 |
 |
 |
|
About RF Cafe
Copyright: 1996 - 2024 BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowl My Hobby Website: |
|
||
|
||
Mac's Radio Service Shop: All Work and No Play
|
||
I have to admit to not recalling ever having heard of
Dagmar; have you? Crack electronics technician "Red" mentioned
her in this episode of "Mac's Radio Service Shop" appearing in the
March 1952 edition of Radio & TV News. I thought Prince
and Cher were the first man (ostensibly)
and woman, respectively, to use a single-name public moniker, but
evidently Dagmar beat them to the punch ...but I digress.
All Work and No Play
Barney stood looking out of the window of Mac's Radio Service Shop at the big fat snowflakes just starting to drift down from the dark sky overhead. "Is this winter going to last forever?" he asked morosely as he walked back into the service department. "I don't think I ever had such a bad attack of winter willies before." Mac glanced up questioningly from the TV chassis on which he was working and then said quickly, "Know exactly how you feel, Red, for I feel the same way myself. Guess we need a little vacation of some kind." "I've got proof I need a vacation," Barney said with a ghost of his old grin. "The other morning I was using a test pattern to adjust the focus on a TV set when suddenly the pattern was cut off and a program started in which Dagmar was a visiting celebrity. For a few minutes I was actually mad because the test pattern had disappeared! Now any old time a red-blooded American boy like me would rather look at the curves of a test pattern than those of Dagmar - well, there can be no doubt but that he needs a vacation from service work!" "You are so right!" Mac agreed with a chuckle. "We really have been hitting the ball pretty hard here in the shop this past six months; but I do not think the amount of work we have been doing is altogether to blame for this sudden I'm-fed-up-to-here feeling that we both have. Part of the trouble comes from the way we have been working. When you first started here, we did a lot of talking as we worked because I was trying to teach you as much as I could as we went along. Then you reached the point where I wanted you to gain self-confidence by licking the problems all by yourself, and we quit talking. Whole hours go by now without our saying a word to each other." "I know it," Barney quickly replied, "and it is not near as much fun as it used to be. I'm gaining self-confidence, all right, but I certainly miss talking over the sets with you and having you give me heck for overlooking something that is obvious or giving me a pat on the back when I pull a bright one." "I miss our chatter, too," Mac confessed; "and I can tell you now that you would be astonished if you knew how often your prying questions prodded me into seeing what was wrong with the set when my mind was a complete blank just before your question nudged me in the right direction." "Well okay then!" Barney exclaimed. "Let's stop 'holding Quaker meeting' and go back to the good old days. You can start right now by telling me what makes this set whistle so loudly on 910 kilocycles. It works all OK on the rest of the band, but it makes so much fuss on the University station on that one frequency that you can't listen to it." Mac flipped over the complaint card attached to the set and glanced at it. "Hm-m-m," he hm-m-med, "says here the customer never noticed the trouble until after he had the phono jack installed on the back. Does that tell you anything, Sherlock?" Barney looked as blank as Laurie Anders of -"The Wide Open Spaces" fame. "What's half of 910?" Mac asked. "455, but what's that - say, that's about the i.f. frequency." "And the phono jack is probably connected across the volume control, which, in turn, is connected to the diode plate circuit of the second detector. At the same time the jack is very near the loop antenna that is resonated to whatever station is being received. When we tune to 910 kc., the strong field about the loop is connected through the lead from the jack directly to the diode plate circuit. Here it mixes with the second harmonic of the i.f. frequency and produces the strong heterodyne whistle as the two slightly-different frequencies are combined by the rectifying action of the diode. The process is exactly the same as is used when you employ a beat frequency oscillator for receiving c.w. stations, except in that case the b.f.o. is fixed-tuned to about the i.f. fundamental frequency and is loosely coupled to the diode circuit so that it produces a whistle on every station received." "That's the cause; what's the cure?" Barney wanted to know. "There are several different ways you can go at correcting the trouble. The main thing is to reduce the coupling between the loop antenna and the second detector diode plate circuit. An r.f. choke in the lead from the phono jack to the volume control would do this, or you might try shielding this lead and moving the jack down into the corner of the back cover so it will be as far as possible from the field of the loop. In general, if you want to avoid birdies in the set, it is a good idea to avoid increasing the possibility of direct pickup by any circuit carrying the i.f. frequency. That's what the person who installed the phono jack forgot when he tied that long lead to the bottom of the secondary of the i.f. output transformer. He would have gotten away with it, though, if we had not had a strong station on approximately twice the i.f. frequency." Barney soon had the jack moved and the lead from it to the hot side of the volume control shielded. This cured the trouble completely. "And now you may return my help - if you can," Mac told him. "See if you have any bright suggestions about this little a.c.