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Unlike many electronics enthusiasts
including hobbyists, salesmen, servicemen, and commercial and domestic users, Mac
McGregor was an early-on believer of the ultimate replacement of vacuum tubes by
transistors. Only a decade old and not yet adopted by a lot of manufacturers (to
their ultimate demise in some cases), transistors were fighting a major battle to
gain acceptance and trust by the public. Not only were transistors still more expensive
than an equivalent vacuum tube, but the reliability was not as good - most times
due to designers not properly accounting for their special needs for protection
against voltage extremes. Once the price of transistorized products fell into parity
with their predecessors, consumers quickly adopted the products because of the markedly
smaller sizes and lower power consumption. Portability for battery-powered radios
and other entertainment-related items was a huge selling point for transistorization,
to which magazine advertisements of the era attest. In this episode of Mac's Radio
Service Shop, Mac is explaining the utility of his newly acquired Battery Eliminator
for use in servicing automobile and household radios that run off of a 6- or 12-volt
deep cycle (i.e., lead-acid) battery.
Mac's Radio Service Shop: The Time Is Now
 By John T. Frye
Aha!" Barney exclaimed as he came sailing into the service shop out of a gusty
March morning and found Mac, his employer, bent over a piece of test equipment on
the bench; "new gadget, huh?"
"Yep," Mac answered as he plugged the cord of the instrument into a socket. "This
is a replacement for our old battery eliminator. I bought it in kit form and put
it together last night."
"What was the matter with the old one?"
"Nothing except that it's outmoded. As you know, we've been hooking a battery
in series with the eliminator to work on twelve-volt sets but that arrangement leaves
a lot to be desired. For some time I've been intending to buy a new eliminator that
would handle both six- and twelve-volt radios, but I was holding out in the hope
someone would build in the extra filtering needed in powering hybrid or all-transistor
sets. In the past few months several manufacturers have come on the market with
just such instruments; so I no longer had an excuse."
"I get you. Since the battery voltage furnished a hybrid or transistor receiver
is equivalent to the plate voltage in a tube radio, it must be well filtered to
prevent hum."
"Your little gray cells are working today!" Mac applauded. "As you can see, this
eliminator has continuously variable output voltages of either 0-8 or 0-16 volts.
Output voltage and current are monitored by the two meters. Notice there are two
sets of binding posts, one on each end of the panel. Those on the left provide moderately
filtered output, with about 2% ripple, for use in battery charging or with vibrator-type
sets. You can take continuous currents of ten amperes at six volts or five amperes
at twelve volts from them. The posts on the right furnish five amperes at either
voltage with very low ripple."
"How is the ripple reduced?"
"By passing the moderately filtered output through an extra L-section filter
consisting of a heavy choke and a 10,000 μfd. capacitor. This brings the ripple
down to less than 0.3%. I am not just taking the manufacturer's word for this; I
measured it with the scope; but let me show you the difference."
As he said this, Mac connected a high-wattage, low-resistance resistor across
the right-hand output terminals and adjusted the eliminator so that it was furnishing
about four amperes of current to the resistor. Then he connected a large capacitor
to one of the tip leads of a pair of earphones and touched the other earphone lead
and the free end of the blocking capacitor to the two output terminals on the left
side of the panel. A very noticeable low-pitched hum came from the phones. He transferred
the connections to the right-hand terminals.
"Hey," Barney exclaimed as he pressed the muff-type earphones tightly against
his head, "I can't hear the least trace of hum!"
"And you shouldn't," Mac said as he turned off the eliminator. "That ripple level
must be very low if you are going to use the eliminator to power all-transistor
sets as I expect to find in many cars next year. Of course, when the variable voltage
feature is not needed, we can always cut the hum still more and provide very good
voltage regulation by floating a battery across the output of the eliminator, but
I don't think that will be needed. In a few cases in working with extremely high-gain
transistor amplifiers we might want to cut the ripple to an absolute minimum. Such
circuits would not require any appreciable current; so it would be very simple to
use an outboard filter section consisting of a high-inductance, low-resistance choke
and a whopping big capacitor of low voltage between the eliminator and the transistor
circuit. About the only time you would need such extreme filtering would be in experimental
work, but it's nice to know a simple addition to the eliminator will provide very
nearly pure direct current.
 "While we're talking about it, I want to mention a couple, of other points to
watch in working on the new hybrid or all-transistor auto sets, in fact, for working
on any set that employs power transistors in the output. With the auto sets, you
must be very, very sure that the voltage applied to the set on the bench has the
same polarity as that provided by the car battery. Reversed polarity will ruin the
transistors almost immediately. After being used to working with interrupter-type
vibrators in which the polarity of the connections makes no difference, you could
overlook this very easily.
