Just as the title of
this installment of Mac's Service Shop, "A Typical Day in the Shop," suggests,
the story is a recollection of the kinds of scenarios that would found in an
ordinary shift in an electronics service business in the mid 1950's. Vacuum
tubes were the norm of the day, as were discrete leaded components and a rat's
nest of wires running from solder lug to solder lug. Printed circuit boards were
beginning to appear in commercial products, but mostly existed in specialty
defense and aerospace applications. You might wonder how many different ways
could there be for simple circuits like biasing and heater element lighting, but
some pretty imaginative variations made their way into radios, television,
record players, and tape decks, and often times that made a serviceman's life
heck. Such was the case here as über-owner-technician Mac admonishes sidekick
Barney for not taking time to consult the schematics for his sick patient. Mac
also take the opportunity to plug the benefits of hiring an expert to do a job
for you even though you might be able to do it yourself.
Mac's Service Shop: A Typical Day in the Shop
By John T. Frye
A man who was just leaving the service shop politely held the door open for Barney
to enter. The youth, at the age when he thought all forms of common courtesy were,
at best, so much wasted motion, or, at worst, performed with an ulterior motive,
looked suspiciously after the neatly dressed man as he carried a small tool box
to his waiting car.
"Well, Mac," Barney greeted his boss, "did you finally get a set too tough for
you and have to call in another technician for a consultation? Why didn't you call
on me? I'm always available."
"That would be like a one-eyed man asking a blind man to lead him," Mac grunted.
"That fellow is a technician all right, but he works on electric typewriters instead
of radios. The on-off switch on Matilda's pride and joy finally wore out, and he
came over to replace it."
"If that's all that was wrong, why didn't you do it yourself?"
"My mind started working in that direction, too," Mac admitted, "and then I brought
myself up short. There I was right on the verge of doing what I criticize so severely
in our customers: tinkering with something I knew little about and that I was not
equipped to service; so I picked up the telephone and called the service department
of the people who sold us the typewriter and asked them to send a repairman over
to fix it. He came right away, and I'm certain I got a lot more useful information
out of watching him work than the bill will total."
"Well, I happen to know that the company manufacturing our electric typewriter
also makes other expensive office equipment used all over the globe. Since they
realize their service technicians represent the whole company and its products in
the minds of many people, they give each fellow an intensive and thorough training
before he is allowed to start work. It occurred to me that if I watched him at work
here in our office, I probably could pick up some good pointers on how a professionally-trained
technician worked to produce a good impression; then I might even try to graft some
of these scientific methods on you."
"Trust a Scotchman to squeeze an education out of a service call," Barney said
with a grin. "What did you learn?"
"You saw for yourself how neat and clean he was - and how courteous. He came
in, introduced himself, removed his coat, opened his toolbox and took out a workcloth
to place beneath the typewriter, put this in place, and went right to work. No time
was lost in idle chatter. In that little toolbox he had exactly the right tool for
every job, and I particularly noted the tools were clean and in excellent condition.
"Even though I promptly gave him my own diagnosis of the trouble, he politely
made his own checks before starting to remove the switch. Two or three other times
I offered him the benefit of my valuable advice - which invariably turned out bad.
He received all this courteously, without comment, and then went right ahead to
do the job the way it should be done; but he was careful not to mention or point
up the fact that I was mistaken. That takes good discipline and will power to keep
from showing a smart aleck how wrong he was!
"I kept trying to engage him in conversation on every subject from baseball to
politics. He answered me each time politely and briefly, but he kept right on working.
His movements were quick and sure but gentle. That typewriter was treated as though
it were a priceless family heirloom. After the switch was installed, he worked it
several times to be sure it was all right. Nothing was taken for granted. Next he
cleaned and oiled the entire machine - not only the working parts but also the portions
that had to do with the appearance of the typewriter. After that, in spite of the
fact that Matilda and I both assured him there was nothing wrong with the typewriter
except the bad switch, he ran a sheet of paper into the rolls and checked the operation
of every key and control several times. And he found several minor flaws in the
typewriter's operation that we could easily see when he pointed them out but that
had gone unnoticed before. For example, he made every key hit with exactly the same
force so the letters were of equal shading. All of these things were taken care
of easily and quickly until finally the machine was doing the job he knew it was
capable of; then he put away his tools and wiped the desk so it was actually cleaner
than when he started."
Barney took in every word of this, although he pretended to pay scant attention;
and Mac knew the boy would try to put into practice the points observed and stressed.
For a while both men worked in silence; then Barney piped up:
"Mac, here's a queer one. This set is practically new, but it has developed a
bad hum. At first I thought it was filter capacitors, but new ones don't help. I've
tried new tubes in the output and audio amplifier stages with no improvement; yet
I know the hum originates here because when I pull any tube ahead of the detector
and audio amplifier tube, the hum is not affected; but when I pull that tube, the
hum stops. A kind of funny thing is that the hum seems to get worse as the volume
is turned down."
Mac noted the model number of the set and went to the service literature file
and pulled out a folder covering the set. He studied the diagram for a couple of
minutes and then turned the chassis over and studied it for a few seconds. Next
he got a new 35W4 tube and substituted it for the one in the set. When this warmed
up, the set played normally and there was no hum.
"Well, I'll be a monkey's uncle!" Barney exclaimed. "I never thought of a bad
rectifier tube causing hum. Thanks a lot. I'll put it back in the cabinet now."
"Not so fast," Mac cautioned as he replaced the old 35W4 in the socket. He waited
until the hum was coming through strongly and then carefully cut a lug loose from
one of the tube sockets so that the resistor and capacitor leads soldered to this
lug were freed from the socket. Instantly the hum quit.
