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Imagine reading an article from a 1958 magazine that references the schematic
for a specific radio manufactured in Germany, and then being able to download a
copy of it for free on the Internet. Such is the case with this Mac's Radio Service
Shop story entitled, "Was Ist Los?" Mac is describing to his sidekick Barney the
difficulty in troubleshooting and repairing a
Metz Transformatoren:
Babyphon 56 that a serviceman had purchased while stationed overseas. The
diagram is of course in German, which requires Mac to pull out a language translation
dictionary. The problem was that many words unique to technical jargon were not
in it. Additionally, units of measure for the capacitors and inductors were not
like U.S. units. Mac noted that many capacitor values were labeled with units of
"u," "n," and "p," for "micro," nano," and "pico." He mentions the "micro" prefix
for the letter "u," but never calls the "n" and "p" by the now-standard terms. Instead,
"n" is a thousand micromicrofarads and "p" is a micromicrofarad. To make reading
the tale more interesting, go to the
RadioMuseum.org
website and download the schematic for the Babyphon 56 to look at whenever
Mac references it.
Mac's Radio Service Shop: Was Ist Los?
 By John T. Frye
Spring had been a long time coming, but finally made it. A warm breeze was wafting
in the wide-open door of Mac's Service Shop this sunny morning and shortly after
eight o'clock Barney himself wafted through that door. His blue eyes had a dreamy,
far-away look in them that promised little in the solder-slinging line for this
day.
Mac, the owner of the shop, was already at work at the bench. A large cream-colored,
gold-trimmed portable was in front of him and he was frowning at a small crumpled
diagram spread out on the bench. At his elbow was a German-English dictionary.
"Hey," Barney exclaimed as his eyes focused on the receiver; "what you got there?
That's a pretty gaudy portable, isn't it?"
"Yep," Mac said without looking up.
"It's a Metz 'Babyphon' 56 receiver made in Germany. An Air Force fellow stationed
in England bought it there and had it shipped home. It has some pretty unusual features."
"Looks as though it might have," Barney agreed. "Do those two little folding
doors in front cover the speaker grille? And what does that 'Babyphon' name mean?"
"The speaker grille is in the end," Mac pointed out as he placed the receiver
on its back; "but take a look in here," he suggested as he lifted the outside edge
of the right-hand door. Inside was a turntable and a little ivory-colored pickup
arm.
"Well I'll be!" Barney exclaimed. "A combination two-way portable and a baby
phono player! Does that thing really work?"
"It certainly does. It plays any 45 rpm record. The motor is d.c. and this rheostat
adjusts the speed. The pickup has a genuine sapphire needle. When the set is plugged
into the 'mains' as the instruction book has it, power for the motor is furnished
by rectified a.c. from a transformer. When you must operate the set away from the
power lines, the tiny motor is run by four medium-sized flashlight batteries connected
in series. The instruction book says one set of these batteries will play eight
hundred sides."
Barney had the back off the receiver by this time and was peering inside.
"What's that little square doohickey back in the corner?" he asked.
"That's a built-in, gas-tight, life-time, nickel-cadmium storage battery that
never requires any servicing except recharging. It has a rating of 1.2 volts and
3.5 ampere-hours and heats the filaments of the eight tubes that are connected in
parallel. Not only does this save dry cells, but floating the battery across the
filament supply when the set is operating on a.c. keeps down ripple and holds the
filament voltage constant. That prolongs the life of the tubes. The battery is charging
at a reduced rate when the set is operating on a.c. and you can charge the battery
more rapidly with the set turned off if you like. A full charge will keep the set
going for fifteen hours. On top of that, if the battery becomes discharged away
from the power lines, you can just slip a standard flashlight cell into this space
in the battery holder and run the filaments off it until you have a chance to recharge
the battery."
"They've thought of everything," Barney remarked. "I see the 'B' battery is a
standard 90-volt unit. What do all those five piano-key push-buttons do?"
"One turns the set off; one turns on the phono player; and each of the three
remaining ones turns the set on and selects a different band. The long-wave band
tunes from 150 to 350 kilocycles; the medium band, from 510 to 1640 kilocycles;
and the short-wave band from 6 to 15.5 megacycles. When the phono button is pushed,
all r.f. and i.f. tubes have their filament voltage cut off. A jack in the back
enables you to run the output of the phono player into an external amplifier. When
you plug into this jack, all filaments are automatically turned off and power is
furnished just to the phono motor. The boys who designed this didn't intend to waste
a single electron!"
"Well, let's hear it play."
"That's the joker; get a load of this."
Mac pushed the medium-band key and the set broke into a loud motor-boating. Manipulating
the bass or treble tone controls or the volume control would change the frequency
of the plopping sound, but no setting of any of them would stop it entirely.
"Since when did motor-boating become a tough service problem?" Barney scoffed.
"Chances are the set has an open output filter capacitor."
"Chances are it hasn't; I tried that," Mac said with a grin.
"Open grid in one of the stages?" Barney offered hopefully.
"Nope; neither can I find any open bypass capacitors, loose shields, or resistors
with radically changed values. I decided to quit guessing and study the diagram
a bit; but I ran into a headache there, too. This diagram is in German."
