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 to remember?
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 it."
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?" he asked.
"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, Red."
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