10 months ago

Electronics-World-1959-05 64 OU know, Mac," Barney said to his boss working at the service 1 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 ' squeaging' 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 68 Service Shop i By JOHN T. FRYE Changer Chatter 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 ,h,;" 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 (Continued on page 103) ELECTRONICS WORLD 8- 12B4A'S 6X4 TI C7,C8 CIO, CII,C12 Ultra -stable OTL Hi -Fi Amplifier ,._. By JULIUS FUTTERMAN 6AN8 2-I284A'S C5,C6 Over -all and under -chassis views of the ultra- stable amplifier are shown here. THE author has been intrigued with output -transformerless (OTL) hi -fi amplifiers for years. He felt that if the output transformer could be eliminated from an amplifier design. while keeping all other phase- shifting components to a minimum, then even with very large ratios of over -all negative feedback there would be no problem of instability. The present model, which will be fully described, is the culmination of several years' work. It is designed to operate with conventional loudspeakers. The only test equipment essential for its construction is a 20,- 000- ohms -per -volt multimeter. The basic design is extremely simple. Referring to Fig. 1, a pentode, V,, is operated as a high -gain voltage amplifier and is directly coupled to the phase - splitter tube V. The cathode load resistor of V. is returned to ground through the output load, which may be the 16 -ohm voice coil of a conventional loudspeaker. Signal voltage developed across the plate load resistor of V. is applied between grid and cathode of output tube V,. Likewise, signal volt - tage 180 degrees out -of- phase, developed across the cathode load resistor of V., is applied between grid and cathode of output tube V,. Careful testing has substantiated the fact that with low values of cathode and plate resistors, the signals from this type of phase -splitter are balanced over the audio -frequency range. The output tubes are connected in series push -pull and are biased for Class AB operation. The load for the amplifier is connected with its high side to the cathode of V. and plate of V, and its low side to ground. Each of the output tubes has its own power supply, consisting simply of a metallic rectifier SR and a capacitor C. Due to the balanced nature of the circuit there is no d.c. in the output load. The potentiometer in shunt with the load has its arm in the cathode circuit of the voltage amplifier tube V,. When the arm of the potentiometer is at ground, there is no over -all negative feedback and the full gain of the amplifier is obtained. With the arm at the high side of the load, there is 100% negative feedback and the gain of the amplifier is essentially unity. Because of the minimum of phase- shifting components in this amplifier, large amounts of negative feedback may be used. The author has constructed amplifiers of this type with as much as 60 db of feedback without any instability! In this circuit, the output tubes are working as cathode followers and thus require large driving voltages from the phase -splitter tube. With this Full construction details on output- transformerless amplifier that boasts a damping factor of eighty. Fig. 1. The diagram shown here is the basic circuit arrangement employed in the output - transformer. less hi -fi amplifier. 1" «--o ou. O May. 1959 69

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