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At the time this Mac's Service Shop episode appeared in a 1957 issue of Radio &
TV News magazine, electronics technicians were beginning to see a lot of transistorized
radio, televisions, record players, and tape recorders showing up in place of the
very familiar vacuum tube models. It was a whole new ballgame. To complicate matters,
biasing, interstage coupling, and tuning circuits were in many ways different requiring
re-learning what a "typical" circuit looked like, and the introduction of printed
circuit boards in place of point-to-point wiring made changing components more difficult.
Delaminating metal traces was easy to do on early PCBs when using the big, high
thermal inertia soldering irons required for larger and more heat-tolerant components.
Author John T. Frye used these Mac's Service Shop stories as a teaching tool
as well as for entertainment. A list of all the episodes I have posted is at the
page bottom.
Electronics magazines of the era published many articles about selenium rectifiers,
including
After Class: Working with Selenium Rectifiers,
The Semiconductor Diode,
New Selenium Rectifiers for Home Receivers,
Selenium Rectifiers,
Applications
of Small High-Voltage Selenium Rectifiers, and
Using
Selenium Rectifiers.
Mac's Service Shop: Servicing "Too-New" Sets
By John T. Frye
Spring was in the air! Buds were swelling to the bursting point on every branch;
robins left off their chirping only long enough to yank elastic worms from the warm
earth; cotton candy clouds sailed serenely in the bright April sky; the breeze wafting
up from the south was warm and caressing. A young man like Barney, the Number Two
service technician at Mac's Service Shop, should certainly have been floating around
in a rosy glow.
But when Mac, the owner of the shop, looked down the service bench at his youthful
helper, it did not seem that young man's fancy was lightly turning to anything gay
and romantic - not from the thoughtful scowl on his face as he stared down at the
transistor portable radio on the bench in front of him.
"What's the matter, Wonder Boy; something got you stumped?" Mac asked.
"In a manner of speaking, yes," Barney admitted with dignity. "This transistor
job has an intermittent condition - with complications! It will play along for a
while and then suddenly go as dead as a hammer. At other times it does not quit
entirely but simply drops away down in volume. Shaking or jarring it will usually
bring it back to normal operation for a short time. The heck of it is that we have
no service data on it. It's too new."
"Are you sure?" Mac asked. "We've got a standing order for service literature
to be delivered to us as soon as the distributor gets it, and the publisher of the
service data gets out the dope on the new receivers mighty fast."
"I'm sure, all right. I was looking in that new index that came yesterday, and
this set is not listed. And I don't mind admitting I need voltage and resistance
values and - above all else - a diagram. I'm still not familiar enough with transistor
circuits to sail through them without some sort of guide, the way I can with a.c.-d.c.
sets. I need a diagram."
"I can understand that, for I need one, too," Mac said. "But let's see what can
be done. In a case like this, I've found that many times a manufacturer will use
practically the same circuit in two or three different models arranged to fit different
cases. Have you checked that possibility?"
"Nope; never thought of it. This is a Model 849. See anything like that in the
index?"
"Here is a Model 842 listed," Mac said as he got out the service sheet on that
set. "Hm-m-m, it uses six transistors and a crystal diode. The battery is nine volts.
How does that check with your patient?"
"Checks fine!" Barney said. "This one has three i.f. transformers. How many does
the 842 have?"
"Same number. This set must be pretty close circuit-wise to the one you're working
on. This one was in a much larger cabinet. Try checking your set against this diagram."
Barney made a few checks with the v.t.v.m. and then announced, "I think I've
spotted some of the trouble. There's supposed to be eight volts on the collector
of this converter transistor, but no voltage is present. The voltage goes into the
i.f. can, but it doesn't come out. Apparently something is wrong with the primary
winding of the first i.f. transformer."
As he talked, he was carefully moving the terminals protruding from the bottom
of the i.f. transformer. Suddenly the set began to play. He found that he could
make it start or stop simply by moving one terminal this way or that.
"The connection on the outside is all right," Barney said; "the trouble must
be up inside on the other end of this terminal. But, man oh man, what a job it is
going to be to remove that i.f. transformer. Look at it! It's not as big as the
end of your finger; a zillion wires go to those terminals; and the case is soldered
to the chassis!"
"Maybe you won't have to remove it," Mac said hopefully. "Use the solder pencil
to heat that terminal up so the solder on it inside the can will be melted as well
as the solder outside. That may do the trick."
Barney followed these instructions; and as the solder on the terminal softened,
the radio began to play. He did not remove the iron for a few seconds longer. After
the solder had cooled, no amount of twisting and pulling on the terminal would make
the set cut out.
Barney started to snap the back of the plastic case in position, and as he did
so the volume of the set dropped away down. It did not take him long to discover
that the ferrite-core antenna lead at one end of the antenna was being pressed so
firmly against some of the turns of the coil that it was shorting them out when
the back was in position, for then this back forced the ferrite-core antenna over
against the pinched wire. A bit of thin plastic tape cleared the trouble.
