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May 1959 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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"Squegging" - Now there's
a word you don't hear every day. It is a shortened version of "self-quenching." As is
often the case in these "Mac's Service Shop" sagas, we get a primer on certain circuit
functions and how to troubleshoot and resolve the issue. You can also usually count on
learning more than one lesson per reading. After replacing the failed component in Barney's
exasperatingly elusive receiver, Mac turns to record changer mechanisms and their bewildering
nature (see my turntable
restoration if you are not familiar with those kind of mechanisms), but the real
message being given is the value of well-written troubleshooting guides from manufacturers.
Even with today's no-user-serviceable-parts-inside products, there are many times a troubleshooting
guide is included as part of the user's manual. That goes for both electronic and mechanical
products. You might laugh at the first step that tells the owner to check to make sure
the electric cord is plugged in or batteries are installed with the proper polarity observed,
but it wouldn't be in there if experience with customers didn't prove its worth. Being
a life-long fix-it-myself guy, I always keep product manuals and go to them for help
when I cannot resolve a problem in short order.
Mac's Service Shop: Changer Chatter
By John T. Frye
 "You
know, Mac," Barney said to his boss working at the service bench beside him, "every time
I begin to think I'm the most as service technicians go, one of these cussed little a.c.-d.c.
receivers takes all the conceit out of me."
"I noticed you were having a rough time with the little monster," Mac said with a
sympathetic grin. "What seems to be the trouble?"
"It oscillates," Barney explained.
"Hear it? That whistle is there all the time, no matter how you tune the set. I've
reached the place where I can't tell if it's an audio feedback whistle, an i.f. on the
rampage, or sort of an over-all oscillation. The fact that I hear the whistle even when
a station is not tuned in makes me think it's the audio; but when I ground the grid of
the i.f. stage through a large capacitor the whistle goes away down, even though it doesn't
change in pitch or stop. That sounds like it can't be the audio or i.f."
"Makes sense," Mac agreed; "is there any way you can stop it?"
"Yes; all I have to do is connect a bypass from oscillator grid to ground and the
whistle stops. So does everything else, of course, for that kills the oscillator."
"Did you check the oscillator grid voltage?"
"I checked all the voltages. Oscillator grid voltage is about twice what it should
be, but I figured that was caused by the unwanted oscillation driving the grid far into
the positive region and making more grid current flow through the grid resistor."
"You check the grid resistor?" "Nope, but I will right now. Say, the thing must be
open! I'm getting a reading up in the megohms instead of the 20,000 ohms I should be
getting."
"There's your trouble. The oscillator is actually blocking or 'squegging' at an audio
rate and producing the musical tone. Is the resistor actually open or is it just a poor
solder connection ?"
Barney held the solder gun to the socket connection of the grid resistor for a few
seconds and then took it away. The set stopped squealing and played perfectly normally.
"Poor connection!" he announced triumphantly. "Let me put this thing back in the case.
I'm sick and tired of looking at it."
"Hold on," Mac said. "Maybe that poor connection was between the lead and the resistor
element. Heat may have caused the lead to expand and bridge the broken connection temporarily.
Give the resistor a shot of that freon gas to cool it down and let's see what happens."
Obediently Barney sprayed the resistor with a mist of the pressurized refrigerant
gas. Instantly the set broke into the same whistling sound as before; and a check with
the ohmmeter revealed the resistance from oscillator grid to ground had returned to near
infinity. Barney snipped out the tricky grid resistor and replaced it with a good unit
from the resistor chest.
While he was doing this, his employer had returned to the record changer on which
he had been working for some little time. Barney heard him muttering to himself and looked
over to see him using a slender pair of surgical clamps to fish the broken pieces of
a flat key-washer from inside the mechanism.
"That's the first time I ever saw that happen," Mac remarked. "That little key slips
in a groove on the end of the shaft holding the main gear of the changer. It broke and
fell on top of the oil-covered gear. The two pieces stuck in the oil as though it were
glue. They wouldn't fall out and you couldn't see them except when the light was exactly
right. With the key gone, the gear could move up on its shaft a bare 3/16" at a critical
point in its revolution. Apparently that was all that was needed to upset the whole changer
cycle. Sometimes it wouldn't trip; other times the set-down point would not shift from
10" records to 7" ones; still other times the crazy thing wouldn't stop cycling. I've
got a hunch - and a hope - that all these troubles will end when I replace the broken
key."
