In our present "No user serviceable parts inside" world of electronic
products, it is easy to understand why very few people have an appreciation
for the technical prowess needed to troubleshoot and repair them.
When reading through these episodes of "Mac's Radio Service Shop"
that appeared in mid last century editions of Radio & Television
News magazine, I am inspired to envy the skills that small
electronics repair shop owners had for working on the old vacuum
tube based radio and television sets. Digital electronics has its
own unique set of quirks and special knowledge requirements to troubleshoot,
but when everything is analog rather than merely being required
to be a '0' or a '1,' tracking down problems in an efficient manner
requires an acquired combination of experience, theory, and luck
- where the luck portion is actually largely a factor of experience
and theory. When I drive past a long ago shut down service shop
with a faded sign offering repair of Sylvania and GE television,
I wax nostalgic over an era of electronics that I missed by only
a couple decades.
Mac's Radio Service Shop: Service Bench Chatter
By T. Frye
A loud slam of the shop screen door announced' that Barney was
back from the service call he had started on only a few minutes
"Mission accomplished!" he said, making a sign of smug self-approval
with circling thumb and forefinger. "It was the old story: they
had moved the TV set across the room and, as a result, had more
lead-in than they needed. Someone had told them that twin-lead could
not be spliced; so, since they thought they might sometime want
to return the set to its original position, they rolled the extra
twin-line into a neat little coil and poked it into the back of
the set. As you can guess, Channel 4 was very blizzardy, and 6,
7, and 9 could not be seen at all.
"As soon as I whacked off the extra lead-in, everything was hokey-dokey.
I sold the man one of these little plastic twin-line splicers and
showed him how he could use it to restore the chunk of line we had
amputated if he ever wanted to move the set back to where it was.
Then I checked the ion trap to make sure it had not been jarred
out of adjustment when the set was moved. Sure enough, it was off
quite a little; so I put it back where it belonged and at the same
time touched up the focus. The guy was most happy to learn that
nothing serious was wrong with his set, and he paid the service
charge without a whimper, even though I had only been in the house
about ten minutes."
"Good boy!" Mac applauded, "I especially like the fact that you
checked that ion trap. Often when a technician locates a simple
trouble, it seems to the customer that he is being charged an awful
lot just to be told that his set was not plugged into the wall socket,
and so forth. What he can't always understand is that if the technician
had not been called away from his bench, he could have made that
five dollar service charge and more without stepping outside the
shop. In such a case, the service charge is really not for what
the technician does at the customer's home; instead, it is to compensate
him for the income he lost by being called away from his bench.
"However, an alert technician can invariably make some little
adjustment on a set, such as touching up the linearity or centering
controls, that will produce an easily-seen improvement in reception.
This minor service usually takes only a few seconds to accomplish,
and it leaves the customer with the happy feeling that he has received
a good return for his money. In this case, he really did, too. If
that ion trap had been left out of adjustment, it could easily have
ruined the picture tube in a short time."
"Oh yeah," Barney interrupted, "there's one other thing. After
the set was turned off, the tube continued to emit flashes of light
like heat lightning every few seconds all the time I was there.
The man said it would continue to do this at a decreasing frequency
rate for as long as two hours after the set had been shut off and
even the plug pulled from the wall socket. I promised to ask you
"That's a timely one!" Mac exclaimed with a chuckle. "Just last
week I wrote a letter to the engineering department of a large kinescope
manufacturing concern and asked about the same thing, for I had
noticed it happening on a set we had in here for service. The engineers
wrote back that quite often a tube will have what they call 'cold
emission' and continue to emit electrons sporadically for some time
after all heat has left the cathode. The filter condensers on the
high voltage circuit retain a charge for a long time and maintain
a potential on the tube electrodes after the power has been cut
off. This potential directs the bursts of electrons to the screen
and causes it to flash. The engineers said they did not know how
to prevent it and did not think anyone else did either. However,
they left the impression that it was nothing to worry about, outside
of the rather spooky feeling it gives you to see a 'dead' set carrying
on in that fashion."
"I had to do a little service on my own set last night," Barney
said as he placed an a.c.-d.c. set on the bench and started removing
a defective filter condenser. "Right in the middle of the wrestling
matches, the whole screen went dark except for a streak right up
and down the middle of the screen about a quarter of an inch wide.
I was feeling mighty low when I saw that, for I figured something
must be wrong with the horizontal portion of the deflection yoke.