"Gwan!" Barney hooted. "Quit trying to pull my leg. A diagram is a diagram in
any language."
"Is that so!" Mac retorted. "Suppose you take a good long gander at this little
diagram that came with the set and explain some things to me."
"Son-of-a-gun," Barney exclaimed; "it does look kind of funny. Do you have it
figured out yet?"
"Not all of it, but I've discovered a few things. This is the power transformer
here. The thick dark lines indicate transformer windings and the light line between
them stands for the core. These little rectangles are resistors. The plain ones
are 1/3 watt. If lines bisect the rectangle both lengthways and across, the resistor
is 1/4 watt. Two dots inside the rectangle mean the resistor is 1/10 to 1/20 watt.
"Capacitors are shown by the usual symbol of two small rectangles side by side
representing the two plates. In a polarized capacitor, the negative connection is
shown by a blocked-in rectangle, while the positive one is left unshaded. Paper
bypasses have both rectangles blocked in. Such a symbol by itself indicates a unit
with a working voltage of 125 volts. A dot beside the symbol means 250 working volts.
Notice this capacitor in series with the resistor from plate to plate of the push-pull
output stage has a little tilde beside it. I'm not sure, but I think that means
a special capacitor for use across a high-potential alternating source.
"The capacity values beside the capacitors on the diagram bothered me for a while,
but I think I have them figured out. The letter 'u' means microfarads. The letter
'p' apparently means micromicrofarads. The letter 'n' must mean 'thousands of micromicrofarads.'
At least a capacitor that is marked 0.047 is shown on the diagram as '47n.' "
"What's this thing that looks like a hatpin drawn through some of the coils and
capacitors?"
"It indicates the coil or capacitor is variable. Notice, too, a single, short,
heavy horizontal line means 'ground.' "
"How about these things that look like dominoes standing side by side?"
"You mean on that tube-layout pictorial? That is the connection panel for the
push-button switches. Notice the letters of the alphabet are spaced across the top
of the vertical rows of dots and a number is alongside each horizontal row. Each
connection is thus indicated by a letter-number combination, and they are so indicated
on the diagram. For example, see how this slider connects 'C5' to either
'C4' or 'C6.' That makes tracing a circuit through the switches
very easy."
"Are those standard American tubes?"
"No, but a slip of paper attached to the diagram says they can be replaced with
American types. It says an American 1AJ4 will replace a European DF96; a 1AN5 replaces
a DF97; a 1AH6 replaces a DAF96; a 3C4 replaces a DL96; and a 1AB6 replaces a DK96."
"Well, what are you going to do about the motor-boating?"
"Easy, Sorrel-Top, easy! I think I'm getting a clue. See how the volume control
and both tone controls are tied in more or less with the negative feedback loop
from the voice coil to the bottom end of the volume control? Remember each of these
controls affected the motor-boating. Now if something were happening to shift the
phase of the feedback, we could easily get just what we're getting."
"Maybe someone connected the feedback wire to the wrong side of the voice coil,"
Barney suggested.
"No; the set was working perfectly when packed for shipment home, and no one
has touched it since it arrived. If we can find the cluster of resistors and capacitors
used in the feedback circuit network, we may find something wrong there. Hm-m-m-m,
this bunch has the right values."
As he talked, Mac was gently moving the resistors and capacitors in a group mounted
on the output transformer. Suddenly the motor-boating ceased and the radio played
loudly and clearly.
"That was real tough," Mac said with a sheepish grin. "Two of the resistors were
rubbing together and shorting out."
"OK, but let me say here and now you are welcome to all the foreign sets that
come in. I've enough trouble when I have good service literature I can understand.
Having to puzzle out the diagram or circuit as well as the symptoms is too, too
much."
"You've got a point there. Something such as this makes us appreciate how good
our service literature is. We are going to have to charge more for working on foreign
sets than we do for domestic receiver service where both our experience and our
service literature help so much."
"Yeah, and I see a lot of those parts are of a special sort that would have to
be obtained directly from the manufacturer. That's another headache that we have
to face."
"Before a foreign manufacturer can expect to do a large business over here, he
is going to have to make it easy to obtain special parts and see to it that the
service technician has adequate service data. Maybe he could arrange to have his
sets covered by the major service data publishers. We won't turn down any foreign
set service, but I don't think it would be wise to solicit this kind of service.
We seem to be getting plenty as it is. Within the last week we have had two German
receivers, a Japanese receiver, and a Dutch tape recorder in here."
"One last question," Barney said as he wiggled his way into his shop coat. "What
were you doing with the dictionary?"
"Trying to puzzle out the abbreviations on these trimmers. See, they are marked
'TOL, TVL, TVK, TVM, TOK, and TOM.' From what I can pick up in this German-English
dictionary, I think 'TOL, TOM, and TOK' indicate the oscillator trimmers, for the
long-, medium-, and short-wave bands respectively. 'TVL, TVM, and TVK' must be the
r.f. trimmers for the same bands."
"Ach! German abbreviations yet!" Barney muttered.
Posted January 21, 2020
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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