As always, the reading of this Mac's Radio Service Shop techno-saga leaves
you a little bit smarter about an aspect of electronics as well as how to
approach business from both the serviceman's and the customer's perspective,
with a bit of humor and personality thrown in for good measure. Author
John T. Frye, who also wrote the long-running and much-loved
Carl & Jerry series, uses service shop proprietor "Mac" McGregor as a
sort of alter ego for passing on his own extensive knowledge of radio and
television servicing. Customer satisfaction is the main theme of this
installment, which entails going beyond just repairing the problem the items
are brought in for. Mac instructs his young wingman technician Barney to run
a series of standard tests on every set in hopes of ferreting out any
potential future problems that might cause the customer to need to return
his radio or TV sometime in the near future. He pitches the policy as a
customer satisfaction measure since repair services tend to be blamed for
the "next" failure even if it has nothing to do with the last failure. It is
one of those things that you never get credit for if there are no problems
so you hope it at lease prevents you from being ruled out when service is
An extensive list of other episodes of Mac's Radio Service Shop is at the
bottom of the page.
Mac's Radio Service Shop: Satisfied Customer Insurance
By John T. Frye
Since mid-morning the doors of Mac's Radio Shop had been standing wide open,
for this had been the first really warm day of the year. Even now in late afternoon
the May sun shone down hot and bright. Children loitered on their way home from
school, with their useless jackets slung carelessly across their shoulders.
The first office workers were starting home, too; and they walked leisurely
along with their topcoats draped over their arms and carrying their hats in
their hands as though to show that the enjoyment of a warm spring day was not
the sole privilege of children.
"You know something, Mac?" Barney called from where he looked out at the
scene from the open doorway of the shop; "this is one time of year that I'm kinda glad my memory is not too good. Every winter I forget just how super an
early summer day can really be; so each spring I have the fun of finding out
all over again."
"Okay, Spring Fever," Mac said with assumed gruffness, "but don't forget
you've still got a set here on the bench that you have been pawing over for
the past couple of hours."
"All right, all right! Lay down the whip, Simon Legree," Barney said as he
sauntered back into the service department. "To tell you the truth, however,
that little cuss about has me whipped. The volume on it is way down, and it
distorts; yet a check of all the usual causes of such a complaint turns up nothing.
All voltages are within 5%; all tubes are good - and just to make sure, I've
substituted for all of them; coupling condensers are neither open nor shorted;
and the speaker magnet has plenty of stuff. A signal tracer shows everything
to be perfectly normal until you reach the plate of the output tube. It sounds
a little fuzzy there, and when you listen across the voice coil it sounds even
fuzzier. I thought it might be a shorted voice coil; so I unsoldered the output
transformer secondary leads and clipped another speaker across them, but there
was no improvement. Any suggestions will be gratefully received."
"If you found everything all right until you reached the output transformer,
and if changing speakers revealed that the trouble was not beyond the transformer,
it would seem that it might be the transformer," Mac commented.
"Neither winding is open," Barney objected. "Ohmmeter checks show the resistances
to be about normal."
"How about some shorted turns in one of the windings?"
"I hadn't thought of that. I suppose it could happen, though. I'll try a
new transformer; but I'm warning you now you had better be guessing right. Changing
the output transformer on this little set is no small chore."
"You don't have to change the transformer to check it for shorted turns,"
Mac said as he switched on the audio oscillator and v.t.v.m. I'll set the oscillator
to 400 cycles, and you connect these leads from it across the primary of the
output transformer. Unsolder one of the secondary leads from the voice coil.
Now check the audio voltage across the primary with the a.c. probe of the v.t.v.m.
Hm-m-m, there doesn't seem to be any. You will notice, though, that when the
leads are unclipped from the transformer primary, nearly four volts of 400-cycle
voltage is present across them.
"And now," he went on as he removed a universal output transformer from its
carton, "let's see what happens when we put this audio voltage across the primary
of a transformer we know to be good. See there: the voltage drops only about
half a volt; but look what happens when we short a couple of the output terminals:
the voltage falls to zero. That's exactly why the voltage is zero across the
primary of that transformer in the set. Some shorted turns in either the primary
or the secondary are acting as an extremely heavy load and lowering the impedance
of the primary so far that our audio oscillator cannot build up a voltage across
"Why bother with the audio oscillator? Why not just use a.c. from the line
"Two reasons: first, the impedance of the transformer would be much lower
to 60-cycle current than to the 400-cycle current at which it is rated, and
the resulting heavier current might cause even a good transformer to overheat
if left on too long. Secondly, the output voltage of the oscillator has very
poor regulation. By that I mean that an increasing load makes the voltage go
down rapidly; and since our test requires a voltage that fluctuates with loading,
this feature of the oscillator is made to order. On top of that, you know that
I never approve messing around with dangerous line voltage unless it is absolutely
Barney soon had the transformer changed and, as the set warmed up, he had
to grab the volume control and back it off in a hurry, as music shook the speaker
cone with great volume. "Nothing the matter with the quality now," he remarked
with a pleased grin as he cocked a critical ear.
"And so we'll have another satisfied customer," Mac observed; "which leads
into another matter I've been wanting to talk over with you. I got to thinking
last night that we ought to take out Satisfied Customer Insurance on every set
we put out."
"Where would a guy buy insurance like that?" Barney wanted to know.
"You don't buy it; you build it into your service technique," Mac replied.
"You see, the average customer measures the quality of the service job he gets
by the length of time his radio or TV set plays before he has to call a technician
again. We know that this is not strictly fair, for a lot of things can happen
to a complex electronic machine that simply cannot be foreseen and prevented.
