we know
it is in the audio section. In the latter event, we can move
our testing equipment from point to point through the audio
amplifier until we reach a place where the signal voltage rises
and falls in step with the volume issuing from the speaker.
Then we work backward from this point at which the volume does
change and forward from that at which it does not - just like
two baseball players closing in on a runner caught off base
- until we have narrowed down the separation of these two points
to the smallest possible circuit distance - say the opposite
ends of a coupling condenser. Then that condenser has to be
the cause of the trouble; and incidentally, coupling condensers
which have poor connections between leads and foil are one of
the most common causes of intermittent sets."
"What if the trouble is ahead of the detector?"
"Use the same 'closing-in' tactics, but your instruments
have to indicate r.f. and i.f. voltages. That means you must
use an r.f. probe on the v.t.v.m. or scope, or you can employ
the signal tracer. Plate, screen, cathode, and a.v.c. bypass
condensers; defective windings in r.f., oscillator, or i.f.
coils; and defective tubes are some of the most common causes
of abrupt changes in the amount of signal voltage delivered
to the detector tube."
"I can see how that would work if the set kept cutting in
and out quite often, but what are you going to do if it won't
cut out when you have it on the bench?"
"The only thing you can do is try to make it cut out," Mac
said. "I usually start by tapping the tubes, especially the
ones with grid caps, such as the 6A7's, 6A8's, 6Q7's and 75's.
Lots of times striking the caps of these tubes smartly on top
with a bakelite rod will cause the volume to change with each
blow; but you want to make sure that the change is not caused
by the jarring of some other defective unit instead of the tube
itself. If tapping the cap of a new tube has no such effect,
you can be pretty sure you have found the villain.
"If the tubes seem OK, I next try to find a bad condenser.
I feed a strong unmodulated signal into the antenna post from
the oscillator, tune this signal in on the receiver so that
I hear the characteristic rushing sound, and then gently tap
the various bypass and coupling condensers. Notice I say 'gently.'
If you go yanking the condensers around roughly, you will have
a lot of little intermittent conditions instead of just one.
When you tap a condenser that has a poor connection between
lead and foil and allow it to vibrate on its leads, you can
usually notice a chopping-up of the rushing sound coming from
the speaker. A good condenser will give forth nothing except
possibly a slight microphonic sound."
"Do you have any other third-degree methods to make these
"Yes, most of them are affected by heat; so I use an infrared
lamp to warm up suspected parts. These lamps have a small intense
heating area that is fine for this job. Also, I try both low
and high line voltages on the set, using the tapped isolation
transformer for accomplishing this. Many a time I have first
struck the trail of an intermittent condition by simply 'wracking'
the chassis a little with my hands. Thumping the chassis in
various spots with a little rubber hammer such as a doctor uses
for testing reflexes is also helpful."
Mac was interrupted by the bellow of the one o'clock whistle
at the laundry across the street.
"Well," he said, "the subject of intermittents is far too
big for us to cover at one session; so suppose you work on the
sets I think will fall under what we have talked about today
for the rest of this month; then we will renew the discussion
and talk about some other kinds of intermittents and some other
methods of tracking down the causes."
"That's oke with me," Barney agreed as he closed his notebook
and slid from the bench; "and in return, if there is ever anything
you want to know about the bees or the birds or the flowers,
just ask old Uncle Barney here!"
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no 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.
Posted November 12, 2015