Sure enough, when a new key had been tortuously inserted in place, the changer worked
beautifully.
Mac heaved a big sigh as he wiped the grease from his hands on a cloth.
"I've tried and tried," he admitted, "but I just can't seem to make myself enjoy servicing
these changers. One thing that sticks in my craw is the fact it takes so long to be sure
you have one working correctly. Practically every changer is a mechanical 'intermittent.'
Some of 'em will act up when they are cold and work perfectly when warm. Others do just
the reverse. Some will play a half dozen records perfectly and then refuse to cycle at
the end of the seventh record. Now and then one will cut up on just one size of record.
The upshot of the whole thing is you can't be confident the darned mechanism is operating
correctly unless you've seen it go flawlessly through separate stacks of all sizes of
records accommodated.
"That's not practical, of course; so the next best thing is to collect all the information
possible from the owner: how does the changer misbehave? How often does this erratic
performance show up? Does it occur only with one size record? If so, what size? Does
it seem to happen more often with a full stack of records on the changer or when only
one or two remain to be dropped? Does it usually occur when the player is first turned
on or after it has been going a while? How long has this condition existed?"
"The only trouble with that is: the average customer is a pretty sloppy observer;
moreover, the one who plays the changer is likely not the one who brings it into the
shop. You know how many changers are simply dumped in here with the comment: 'My kid
says something's wrong with this thing. Fix it.' "
"Yeah, I know; and when that happens, about the only thing to do is to make a dive
for the record-changer manuals. Without service data on a particular changer, you can
waste hours and hours."
"You can ditto that. My favorite use for the manuals is as sort of a 'mug book' to
identify a changer that doesn't carry a make and model number - something that happens
too darned often. By leafing through the manuals and looking at the pictures, I can almost
always find one exactly like the changer in front of me."
"True," Mac agreed; "but changer manuals do a lot more than help locate a particular
changer. A feature that's of really basic value is the description of the complete change-cycle
from the moment the trip device is actuated until the needle sets down on the next record.
With only this and plenty of horse sense, a technician could, in time, spot the cause
of any difficulty. All he has to do is mount the changer and arrange his light so that
he can view what goes on beneath the turntable. Then he revolves the turntable slowly
with his finger while he watches carefully to see if each of the actions described in
the change-cycle actually takes place in the proper manner and sequence. When something
fails to happen, or when it happens at the wrong time, with any luck he has found the
trouble."
"You sure have to turn the thing by hand," Barney remarked. "Trying to spot trouble
while the motor is doing the turning is like trying to watch all three rings of a three-ring
circus on speeded-up film."
"Once in a great while, though, something will show up when the motor is doing the
turning that will not be there when the turntable is rotated by hand. I had a case of
that last week. The customer admitted that the changer had operated perfectly until he
decided to 'give it a good cleaning and oiling.' In the process he carefully cleaned
off the heavy grease from around a small pawl on the rim of the main drive gear that
had to be pushed outward to start the change cycle. This grease was put there intentionally
to keep the pawl from moving out under vibration and centrifugal force as the drive gear
made its revolution. With the damping grease gone, the pawl did just that and kept the
changer cycling. It would not move out when the turntable was revolved by hand. Very
fortunately, in the list of possible difficulties given in the changer manual, the lack
of this grease was mentioned as a likely cause of continual cycling."
"That 'Trouble Chart' in the service data is my favorite reading," Barney confided.
"I love the way they list first the 'symptom;' then the 'possible causes'; and finally
the 'remedy.' Nine times out of ten you'll find the cause and cure for any particular
fault listed in these charts, no matter if it's turntable wow, recycling, failure to
cycle, improper set-down point of the needle, dropping two records at a time, improper
turntable speed, or what have you. When you have a specific complaint to go on, that's
the place to look."
A broad grin spread across Mac's wrinkled face. "Sure is funny," he commented, "how,
when we have all this good help, we still both hate to work on changers and will work
on practically anything else in the shop first."
"It is queer," Barney agreed. "But I kind of think part of it is that we unconsciously
consider working on record changers sort of infra dig, or beneath our dignity, as my
Latin teacher used to put it. Changer work is 'mechanical work'; and we are proud and
haughty 'electronic technicians!'"
Posted September 11,2018
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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