If there was another episode of
Mac's Radio Service Shop where Barney was the primary teacher and Mac was the student,
I don't remember what it was. In fact, this is about as total of a role reversal
as there can be. First, Mac admits to having chased a presumed oscilloscope issue
down the proverbial rabbit hole only to realize the cause of the problem was totally
unrelated. Then, Barney produces a nifty device meant for recording telephone conversations
and demonstrates to Mac a couple ingenious applications he discovered that were
handy for troubleshooting television sets. When reading Mac's description of using
a magnet to alter the electron beam in a CRT, it reminded me of how cool it was
on the CRT displays to run a magnet across the face of the tube and drag the beam
around it. If you used too strong of a magnet the phosphor dots would exhibit a
rather impressive "memory" (aka retentivity) that could cause you to fear having
injured the CRT. My parents weren't too happy the first time I demonstrated the
effect for them on the family TV set. The other cool thing with the old color CRTs
was to use a magnifying glass or jeweler's eye loupe (preferred) to inspect the
individual red, blue, and green phosphor dots on the tube face. Are you old enough
Mac's Radio Service Shop: New Uses
By John T. Frye
Barney came through the door of the service department at his usual loping gait
to find Mac bent over the bench, but the youth stopped short in his tracks as his
employer straightened up and turned around.
"Hey, man, where did you get the eye-crutches?" Barney exclaimed. "I thought
you were Mr. 20/20 himself."
"So did I," Mac admitted ruefully as he slid off the horn-rimmed glasses and
put them in a pocket case; "and the way I found out different is a joke on me too
good to keep. The other day I was working with the scope when I suddenly noticed
something was wrong with it. The sine-wave trace sort of opened out at the knees
of the curve so that two very closely spaced traces could be seen at these points.
They merged together on the down-sloping portions. I made a couple of quick checks
and found out that a straight horizontal line, when properly focused, became two
closely-spaced parallel lines. This was not true of vertical lines. When I removed
all signals from both amplifiers and cut their gain controls clear back, instead
of the tiny, beautifully round little dot of which I had been so proud, I saw two
dots, almost touching, one above the other.
"You know I simply can't bear to have anything, no matter how minor, wrong with
my scope; so I immediately set out to discover what had happened. The first thing
I did was to spend about a half hour working with the spot-shaping, focus, and intensity
controls trying to find a combination of settings that would get rid of the annoying
condition; but nothing I did would banish the two dots. Next I decided a small signal,
such as hum produced by a heater-cathode short in one of the vertical amplifier
tubes, must be reaching the vertical deflection plates; so I connected these plates
directly together. This made absolutely no difference on my double-dot trace and
it left me with only one nasty probability: in spite of the special metal shield
around the CR tube, a magnetic field must somehow be affecting the trace. For the
life of me, though, I could not see how this could have developed all at once.
"Checking for magnetic deflection on an electrostatic tube is not too simple
a job. I moved the scope to different locations to make sure the deflecting field
was not being produced by something outside the scope; but since the condition persisted
unchanged, no matter where the scope was placed, I concluded the field had to be
produced by the instrument itself, probably by the power transformer. I have some
sheets of special magnetic shielding material and I tried inserting these between
the power transformer and the scope gun. I even wrapped a sheet of the material
completely around the outside of the CR tube. Same difference; nothing helped.
"Then I tried holding a powerful little magnet near the neck of the CR tube so
as to distort the a.c. magnetic field I felt sure was causing the trouble. By positioning
the magnet various ways, I could move the elongated spot about on the face of the
tube; but I could not change the position of the dots with respect to each other.
This simply was not right. I knew from experience that I should be able to change
the slope of the short line made up of the two dots by moving my magnet about.
"As I was pondering this mystery, I noticed something else odd: when I backed
away from the face of the scope some two feet or so, the dots seemed to move together.
Curious about this, I got my jeweler's loupe and looked closely at the dots - or
rather at the dot, for with the loupe only one dot was to be seen! That was all
the clue I needed. I drew a straight line on a piece of paper and held it close
to my face. When it was horizontal, I saw two lines; when it was vertical, I saw
only one. I realized I had spent several hours in a vain attempt to locate a scope
defect that was all 'in the eye of the beholder!'
"First thing the next morning I went to an oculist. He said the condition was
not at all unusual for a person 'your age' - I hate that phrase! - and that I needed
glasses for reading and close work. He must be right, too, for they certainly help
in working with these darned little printed circuit transistor receivers or in trying
to spot a broken wire in a coil."
"Well cut off my beam current!" Barney exclaimed with a grin. "I know you use
that precious scope for doing just about everything around the shop, but I never
thought you'd use it to test your eyes. Put on those cheaters again and let me look
at you. H-m-m-m, I believe they make you look smarter and more dignified. I think
I'll have to call you 'Mr. McGregor' when you've got them on."
"You do and I'll wring your Irish neck," Mac promised.
