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"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 the November 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 that predated magnetic tape types. The "20 Questions"
theme is from the game where the player attempts to guess the answer by asking a
series of questions to narrow 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
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."
Figure 1 - Mac's 'One-String Fiddle."
"Now that you are sure you guessed right, what are you going to do about it?"
"Barney wanted to know.
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.
Figure 2 - Using Mac's "One-String Fiddle."
"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.
"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'!"
Posted October 2, 2020
(updated from original
post on 9/2/2015)
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
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 April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
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