"For the umpteenth time," Mac said. wearily, "I repeat: study the diagram whenever
you come up against an unusual symptom. In this case the unusual symptom is that
business of the hum being worse when the volume is turned down. The diagram shows
the bottom of the volume control does not go directly to ground, as is usually the
case, but instead returns to 'B-minus' through this 2200-ohm resistor. A 0.05 μfd.
capacitor connects to the junction of the control and this resistor, and the other
end of the capacitor goes through this 4700-ohm resistor to the cathode of the output
tube. The cathode resistor of this tube is not bypassed, and some of the voltage
developed across it is fed back through the resistor and capacitor to the bottom
of the volume control, thus supplying negative feedback to the grid of the audio
amplifier tube fed from the sliding contact of the control.
"The hum's growing worse as the sliding contact approaches the bottom of the
control made me think the hum was being fed in at this point. About the only way
it could get there would be through the .05 μfd. capacitor connecting there. When
you look at the chassis wiring, you see that this capacitor and the 4700-ohm resistor
tie together on this blank lug of the rectifier socket. I suspected that something
may have happened inside the rectifier to cause the 'blank' pin to be hot with a.c.
When a new tube cleared up the hum, I was sure this was the case. Replacing the
old tube and then removing the capacitor-resistor junction from the socket to again
have hum-free reception cinched things. Just put in a tie-point for the connection
you have taken off the socket and leave the old rectifier in. It can cause no further
trouble, and neither can any other rectifier that may develop this fault."
"I don't think much of tying leads to empty socket lugs;" Barney remarked as
he started mounting a tie point on the chassis.
"Neither do I when those lugs have a tube pin connected to them," Mac agreed
as he went to answer the telephone; "and tying a sensitive grid connection to a
rectifier socket with its high-voltage a.c. is just asking for trouble."
When Mac came back from answering the phone his face was wearing a satisfied
smile. "That's the kind of call I like to get. It was Mr. Rudy just calling to say
his set was working fine and thanking me for clearing up his trouble. Most customers
only bother to call when they want to gripe.
"Mr. Rudy's was a rather interesting case. He brought in his set about a month
ago, and it was seemingly a simple repair job, for all that was wrong was a shorted
plate bypass capacitor together with its charred decoupling resistor. Both were
replaced and everything worked fine. I let the set run for a couple of hours and
then took it back. The next day Mr. Rudy called and said it was dead again with
exactly the same symptoms as before; then he told me he had had trouble with this
set time and again and that he was about in the notion of junking it.
"Well, I picked it up and found that another high-voltage bypass capacitor had
gone west, taking a resistor with it; furthermore, I noticed now that several other
'B-plus' bypass capacitors had been replaced previously. A new capacitor and resistor
restored the receiver to normal operation, but I did not take it back. Instead,
I called Mr. Rudy and asked him if he burned out a great many light bulbs in his
house. When he emphatically said that he did, I suggested he call the light company
and have them put a recording voltmeter on his line for twenty-four hours. The company
did this and found the line voltage hit peaks of around 127 volts. The fellow who
installed the recording voltmeter told Mr. Rudy he doubted this would cause trouble
in the radio because it would only raise the voltages something less than ten per-cent
above normal, and that would still leave the bypass capacitors with a voltage rating
leeway of 150 volts or so.
"It was true that the highest voltage in the set when it was operating normally
on a 117-volt line was 250 volts and the capacitors were all rated at 400 volts;
but when a voltmeter was placed across one of the new capacitors and the set was
turned on, the voltage soared to 375 volts for several seconds before finally settling
down to the 250-volt figure. You see the set uses a filament type of rectifier while
the rest of the tubes are of the cathode type. That means the high voltage power
supply runs virtually unloaded until the tubes warm up and start drawing current.
Since series dropping resistors to screens, etc., only perform their voltage dropping
function when current is being drawn through them, bypass capacitors at the ends
of these resistors were subjected to the full high voltage during this warm-up period,
even though the working voltage at these points might be below a hundred volts.
"Next I ran 127 volts on the set from our tapped isolation transformer. When
I did this, the voltage on the capacitors soared to 430 volts before the warming
cathodes pulled it down. That easily explained why the receiver was pop-ping all
the capacitors. Upon my advice Mr. Rudy had the electric company change the taps
on the pole transformer feeding his house so that his voltage was down around 117
volts where it belonged. He reports he has not had a bit of trouble with the set
since and he has not replaced a single light bulb since the voltage was lowered."
"That's interesting," Barney remarked. "I'm so used to seeing the bad effects
of low voltage, especially on TV sets, that I never thought about high line voltage
giving trouble. Come to think of it, though, practically all TV sets use either
filament or selenium type rectifiers; and in either case the output of the low voltage
supply runs unloaded until the tubes warm up and start to draw current. From now
on I'll be suspicious of this condition when I run across any sets that seem to
be blowing too many capacitors."
"And don't forget that high line voltage is hard on tubes, too, just as it is
on light bulbs," Mac pointed out. "Be suspicious of high line voltage when a set
burns out a lot of tubes. Many TV sets have a tapped primary on the power transformer
to compensate for line voltages that are abnormally high or low; but it is better
to have the electric company correct the voltage fed to the house, if they can.
If this is done, all the other electrical equipment in the house will be relieved
of the strain imposed upon it by improper voltage, while changing the taps on the
TV receiver transformer will only help it."
"Check!" Barney acknowledged.
Posted August 21, 2020
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive 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 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
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.