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Electronics-World-1959-05

www.americanradiohistory.com I 300 By BOB ELDRIDGE another day in the shop / Students Jack and John make headway on their own, but learn more from Tommy's experience. EDITOR'S NOTE: HMG on the list of articles readers told vs they liked last year Iras Eldridge's "A Day in the Shop." It involved a couple of student Ivri,, irin ns Who, in return for some miscellaneous chores. were permilted to nose about an established service shop to pick up what they could from experienced twin shooters. Into this steam tens woven onuel, practical bench k uo w- tome. I 'silt I/ the Saute cast of clla raeter.V. Ill,' same ru ,rlh nr matches his first periotmt n uee here. EVERY once in a while Jack and John, a pair of up- and -coming student technicians, come in to help out in our shop. They do a useful job in routine cleaning and checking chassis that are ready for servicing, mean- while keeping interested eyes on Tommy, our henchman. Tommy, for his part, takes time out once in a while to explain just what he is doing, and why, with the sets on the bench. Tommy phoned in one morning recently to say he would be coming in to work half an hour late. Seizing the opportunity, John piled right in checking the tubes on an Emerson chassis from a model 720D. 132 suss 560K NOR He likes to have the set running while doing this, because he hates waiting for each tube to warm up from dead cold in the tester. He used to have troubles sometimes. when he pulled the tubes in the wrong order, but now he plays it safe and removes the rectifier tube first, leaving it out. As he says, "If there's no 'B +' in the set. you can do what you like with the tubes without harming any because you have unthinkingly pulled the one that drives it." Anyway (to get back to the Emerson) when John started pulling tubes for testing. the first one attended to after the rectifier was the 6V6 audio - output tube. He noticed that it wasn't as blistering hot as he had expected and, being a methodical sort of fellow. he wrote on the check -out slip (which already bore the message, "No sound, not tubes." contributed by the outside technician) a note of his own: "6V6 checks normal but doesn't get hot in the set." Fig. I. Two G -E receivers using this circuit exhibited similar symptoms. C355 35 Jz t/2 12ÁU1 PHASE DET ;R352 :120K R36 66 C368 00 31LLL i0 PPEP R355 ME" C354 60 Ill 12AU7 REACTANCE -s 3 3 R356 00 220K 240V +240V 2 4355 470 00?6I/ 1`----- C} 9 s R364 66K L351 N MOLD 00 C36 1/2 124U7 NOR SSC 33K "'65 1/2 12AU7 + OISCH C3 . 4 9 C352 R3S6 2365 10K 661( YN/V. .vJJVvV Now one of the useful chores the two boys do is to hunt out the schematic for each set awaiting service. A look at the audio -output circuit on the schematic made John feel that this one should he so easy that it wasn't worthy of Tommy's august attention. John belongs to the "play-it-safe, static- test -first" school of servicing. so he set to work with the ohmmeter and in two minutes flat he came up with an open 470 -ohm cathode resistor (see Fig. 4). He soldered a new resistor in place just in time to hand over the set to the late- arriving Tommy for a routine check. Except for a minor adjustment to the phasing coil, the set performed dead on the ball. Thus the first job of the day was on the hot -run bench by 9:45 a.m. No Vertical Sync Meanwhile Jack had been preparing a Majestic 110 chassis. The job sheet bore the legend "No vertical lock." The only tubes weeded out by Jack's efforts on the tester were a couple of 6AU6's. Have you noticed that the 6AU6 is often a candidate for replacement in sets where all other tubes are still going strong? On the bench. the set performed just as noted. Everything was fine except that the picture would roll up or down without even hesitating as it wént round for another turn. "Well," said Tommy, "what can we see? The oscillator is capable of running at the correct speed and the sync separator must be popping out pips all right, or the horizontal would be haywire. There's not much in between the sync separator and the oscillator except the integrator network, so we may as well tack in a new one." (The cir- ELECTRONICS WORLD