-d.c. puzzler. When I first turn it on, it plays with good volume and has good sensitivity; but after it runs a few minutes, the volume slowly dies away, and the only station I can pick up is the local one. I checked the tubes the first thing, and they are all right. Plate and screen voltages stay right up close to the recommended values. The change in volume is far too gradual for it to be condenser trouble." "Did you check the filament volt-ages?" Barney asked with a smug look. "No, but the filament current seems to be about normal as near as I can see by looking at the brightness of the 50B5." Barney switched off the bench lights and looked closely at the set. Then he switched the lights back on. "Check the filament drop across that 12BA6," he ordered importantly. Mac turned his head aside to conceal a grin as he obediently hooked the v.t.v.m. across the filament prongs of the tube. "Well I'll be -" he exclaimed. "There's only about five volts drop here." "I thought so," Barney said complacently. "I had one just like that the other day except that a 12BE6 was the joker then." "What do you think happens?" "I think that a loop of the filament shorts out after the tube reaches a certain temperature. That cuts down the heat delivered to the cathode and reduces the emission." "Why didn't the tube checker burn out the part of the filament being heated ?" "Because you checked the tube the first thing, before it got hot enough to short out. I'll bet if you yank it out of the set and pop it into the checker before it has time to cool down it will burn out now." Mac quickly jerked the tube from the set and stuck it into the tube tester. The filament glowed brilliantly for a second or so and then went dark. "Aw, quit trying to look as though you just invented perpetual motion," Mac said in mock disgust at the self-satisfied look on Barney's face. "Say, Boss, not to change the subject," Barney said with more interest and enthusiasm than he had shown in weeks, "but where were you last night? I called and called because I wanted to double-check with you on some transmitter trouble on Channel 6 that was making the picture cut some funny didoes, but nobody was home." "I was over fixing Old Man Bennett up with an earphone on his TV set. He is pretty hard of hearing, you know, and all he was getting out of his set was what he could see. Wrestling matches and prize fights were about all that made sense to him." "Couldn't he listen with his hearing aid ?" "Not to do any good. If you ever played with one of those things, you would know that the microphones they use do the same thing any microphone does: exaggerates echoes. You've doubtless noticed that a person standing a few feet away from a broadcast microphone in any room except a studio always sounds as though he were talking in a huge hall, even though the room may be quite small. Exactly the same thing happens when you talk to a person wearing a hearing aid from a few feet away from him, and this I echo is just enough to confuse a person whose hearing is not up to par. When Mr. Bennett sat right up against his TV set, he could hear pretty well; but when he backed off far enough so that the picture looked good, he could not understand what was being said." "What kind of an earphone did you use?" "A regular hearing-aid earphone that would snap into the moulded plastic earpiece he has. It so happened that he had an old hearing aid he no longer uses, and I got the earphone from that. I measured it and found that it had a d.c. resistance of about 30 ohms; so I tried it right across the voice coil of the speaker, and it worked beautifully. When the set is adjusted to just normal room volume from the speaker it is also just right for Mr. Bennett and his earphone, and the extra load represented by the earphone is so. light that you cannot notice any difference when it is connected across the speaker. I ran a couple of leads from the voice coil to a jack on the back of the cabinet. From a plug in this jack a length of lamp cord runs down through a small hole in the floor behind the cabinet, across the joists in the basement, and then back up through another small hole in the floor to a jack mounted on the baseboard right beside Mr. Bennett's favorite chair across from the TV receiver. A few feet of flexible cord and a plug allows him to plug his ear-phone into either this jack by his chair or the one on the set." "Why both jacks?" "Well, usually his wife tunes the TV set, but there might be times when he will have to do this for himself. Then all he has to do is pull the earphone plug from the jack beside his chair and walk over and plug into the jack on the receiver. When he has the set correctly tuned, he can replace the lampcord plug in the cabinet jack and go back and plug his earphone into the baseboard jack. On top of that, the jack on the cabinet allows the lampcord to be disconnected when it is necessary to pull the set away from the wall." "What did he think of it?" Mac smiled reminiscently as he replied, "Barney, when I saw that old man sitting there chuckling and slapping his leg at some of Bob Hope's fast-talking nonsense, I felt I had been repaid for all of the headaches we have in this wacky business. The few minutes spent attaching that earphone to the TV set meant hours and hours of pleasure and entertainment for that old fellow." "Yep, Boss," Barney agreed, "this radio and television game is a pretty good one at that. There are times, of course, when a fellow feels a little low and discouraged as I did a couple of hours ago - although I'll be darned if I ·can see why now - but most of the time I feel as I do right this minute when I can hardly wait to get at the next set." "Hold that mood!" Mac shouted as he dashed across the room, snatched a small set from the set-to-be-repaired group, and rushed back to place it on the bench in front of his broadly-grin-ning, red-headed assistant.
Posted June 11, 2015 Mac's Radio Service Shop Episodes on RF Cafe This series of instructive 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 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 Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
|
||
|
 |
 |
 |
|
Please Support RF Cafe by purchasing my ridiculously low−priced products, all of which I created.
These Are Available for Free |