I reasoned that if anything was wrong with any other part of the
horizontal deflection circuit, there would be no high voltage, for
the set uses a horizontal flyback type of high voltage circuit;
and that would mean there would be no illumination of the screen
"However, with the same dopey impulse that makes a man who does
not know from nothing about automobiles get out and lift the hood
and peer beneath it when his car won't start, I pulled the set out
from the wall and looked into the back of it. The first thing I
saw was a tiny but very bright little spark on top of the 6BQ6G
horizontal output tube inside the high-voltage cage. Right away
I shut off the set and poked the tip of my solder gun through the
louvers of the cage and reheated the solder on that plate cap; then
I turned the set back on, and it took right off and played all right
all the rest of the evening."
"That was a queer one," Mac mused. "Apparently the expansion
and contraction of the tube broke the solder connection between
the cap and the plate lead, but the separation was so tiny that
the current arced across it; and, while this current was too low
to sustain any appreciable amount of deflection, it was sufficient
to produce enough voltage to illuminate that narrow streak on the
For a little while there was silence in the shop as each of the
men became engrossed in his work. Mac was installing a new quadrature
coil in a gated-beam TV sound detector, and Barney was installing
new filter condensers in the small set. Barney was through first,
and after he had cleaned both cabinet and chassis and put the two
together, he placed the receiver on the secondary bench and snapped
it on for the thirty-minute check that Mac insisted each set must
pass before leaving the shop. For several seconds there was no sound
at all, and then the set snapped on with a burst of music. Barney
turned the volume down and picked up another set from the to-be-worked-on
"You satisfied with that set?" Mac asked without looking up.
"I know from your tone that I'm supposed to say No," Barney replied;
"but I don't seem to see much wrong with it. It's a little slow
warming up is all."
"How about that popping on so abruptly?"
"Probably an oscillator tube cathode that comes up to operating
temperature a little after the others," Barney suggested.
Mac shook his head. "I don't think so. The sound came on too
abruptly. Seemed as though something had the audio completely choked
off until the set reached a certain temperature."
He picked up a little rubber hammer such as doctors use for testing
reflexes and walked over to the set. Removing the back, he tapped
the 12SQ7 first on one side and then on the other. Suddenly the
set stopped playing as abruptly as it began, but another rap on
the duo-diode-high-mu triode started it once more.
"The grid is probably shorting to the cathode," Mac explained
as he tossed the tube into the discard box and put in a new one.
"In these high-gain triodes the spacing is very close, and quite
often a short will occur at one particular temperature. This tube
was shorted until it got just so hot, and then the short disappeared;
but the slightest jar was all that was needed to make it short again.
"Never let one of these sets that come on with a bang go out
of the shop," he concluded. "Even if it is a slow-starting oscillator,
find out why it is sluggish and correct it, for the oscillator that
starts slowly this week probably won't start at all next."
""Okay," Barney said with a shrug of resignation, "but if you
keep on giving me symptoms of coming trouble for me to keep in mind
and correct before letting the set go, I'll be getting out about
two sets a day."
A quick frown went across Mac's face.
"I'd rather have you put out one set a day and do it right than
put out a dozen cobbled-up jobs that will probably bounce or lose
customers or do both," he said sharply. "Eventually I want you to
learn to turn out good work fast; but if you will just keep on concentrating
on doing your best on every set, the speed will come automatically."
"Now don't get your Scotch up," Barney said soothingly. "I was
just popping off without thinking. I know you are doing your best
to make a really good technician out of me, and I want you to keep
right on doing it. For example, what do you suggest about this job
here. The lady says that this set has a very annoying whistle in
it at times, but I've had it playing for three hours without hearing
He turned on the little three-way portable, and instantly it
emitted a little pinging sound and then started to play perfectly
"Put it on the isolation transformer and run the line voltage
down a little," Mac suggested.
Barney obeyed, and at one setting of the line voltage the set
started to howl with the characteristic musical-saw sound of a microphonic
"There's your whistle,''' Mac told him. "As you should know by
now, you can't place too much importance in the words a customer
selects to describe the way a set sounds. We'd call that a howl,
but to her it is a whistle. It only happens when the microphonic
tube - and you can find which one that is by tapping them - is at
a certain critical temperature. That is why it emitted that little
pinging sound as it passed through that critical temperature during
the brief warm-up. When we backed the line voltage down until the
filament temperature was again at that point, the microphonic condition
was sustained. At her home, probably the line voltage often stayed
close to this critical voltage-figure for some periods of time,
and then her set kept right on whistling - I mean howling!"
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.
Posted February 22, 2016