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Mac's Radio Service Shop: Barney Plays "Twenty Questions"
November 1948 Radio & Television News Article

November 1948 Radio & TV News
November 1948 Radio & Television News Cover - RF Cafe[Table of Contents]

These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.

"The Whistler and His Dog" is one of those tunes that you have probably heard dozens of times but never knew the title of it (video at bottom of page). It is mentioned in this installment of "Mac's Radio Service Shop" from a 1948 edition of Radio & Television News magazine. Barney is said to have been whistling it while replacing an output transformer on a receiver-recorder... a wire recorder at that. The "20 Questions" theme is from the game where the player attempts to guess the answer by asking a series of questions that narrows the possible results until only the correct one is left - aka deductive reasoning.

BTW, I'll bet "The Syncopated Clock" is another tune you've heard many times but didn't know the title of it.

Mac's Radio Service Shop: Barney Plays "Twenty Questions"

By John T. Frye

Mac's Radio Service Shop: Barney Plays "Twenty Questions", November 1948 Radio & Television News - RF Cafe

The November morning was overcast, and there was a cold damp wind blowing the scattered leaves about the streets; but this had no effect on Barney's cheery whistling as he stepped into Mac's Radio Service Shop and found Mac already at work at the bench.

"The top of the morning' to you, Boss," he greeted as he hung up his jacket and slipped into a shop coat. '''What great electronic problems await my solving this morning?"

"Well, if it is not asking too much of you, Marconi the Younger, you might put a new output transformer in that set on the other end of the bench."

Barney, still whistling, went to work removing the old output transformer from the speaker. Mac, who was working on a wire-recorder-receiver combination, switched the output of the signal generator into the signal tracer so that the 400 cycle note came clear and strong through the speaker; and then he said to Barney, who was getting in some very hot licks on The Whistler and His Dog, "Would you mind muzzling that dog for a couple of minutes while I get this audio oscillator on the wire?"

Barney remained mute while Mac put about three minutes of the 400 cycle note on the wire through the microphone, but the instant the "rewind" switch was snapped, he burst forth with, "Why are you doing that?"

"The complaint on this outfit is that music does not sound good on it, although voice seems to be natural. I feel confident the trouble is caused by 'wowing' as a result of uneven pulling of the wire. The quickest way to detect a condition like that is to listen to a sustained note of unvarying pitch that has been recorded. If the speed of the wire changes this will show up as a change in frequency of the recorded note. Catch ?"

"I catch," Barney said; "but why doesn't it affect the voice recording?"

"It does, but because the voice is continually changing frequency itself, it is harder to detect this kind of distortion with speech on the wire."

"It seems to me that if the wire changed its speed during the playback exactly as it had during the recording, you would not notice anything wrong."

Mac's face wrinkled into the pleased grin that a shrewd observation on the part of his apprentice always evoked. "You are exactly right; but since this change in speed is caused - I think - by slippage in the drive mechanism, it would be a very rare coincidence if those changes were in step."

The recorder was switched to "Play Wire," and the audio note came through with a rhythmic rise and fall of frequency that clearly indicated the inspiration for that term "wowing."

"Now that you are sure you guessed right, what are you going to do about it?" "Barney wanted to know.

Mac's 'One-String Fiddle' - RF Cafe

Figure 1 - Mac's 'One-String Fiddle'

Without replying, Mac loosened a jam nut on the back of the frame of the wire-transporting motor and carefully adjusted the screw that this freed. As he did so, the note improved, although there was still some deviation. Once more he recorded the 400 cycle note, and this time when it was played back, it came through as clearly and as unvarying as it had come from the speaker.

"That adjustment, which is a ticklish one, regulates the 'motor torque,' " Mac explained. "If it is a little too loose, there is some slippage, and the wire is pulled unevenly, both on 'Recording' and on 'Playback.' "

"What if it is too tight?"

"When you change from 'Rewind' to 'Playback,' the reversal is so abrupt that the wire is broken. Usually, though, the change due to wear is toward the 'too loose' direction, and about a quarter of a turn of that adjusting screw in a counter-clockwise direction will take the 'wow' out."

While Mac was putting the wire-transporting mechanism back in the recorder case, Barney installed a new "universal" output transformer on the speaker frame and connected the primary winding. Then he looked into the service manual to see what the impedance of the voice-coil was so that the proper taps of the secondary could be used, but the information was not given.

"Hey, Mac, how am I going to match the transformer to the voice-coil impedance when I don't know what the coil impedance is?" he asked.

"We use the one-string fiddle for that," Mac told him gravely.

"The what?"