"I'm going to remember that trick of looking up a model similar to the one that
puzzles me," Barney promised.
"It won't always work that well," Mac warned; "but even if we had not found a
similar model in the service literature we should not have been whipped. We still
could have gone to the magazine stacks. Service magazines are continually publishing
diagrams of interesting new sets, and I remember seeing several articles recently
on new transistor receivers. I should not be a bit surprised if we could find this
very set, together with a discussion of the various circuits in it, described in
a recent issue of Radio & TV News. We need all of these tricks because we get
quite a few of these brand-new sets that are still in warranty from that department
store across the street."
"How come we get that business?" Barney asked curiously. "Why don't they return
those sets to the distributor to be repaired. After all, they are still in guarantee."
"The manager explained that when the trouble is minor he'd rather have us take
care of it quickly than deprive the customer of the set while it is packed up, shipped
back to the distributor, repaired, and returned. He says that with postage, insurance,
and paying help to pack and unpack the set, he often comes out ahead to let us do
it. If we run across an expensive defect, we charge him an estimate and suggest
that he return the set; but when it is a simple matter, we just go ahead and repair
it. He takes our advice about what to do."
"Record players can be another headache," Barney commented. "Quite often the
changer is made by a different manufacturer than the one whose name is on the player
or combination; and in this case the changer manufacturer frequently leaves the
make and model number off the base plate. That leaves you in a pickle when you need
service data on the changer.
"But I've pretty well licked that problem since you bought that complete set
of record-changer manuals. I simply leaf through them, looking at the pictures of
the changers, until I find one exactly like the one giving me a hard time. Lots
of times, of course, I have a pretty good idea whose baby it is just from looking
at it; but those manuals are worth their weight in gold when it comes to identifying
an unfamiliar changer. And believe you me: most of those Rube Goldberg contraptions
are hard enough to work on when you have exploded diagrams, descriptions of the
change cycles, and possible trouble charts to help you. Without these things, you
are darned near helpless."
As he finished this lecture, Barney walked over to see what Mac was doing to
the TV set on the bench. He had removed a couple of stacks of selenium rectifiers
from it and was bringing the leads that had gone to the rectifiers underneath the
chassis up to what looked like a double fuse-clip mounted top-side.
"What do you think you're doing?" Barney demanded.
"I'm changing over to the new silicon rectifiers recently brought out by Sarkes
Tarzian," Mac explained.
"Where are they?"
Mac brushed aside an empty 12BE6 tube carton and revealed a couple of little
units about the size of cold capsules.
"You mean them are going to take the place of them?" Barney demanded as he jabbed
his forefinger first at the tiny silicon rectifiers and then at the discarded selenium
units.
"Ignoring the bad grammar, yes," Mac replied. "Those little Type M500 silicon
diodes are rated at 500 milliamperes at 130 volts; and from what I've been reading,
they can really take it."
"Looks as though they'd melt right down," Barney muttered. "They don't have the
heat-radiating surface selenium rectifiers do."
"Neither do they generate as much heat," Mac replied. "They have a very low reverse
current leakage, and the forward voltage drop across them is extremely low. Since
the heat generated in the rectifier is directly connected with the forward resistance
of that rectifier, when this resistance is lowered, the rectifier heating goes down
with it."
"If they have a low voltage drop, I'd think they'd put out more voltage than
do the selenium jobs," Barney observed.
"You think right. Actual tests made on TV sets using half-wave voltage doubler
circuits showed that when selenium rectifiers were replaced with silicon units,
the full-load output voltage was upped from 15 to 30 volts."
"Wouldn't that tend to upset things, maybe pop filter and bypass capacitors,
and so on?" Barney asked.
"I was wondering about that, too. However, I doubt you'd have trouble in most
cases. The parts you mention ordinarily have more than enough safety margin to take
care of the extra voltage. We have found out that most sets work better with the
voltage a little high than they do when it is a little low. But there are bound
to be some cases, where say the line voltage is above normal to start with, when
replacing selenium rectifiers with silicon units may cause a marginal capacitor
to give up. But it would probably have gone before long anyway."
"Do you think they will last longer than the selenium units?"
"I'm not going to stick my neck out on that one. I still remember that when selenium
rectifiers came on the market I took the word of the "experts" and passed along
to my customers the information that a selenium rectifier would never need to be
replaced, that it would last the life of the set. Remembering, with a red face,
how wrong I was about that, this time I'm going to wait and see. However, I'll say
this: all tests seem to indicate silicon rectifiers may deliver what selenium rectifiers
promised. The silicon units seem to suffer no loss in output voltage after many
hours of hard usage, and they can operate in higher ambient temperatures than can
selenium rectifiers.
"Still, I'm not going to say they will not need replacing. I'll just say this:
if they do have to be replaced, slipping the old ones out of these little clips
and slipping a pair of new ones in place is going to be a whale of a lot easier
than replacing those soldered-in, under- the-chassis seleniums!"
Posted November 13, 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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