Mac's Service Shop: Tape Recorder Tips |
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Barney, Mac McGregor's trusted technician sidekick, would be in a heap of trouble in today's workplace. His complimentary and sincere remarks to office secretary Matilda are considered as harassment and even misogynistic by current standards. The unintended consequences - or maybe they are intended - has been to cause tense and guarded environments where traditional interpersonal behaviors and attitudes are avoided rather than risk offending someone and paying a steep price for it. Mentioned in this "Tape Recorder Tips" from a 1958 issue of Radio & TV News magazine is the chemical compound carbon tetrachloride, aka carbon tet. At one time it was commonly used as a cleaning agent because of its ability to efficiently clean oily and fatty residues. Tape recorder heads, rubber drive wheels, and metal guide posts get gunked up fairly quickly when the machine is used a lot, so service shops and recorder/player owners used it. At some point it was determined that a health hazard existed, and usage dropped considerable, and the next best thing - isopropyl alcohol - was substituted. I know as a not-so-active-anymore postage stamp collector that carbon tet was/is used to expose watermarks on on gummed, mint condition stamps without compromising the integrity. Mac's Service Shop: Tape Recorder Tips
This isn't going to do my figure a bit of good," Matilda announced as she dug the little wooden spoon deep into one of the chocolate sundaes Barney had brought back with him from a service call. He and Mac, the owner of the shop, were sitting on the service bench. Matilda was perched on a high stool in front of them. "There you are!" Barney mumbled with his mouth full of ice cream. "Matilda here, with not a surplus ounce, worries about her figure; but when I was buying these sundaes, there were two or three babes standing in line who looked big enough to eat hay." "Thank you, Barney - I think," Matilda said; "but that wouldn't be a sneaky way of calling me skinny, would it?" "You know doggone well it's not," Barney said with an aggrieved air. "I wolf-whistle at you every chance I get, don't I? And I'm mighty careful who I whistle at. I've got a reputation for taste to maintain in this man's town." "Yes, and if you don't stop those leering whistles I won't have a reputation at all," Matilda said with a dazzling smile that belied her reproof. Mac set his little paper bucket down on the bench beside him and stretched luxuriously. "That really hit the spot this sizzling day, Flame Head. It was mighty thotty of you and it leaves me in a gabby mood. I've been wanting to talk to you about tape recorder servicing. Think you can listen and eat at the same time?" "I read you loud and clear; over," Barney answered. "The whole thing is tape recorders have improved tremendously these past few years. They are capable of excellent musical reproduction; and now that the hi-fi crowd has taken them over, we have much more critical customers to satisfy. That means we must be more critical, too, so that every recorder leaving the shop is delivering the best performance of which it is capable." "Sounds like sense," Barney observed. "Always start by demagnetizing the heads. Magnetized heads will produce a five to ten db increase in noise and distortion; but what is worse, such heads will gradually erase high frequencies from tape passed over them. I don't want our expensive test tapes ruined by using them on magnetized heads. By the same token, we should explain to the customer that a magnetized head is as bad for his prize tapes as a worn-out needle is for his fine records. Then we should sell him a head demagnetizer. "Next, use the test tape to check head alignment. Don't forget that if a head is badly out of alignment, such as might occur when a head is replaced or when someone has tampered with the alignment adjustment, false peaks may be found on either side of the true azimuth position. These false peaks, however, will be 15 to 20 db below the correct position; so you will have no trouble locating the true azimuth setting if the adjustment is moved far enough to be sure you are not stopping on one of the false peaks." "One thing that's always puzzled me is how you check the bias oscillator frequency," Barney said. "The frequency is too low for our grid-dip oscillator to reach; it's below the lowest frequency put out by the r.f. signal generator; and it's higher than the 20,000 top of our sine-wave generator. Just how do you check it?" "Probably the best way is to feed the audio generator into the horizontal amplifier and the signal from the bias oscillator into the vertical amplifier of the scope. Then adjust the audio generator frequency until you get a stationary Lissajous pattern. In getting this pattern, start at the upper limit of the audio oscillator and come down. The oscillator frequency can then be read as the indicated multiple of the audio generator frequency. For example, if counting loops of the Lissajous figure indicates a 4 to 1 ratio and the sine-wave generator is putting out 16,000 cycles, you know the oscillator is working on 64 kilocycles. While you're at it, turn on the horizontal saw-tooth sweep and examine the waveshape of the bias oscillator output. It should be close