"The one-string fiddle," Mac repeated as he opened a cabinet and took out a narrow piece of wood about a foot and a half long with a slender wire stretched along the length of it (See Fig. 1). One end of the bare wire went beneath a binding post, and there was another binding post at one side of the little board with a flexible lead terminating in a tiny battery clip attached to it.

"That thing looks as though it would suck eggs," Barney muttered. "How can it help in finding the voice-coil impedance ?"

"Just watch, impatient little man, and you will see," Mac said as he switched on the v.t.v.m. and brought a couple of leads from the audio output of the signal generator and clipped them across the volume control.

"That wire is resistance wire, and there is about twenty ohms worth of it there. I am placing this simple rheostat - for that is what it really is - and the voice-coil in series across any two convenient taps of the output transformer, like this": and he sketched Fig. 2 on the blackboard at the end of the bench.

Using Mac's 'One-String Fiddle' - RF Cafe

Figure 2 - Using Mac's 'One-String Fiddle'

"Now I attach the ground lead of the vacuum-tube voltmeter to point B and check the audio voltages that appear between this point and A and this point and C. By sliding the little clip of our one-string fiddle up and down the resistance wire, I can reach a point where the meter will show exactly equal voltages across the resistor and across the voice-coil."

"That means that the impedance of the resistor is the same as that of the voice-coil, huh?"

"Right; and while the impedance of the voice-coil is made up of its d.c. resistance and its a.c. inductance, the impedance of our resistor is practically that of its. d.c. resistance alone, for a straight wire like this has negligible inductance at 400 cycles."

"So all we have to do is to measure the resistance with an ohmmeter, and we know the impedance of our voice-coil," Barney said; "but why did you use the vacuum-tube voltmeter? Wouldn't the multimeter have worked just as well?"

"Yes, for we are not interested in absolute values. All we need to know is that the two voltages are equal. I use the vacuum-tube voltmeter through force of habit When measuring an a.c. voltage other than 60 cycles. Copper oxide rectifier meters are quite frequency-conscious, and they are calibrated at 60 cycles; so they are not too reliable at other frequencies."

"You use this special kind of rheostat in order to get away from the inductance you would find in an ordinary spiral-wound job. That I get; but wouldn't the impedance of the voice-coil change if you used some frequency other than 400 cycles?"

"It would, but unless stated otherwise, all audio equipment impedances are given for 400 cycles. Your matching chart for the output transformer is worked out for 400 cycles."

While Mac watched, Barney turned on the receiver and carried out the procedure outlined. It was found that the voice-coil impedance was very near six ohms; so the proper taps were connected.

"Say, Mac," Barney suddenly said, "I have to give a little talk tonight at the Y. Each of us is supposed to talk on 'What Makes a Good ___ '; and we fill in the blank with what we want to be. I want to be a serviceman; so how's about giving me some pointers?"

Mac rubbed his chin for a few seconds, and then he said slowly, "I think that the ability to play the game of 'Twenty Questions' is what it takes to make a good serviceman."

"Come again," Barney said in bewilderment.

"You know the game. One person thinks of an object, and the idea is to identify it by asking not more than twenty questions. The cleverest person is the one who can name the object with the fewest questions.

"Radio servicing is like that. Instead of an unknown object, you have an unknown trouble to find. You ask yourself the questions, and you make tests to find the answers, The good serviceman is the one who can spot the difficulty with the fewest number of tests.

"You will remember that the classical first question in the game of 'Twenty Questions' is: 'Is it animal, vegetable, or mineral?' That is a fine question, for it immediately cuts down the field to be investigated by two-thirds. The serviceman who puts an audio signal into the first audio stage of a 'dead' receiver and hears it coming out of the speaker has done about the same thing, for he knows that the trouble is ahead of this point.

"The main idea is not to ask unimportant questions that waste time. For example; instead of finding out what kingdom the object was in, a foolish person might ask, 'Does it wear a hat?' The answer would. be 'no' whether the object was a turnip, a lump of coal, or a red-headed radio apprentice, and that question would be wasted. The fellow who simply prods around in a set checking condensers, resistors, voltages, etc., without any plan or purpose is playing the game in the same stupid manner. I am more impressed by the head-scratching, diagram-studying serviceman than I am by the fellow who thinks he is not working unless he has a pair of test-prods in hands."

Barney, who had been scratching down notes on a torn-open volume control box, looked up with a grin. "Thanks a lot, and I'm after thinking 'tis a fine speech you are going to be making with my mouth this evening, Mr. McGregor," he said; "and I hope you'll be remembering after this that when you catch me staring off into space, I am not thinking about Margie; I am simply playing 'Twenty Questions'!"

 

"The Whistler and His Dog" (Wikipedia)

"The Syncopated Clock" (Wikipedia)


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 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 Electronics 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 September 2, 2015

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