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MECA Electronics

Mac's Service Shop: Changer Chatter
May 1959 Electronics World

May 1959 Electronics World

May 1959 Electronics World Cover - RF Cafe 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.

"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

Mac's Service Shop: Changer Chatter, May 1959 Electronics World - RF Cafe"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 stories was the brainchild of no 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 Electronics 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.

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