to a true sine wave for best operation. "Sometimes the service manual gives you the bias or erase voltage that should be present across a head winding. Ordinarily these must be read with a true a.c. v.t.v.m. Make sure that the instrument you use has an upper frequency limit adequate for measuring the oscillator frequency. Some v.t.v.m.'s, intended chiefly for reading line frequencies and audio frequencies, will not give a true indication at the 50-100 kilocycle frequencies involved here. If you're not sure of your v.t.v.m., the scope can always be used with a voltage calibrator to check the amplitude of the voltages with sufficient accuracy. "Keep in mind, too that some recorders change the voltage across the head with the speed of the tape. For example, one Bell recorder specifies 35 volts bias at 7.5 ips, 14 volts at 3 3/4, ips; and 10 volts at 1 7/8 ips." "What if erase and bias currents rather than voltages are specified?" "Put a 10-ohm precision noninductive resistor in series with a lead going to a head and measure the a.c. voltage drop across it. Multiply this voltage reading by 100 to get the current in milliamperes. For example, if you get a reading of 0.3 volt, that means the current is 30 ma." "Isn't there some quicker way of checking the bias oscillator without going through all that stuff?" "Well, a quick and dirty check of oscillator performance is to measure the d.c. voltage developed across the grid leak of the oscillator with a v.t.v.m and compare this reading with the data given in the service manual. If the voltage is too low, try a new oscillator tube and check the other voltages applied to the tube elements against the voltage data supplied in the service sheet. If the voltage is too high, maybe the grid leak has changed value. More likely, the head winding may be open or a switch may be making a poor contact so that the oscillator is running unloaded. If you check the head winding for continuity with an ohmmeter, just remember this d.c. current is certain to magnetize the head; so use the demagnetizer again before running tape across the head." "What should I use to clean the head and the rubber drive wheels? Some manuals say to use alcohol; others say carbon tetrachloride." "You know how I feel about carbon tet. That stuff is deadly dangerous when it is used in such a way that you inhale the fumes. However, many manufacturers specify that it be used to clean the tape residue from heads, capstan, and guides. It is a good solvent for the material that collects on these. On the other hand, some manufacturers say never to use carbon tet on any of the rubber drive wheels or pressure rollers. They insist that alcohol be used in such places. Still others say that carbon tet is perfectly all right for use in these places. I suspect that the difference lies in the kind of rubber used. Always follow the manufacturer's instructions to the letter. When in doubt, I believe it is safer to try alcohol." "Check," Barney said as he spooned the last drop of his sundae from the paper bucket. "Many of the recorders have a hum balance control," Mac went on; "and there's a lot more to adjusting this than just twisting a screw. In the first place, the recorder should be in completely assembled operating condition when this adjustment is made. This adjustment is intended to 'buck out' various sources of hum induced from motor, transformer, choke, and other parts of the recorder. "But even if the tape recorder is all together, you still have to be sure that it is not picking up hum from some external source, such as an electric clock motor, a fluorescent lamp ballast, etc. The way to check on this is to note whether or not the level of the hum changes when the tape recorder is moved about. If it does, you can be sure it is coming from an external source." "Seems to me trying to judge the hum by ear would be pretty tricky." "Don't try it. Hook our a.c. v.t.v.m. across the speaker voice coil and set it to a low range. Usually the hum should be less than 0.1 volt with the volume and tone controls full on. Keep your eye on the meter while making all checks and adjustments regarding hum. It will give a more sensitive indication than your ear. Try reversing the line cord, too, and adjust the hum control again. If the hum can be reduced to a lower level with the cord in one way - due, no doubt, to some of the fields cancelling themselves out - adjust the hum control with the plug in this way and mention to the customer that the hum will be lower when the plug is properly inserted." "One final question: is there any easy way to run down the source of wow?" "Not really. There are some things that help. One is to have the tape transport mechanism arranged where you can see it while a tape with a steady tone is passing across the heads. Watch carefully and see if you can detect a connection between the wow and the rotation of the capstan, pressure roller, or speed roller. If you can spot any such relationship, the battle is half won. All you have to do then is find out why that particular bit of the mechanism is running at an uneven speed." "OK," Barney said. "Now that I've had food for the body and food for the mind, I'm rarin' to go!"
Posted August 31, 2020 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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