The only real test of a service job is how well the radio or television set
performs when it is delivered.
"However," he went on, "if you want to stay in business, you have to do your
best to satisfy the customer's idea of good service; and there really are several
things you can do to a set while it is on the bench that will go a long way
toward reducing the likelihood of more trouble showing up in the immediate future.
These are the things that I want you to include as a part of regular checking
routine of any radio you put on the bench."
Barney perched himself on the service bench stool and entwined his long legs
through its supports. Pulling a pencil and scratch-pad from the pocket of his
service coat, he moistened the lead with the tip of his tongue and poised the
pencil over the pad.
"Shoot!" he commanded out of the corner of his mouth in his best Humphrey
"Well, first, I want you to make certain none of the controls are on the
verge of causing trouble. Check volume and tone controls while the set is cold,
for often trouble will show up in variable resistors then, but will disappear
when the set is warm. If the change in volume or tone is not smooth and quiet,
if there are any abrupt changes in control action or any tendency to create
scratching sounds or whistles when the knob is moved, change the control. Do
not try to doctor it with cleaning fluid."
"Check," Barney acknowledged. "Try the tuning action for slippage while the
set is cold and again after it has warmed up. Some dial cords and belts will
slip when warm; others, only when cold. While you are checking, move the knob
very slowly and note if the pointer responds immediately and positively to every
slight movement of the knob. If it does not, if you have to jerk the knob or
press it up or down or sideways to start the pointer, correct this condition;
and the best way to start is by installing a new cord. Do not increase the cord
tension beyond normal or roughen the shaft or use dope on the cord, for usually
such measures are only temporary cures, and the trouble will soon show up again."
"Use the v.t.v.m. to check bias voltage between cathode and signal grid of
each output tube. If this voltage is not up to rating, find out why. A leaky
coupling condenser is frequently the cause; so clip the grid end of the condenser
loose and check it for positive voltage. If any is present, change the condenser.
Sometimes an output tube will develop a positive-creeping voltage on the grid
even when the coupling condenser is cut loose and the grid-supply voltage stays
the same. Such a tube is defective and should be replaced. With these precautions
you will often be able to nip trouble in the bud before any actual audio distortion
can be noticed.
"Use the vacuum tube voltmeter to check the grids of tubes controlled by
automatic volume control for the development of a good a.v.c. voltage when a
local station is tuned in. Leaky bypass condensers on the a.v.c. line will cause
strong stations to overload, and they are often overlooked in routine servicing
unless they develop direct shorts.
"Next feed a modulated signal into the receiver and use the a.c. scales of
your v.t.v.m. to check the push-pull output stage for balanced input and output.
You will be amazed how unbalanced you will find some of these. Correction is
usually a matter of changing tubes, coupling condensers, or resistors that have
changed value. A lopsided output stage is incapable of really good quality."
"Check," Barney repeated as Mac stopped for breath.
"Be sure and check for hum. Listen to the set tuned to a dead spot on the
dial with various settings of the volume control. A slight increase in hum as
the volume control is advanced is normal, but at no setting of the control should
the hum be more than barely noticeable. Incidentally, always listen for hum
with your ear directly in front of the speaker. If you listen at certain critical
angles off to one side, the hum vibrations from the front and back of the speaker
will cancel and the hum will not be heard. Always find the cause of excessive
hum and remove it. Do not try to kid yourself that the set 'has probably always
sounded like that.'
"I think you'll find that checking filter condensers by measuring the pulsating
voltage across them with the a.c. portion of the v.t.v.m. is better than bridging
with another condenser. This last practice produces a surge that often temporarily
cures the hum and makes it impossible to be sure as to what was the cause. The
v.t.v.m. probe does not do this, and after a little practice you will be able
to tell by reading the meter whether or not the condenser is doing the filtering
job it should.
"You have to keep in mind, however, that open grids, cathode-to-filament
shorts, and other less-common conditions can produce hum. For example, one kind
of hum is only heard when a strong station is tuned in. A nasty characteristic
of such a tunable hum is that often it will be very bad on a particular station
when the set is used in one electrical outlet but will disappear altogether
when another outlet is used. In the latter case, though, it will usually be
found that the hum will reappear on another station carrier at a different portion
of the dial. That is why it is a good idea to listen to three or four strong
stations at different parts of the dial when checking for such a hum. As you
know, a very common cause of this condition is a defective line-bypass condenser.
"And finally," Mac finished after stopping to draw a long breath, "always
give each set a ten-minute run at 125 line volts and then another ten minutes
at 105 volts. If nothing shows up at these voltages, we can rest assured we
have done our best to anticipate any trouble in the immediate future."
"How much are we going to tack on to our usual service charge for making
these extra checks?" Barney asked.
A quick frown creased Mac's face. "Not a nickel!" he said emphatically. "If
you insist on looking at it from the what's-in-it-for-me angle, the thorough
checking will doubtless turn up additional needed repair work, and the increased
customer-satisfaction should bring in more repeat business; but when I was thinking
about the matter, my only concern was how we could do a better job of servicing.
"This present tendency to concentrate on the reward instead of the work is
beginning to get under my hide a little," he went on. "I'm just old-fashioned
enough to believe that if we put all our efforts into doing a really fine job
of servicing, our income will take care of itself. Right now, when everybody
is beginning to complain that it is impossible to find a workman who is conscientious
and takes pride in his work, is a pretty good time to start proving them wrong."
"Check!" Barney agreed.
Posted July 29, 2019
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
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.