"That reminds me I've been wanting to show you some new uses I found for this
gadget," Barney said as he took from his pocket a little flat, black, rectangular
object with a wire coming out one end of it.
"It's a telephone pickup. You place this under a telephone and connect this shielded
wire to the microphone input of a tape recorder. Voice currents going both ways
through the 'induction coil' of the telephone induce currents in the turns of this
device that feed the recorder amplifier. This pickup is very sensitive to any sort
of magnetic field."
"Quit talking as though I were a not-quite-bright child," Mac growled. "I know
how a telephone pickup works."
"OK, OK! but let me show you some uses for it you don't know," Barney said as
he connected the leads of the shielded cord to the vertical input connections of
the scope. He turned up the signal generator feeding a little a.c.-d.c. receiver
Mac had been testing on the bench and laid the pickup on top of the cabinet. The
400-cycle modulating wave appeared on the scope screen. The smallest variation in
the signal delivered to the speaker was instantly apparent as a change in the size
of the scope trace.
"You know it's almost impossible to get at the speaker voice-coil connections
on lots of sets, especially transistor receivers," Barney explained. "With this
arrangement serving as an output meter, you don't need to. Whenever the pickup is
brought within several inches of the output transformer, any signal passing through
the transformer is picked up and transferred to the scope. What's more, by examining
the shape of the sine wave as well as its size, you can check the receiver for distortion
at the same time you're going through the alignment."
"That would speed things up in many cases," Mac said thoughtfully.
"And it's a handy-dandy little a.c. field detector," Barney went on as he held
the pickup within about three feet of an electric clock on the wall. The straight
line on the scope screen wrinkled up into a very distorted sine wave as Barney adjusted
the sweep to sixty cycles. "Using this thing as a probe, you can tell where a magnetic
field is strongest and weakest. That often helps in placing transformers, discovering
how hum is getting into a phono pickup, etc. And, by looking at the scope, you can
tell the nature of an unknown field and make a shrewd guess as to what is producing
As he finished speaking, Barney disconnected the pickup from the scope and connected
it to the input of the a.c. v.t.v.m. The range switch was set to 0.01 volt full-scale.
"I want you to see how sensitive this arrangement is to any pulse produced field,"
Barney said as he picked up a metal-cased flashlight and held it near the pickup.
Every time it was switched on, the meter pointer jerked smartly. "Give you any ideas?"
"Yeah. That arrangement could be very handy in locating an intermittent or poor
connection, especially in any sort of transformer winding. With d.c. passing through
the winding, the slightest variation in current would show on that meter."
Barney looped a test lead from the ohmmeter carelessly around the pick-up and
touched the prods together. Even though the ohmmeter was on a range that sent one
milliampere of current through the leads, the meter kicked every time the prods
touched each other. "Just wanted to show you how sensitive the thing is," he explained.
"You can detect a very small change in current through even a single wire lying
against the pickup and you can do it without making any direct connections to the
circuit. Of course, if you want to hear what the pickup is getting instead of seeing
it, all you have to do is connect it to the high-gain input of the signal tracer."
"I'm sold," Mac said with a grin; "you've just invented a new probe."
Barney started to speak and then stopped. Finally, though, he looked up at Mac
sheepishly and said:
"I didn't intend to tell this on myself, but after your confession I think I'll
tell you my experience with that blessed scope. When I was cleaning up the bench
the other day I noticed the transparent screen in front of the tube had collected
a lot of finger smudges; so I took it out of the bezel and gave it a good washing,
wiped it dry with a clean paper towel, and replaced it. Wanting to see how much
difference this made, I switched on the scope. When no trace appeared, I increased
the intensity, double-checked the positioning controls, fiddled with the focus control,
and made sure both amplifiers were turned clear off; but not the slightest trace
of a spot or pattern could be seen.
"Man, I was really sweating, for I was convinced that somehow I had clobbered
your scope. I couldn't make up my mind whether to join the Foreign Legion or volunteer
to be the first man shot to the moon! To see if I could detect any sign of fluorescence,
I turned out all the shop lights and removed the calibrated screen again. There
was the trace just as nice as you please!
"It didn't take me long to discover that when I tried to replace the screen in
front of the tube the trace would slide off to one side like a glob of mercury under
a finger tip. That transparent screen was carrying a king-size static charge, doubtless
induced by the brisk rubbing with the paper towel. The nasty part was I couldn't
get rid of the charge. I tried rubbing it all over with my hands, holding it under
running water, and squeegeeing it on the metal bench; but the more I did the more
charge it seemed to collect. It picked up lint and dust and began to look as though
I was trying to tar and feather it. Finally I got an inspiration: I rubbed it with
the anti-static cloth we use on records, and that did the trick. I was sure glad
to be able to see that trace again through the screen!"
"I had exactly the same experience," Mac chortled, "but I never thought of the
cloth. I just waited until the charge leaked off and that took a long, long time.
"Much as I hate to break up this Kaffeeklatsch, it's really time we got to work,
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
Popular 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 August 13, 2019