cuit is shown in Fig. 2). This proved to be a had guess and the fault remained unchanged with the replacement printed -circuit network in place. "Whip it out," said Jack, "and I'll put it back in the drawer." "Oh no we don't," reproved Tommy, "we leave it right there until the trouble is found! What if we should have two faults at the same time. one maybe caused by the other? We leave any substituted parts in the circuit, then take 'em out one at a time when the set is working properly. What is there left to check now ?" he continued. "A couple of capacitors and the 33,000 -ohm resistor. It's easier to measure the resistor, so let's start with that." The ohmmeter registered the resistor as being about 35,000 ohms so the clipped end was soldered hack in place and one end of the .0022-µfd. capacitor (input end of the integrator network I was disconnected. "Check it with the ohmmeter," said Tommy. "It would have to he shorted or leaking badly to produce a fault like that. If it was open, the lock would be more or less normal." The capacitor showed a dead short. Jack was puzzled to see Tommy replacing the 33,000 -ohm resistor, after soldering in a new .0022-4d. capacitor and the original integrator network. "Well," explained Tommy, "that's a matter of safety first, really. 'B +' current has been running to ground through that resistor and it may have been damaged in the process. You know how the boss feels about callbacks!" John was scribbling on a piece of paper. "Look here," he queried, "doesn't 135 volts across 33.00() ohms only produce half a watt anyway ?" Tommy looked thoughtfully at the schematic. "Bet you a coffee break there's much more than that across it sometimes." He attached the probe of the v.t.v.m. to the "hot" end of the resistor, clipped the common lead to the chassis and switched on the set. Up and up swung the needle to read nearly 900 volts and then it eased slowly down to settle at 120 or so. "Let's go!" exulted Tommy (who seldom loses a bet ). "What would you expect from a directly heated rectifier feeding a pile of indirectly heated tubes ?" John trailed along on the way to the coffee shop, looking in bewilderment at the schematic. When they were settled in the booth he could contain himself no longer. "I don't get it!" he blurted. "What's this midget relay thing for then ?" (RL, in Fig. 21 Tommy took the schematic and studied it in silence. "That's a good question," he finally admitted. "We'll have another look when we get back to the shop. Let's forget it for ten minutes." Back on the bench the mystery was speedily solved. The .0047-ufd. capacitor across the midget relay contacts was completely shorted. "That was a bit of luck," said Tommy, when a new component was in place and the set was on test. "I think probably that was the original fault, and the .0022 May, 1959 went down as a result. I don't think you should pay for the coffee after all, John -the boss should! We might have had a sticky callback on that one but for your curiosity!" As we said - Tommy seldom loses a bet! Two With The Same Chassis The boys had readied two G -E "Stratopower" chassis next for attention, each with more or less the same symptoms. The first one was way off horizontal frequency and could not be corrected by adjustment. The other one could just be brought to the correct horizontal speed, with the oscillator slug unusually deep in the coil, but sync was then very unstable. Fig. 2. The primary defect in this Majestic circuit was easily found and cured. However, an after - thought led to a recheck with interesting results. Another, hidden fault was found and cured! 1 Fig. 3. The circled ground strip, outside the tube shield, may mean trouble. Fig. 4. First lob of the day, handled by John, involved this Emerson circuit. 205. AUDIO OUTPUT 470 210V 1500 TBOyFD. +245V Fig. S. The reactance tube is in shunt with the oscillator tank, through REACTANCE TUBE ft C358 OSC. TUNING .0047 "Let's tackle the totally wrong one first," said Tommy. Like most technicians, he prefers the definite failure to the marginal one. "There's a beautiful schematic for this model, with waveforms and voltage and resistance measurements all over the place. Make a check with the v.t.v.m. on those two 12AU7 bases. Don't forget to short the antenna terminals to keep the local signal out of the set while you are measuring," John undertook the job and soon came up with a list of readings that showed the only serious discrepancies from the normal voltages were on the cathode, grid, and plate of the phase detector (Fig. 1). These points were ooz; 33K RLI DGET RELAY MM.-0. K 375K low 250 + VERT INT. NETWORK 33K ß5v + 4C all slightly positive, instead of being from 4 to 6 volts negative as called for in the manual. "What now ?" asked Jack. "Well, according to the circuit diagram," said Tommy, "there is nothing connected to these points that could produce positive potential. That resistor R.,I goes away to the grid of the oscillator, which is bound to be negative: the grid current flowing in an oscillator tube produces negative voltage at the grid terminal. Change the 12- AU7 first, just in case there's something wrong that didn't show up on the tube tester." With a new tube in the socket and the fault still present, John was studying the schematic. "Maybe that 3300 - ,uµfd. unit from the sync section is lacking." he offered. Tommy looked at the list of voltages on the bench. "I doubt it because, if it was, you would expect to find pin 8 more positive than pin 6. It's closer to the point of connection you see. But we can't be sure of that, so clip it and have a look." "No change," reported John. "Maybe I can follow up this 'most- positivepoint' idea. The most positive voltage is on pin 6. It can't be coming through the 100,000 -ohm resistor (R,4) because pin 8 is on the other end of that. The other end of the 1- megohm job (RASA) is pin 2 of the other 12AU7, and that's reading negative. What about this 10,000 -ohm fellow (R,S) ? "Whoops!" He grinned happily. "Now I think I'm getting somewhere. This point is more positive, so I can follow along through the 6800 -ohm resistor (RS). Yep! That's even more 133

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