"Make sure, too, that the output transistors are working into a proper load at
all times. Running a signal into them with the speaker disconnected is a fine way
to ruin them. And you've got to be careful what you do with your test prods when
you're working on one of these sets. They are as touchy in that respect as battery
portables, but for a different reason. With the portables, you have to be careful
that you do not accidently short the high plate voltage across the fragile filaments
and blow them. With the transistor sets, it's not the voltage as much as the almost
unlimited current provided by the battery that you have to treat with respect. A
test prod that accidentally grounds the base of an output transistor can ruin the
transistor while you are batting your eyes. That's why you must never try to use
the old circuit-disturbance type of troubleshooting in which you ground a tube grid
and listen for a click in the speaker in working on transistor sets. Ground a base
and the click you hear will probably be the last."
"I'd think that a battery-eliminator, being fused and providing only a limited
amount of current, might provide some measure of insurance against that kind of
damage."
"It does, but don't depend on it. A charged 10,000 μfd. capacitor can put
out a lot of current for a few milliseconds and a fuse takes an appreciable time
to melt. Let's just say you'd be less likely to ruin a transistor when the set is
being powered by an eliminator than you would when a battery was furnishing the
current."
"Sounds to me as though we're getting pretty transistor-minded all at once."
"That we are and I think this is precisely the time we should. We have to keep
in mind that the attitude of a practical service technician toward new electronic
developments has to be considerably different from that of an experimenter or hobbyist.
The latter start working with the new device as soon as they can get their hands
on it, but the wise service technician knows that he cannot do this and still take
care of his bread-and-butter job the way he should. He has just about all he can
do to keep abreast of the new circuits that are constantly being thrown at him by
radio, TV, and hi-fi manufacturers. When and if the new development proves itself
and is incorporated in the equipment the technician must service, then it is high
time he becomes thoroughly familiar with its possibilities, its limitations, and
its general behavior.
"As far as transistors are concerned, I believe that time is right now. We both,
of course, have kept abreast of the theory and general development of transistor
circuits through the fine magazine articles that have been published on the subject,
and we have had no trouble taking care of the few transistor radios that have drifted
into the shop so far; but from here on in we can expect to meet transistors more
and more often. Now is the time to get any special equipment needed to service transistorized
equipment, and now is the time to start getting all the practical experience we
can with transistors. What's happened to your beagle instincts? Over there on the
end of the bench is a new transistor-tester you haven't even noticed yet!"
"Well so it is! I have read, though, that transistors fail very seldom as long
as they are not abused. In fact, it is said that in a tube set that won't work,
the first thing you suspect is a bad tube; but in a transistor set, the transistors
themselves are the last thing to test."
"That's probably true, but I expect to use the transistor tester in a lot of
ways. For one thing, it is often necessary to use matched transistors and this tester
will allow us to select such matched pairs. Also it will permit us to sort through
our stock of transistors - pretty limited, I'll admit, at present - and select the
best transistors for particular jobs. For example, we can sort out low-noise units
for the front end of amplifier circuits, save the high-gain transistors for places
where gain is important, and so on."
"The whole discussion makes pretty good sense," Barney admitted. "I was reading
the other day where a Mr. Fancher of General Electric said that while transistors
are used in only about 12% of new electronic equipment built this year, they will
go into 80% of the equipment built in 1967. Looks like it's time we got on the bandwagon."
"Check! I'm also collecting all the transistor data I possibly can from the various
manufacturers," Mac said as he waved at a new shelf on the wall. "Everyone of them
puts out some printed information on their products and various applications. These
vary from simple characteristic leaflets to complete booklets on the theory of transistors,
suggested circuits, etc. Every time I see something new is published I order it."
"I see some books on that shelf that are not concerned with transistors," Barney
observed.
"That's right. I'm trying to select a bunch of books that will help us with the
'dogs.' Each book covers just a specialized portion of a circuit or piece of equipment.
You'll find them on servicing sweep circuits, video circuits, a.g.c. systems, sync
circuits, TV tuners, hi-fi systems, tape recorders, and record changers. When we
get a real tough problem in the shop and know that the trouble must lie in a particular
portion of the equipment, we need exhaustive information on the problem to make
sure that we are not overlooking a possibility. That is where the books are well
worth their cost. They provide a concentrated review of everything we should know
about that particular circuitry or mechanism. I know from experience that leafing
through one of these books can often spotlight a source of trouble I was completely
overlooking. I intend to keep adding to that shelf. For example, I intend to get
a book on color convergence procedures and another on industrial electronics as
soon as I can."
"Good," Barney applauded. "I can see where reading through one of these books
will be just like having someone say, 'Did you try this?' and 'Did you try that?'
But where was your imagination when you built that shelf? Why didn't you build it
in the shape of a dog house?"
Posted February 14, 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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