Mac's Electronics Service: Intermittent Roundup
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Just about anyone who has done a fair amount of troubleshooting on electrical and/or electronics equipment has experienced the frustration (to put it mildly) of trying to identify the source of an intermittent problem. As Mac points out in this "Mac's Electronics Service: Intermittent Roundup" story from a 1962 issue of Electronics World magazine, the causes can be electrical, mechanical, or thermal. A combination of two or all three might even be cohorts in making your life miserable. If poor power cord or battery connections are not the culprit, then I typically start exercising switches and twisting knobs in search of any one of them causing an unexpected change in the functionality. Following that, I'll open the device and sniff and look around for obvious failures. Next comes unplugging and plugging back in all the internal connectors (multiple iterations) in case a contact is contaminated. If no joy is produced by that point, it's time to pull out the DMM and start checking easily determined voltages from power supplies. After that the job gets exponentially tougher - on to the oscilloscope. Having a schematic available definitely makes the job easier, but with pre-IC equipment you can often traces signal paths to figure out what is going on. I make no claim as a troubleshooting expert, but I have successfully fixed many things in my 64 years (to date). A lot I haven't been able to fix, but that number is far less. p.s. My latest success was with an LP gas-powered Generac whole-house backup generator system (c1986) on my daughter's property. It failed to kick in when the power went out during a severe storm. After checking for obvious problems, I sprayed starter fluid into the carburetor and it fired right up and ran for about 10 seconds. Therefore, it appeared to be a fuel starvation issue. LP Gas was getting to the generator input as a check of the pressure there showed, but it wasn't getting to the engine (a sniff check while cranking revealed no LP gas odor). After consulting a wiring diagram found online, I found a gas supply solenoid between the output of the regulator and the engine. A signal from the computerized control board activates it, and the signal was there. I removed the two wires (very hard to get to, of course) from the solenoid, cleaned both sides of the contacts, and pushed them back on. The starter switch was mashed and voila! the engine fired right up. It's been working fine ever since. Sometimes you just get lucky. Mac's Electronics Service: Intermittent Roundup
Barney's Irish temper had a short fuse, and it was sputtering. "Doggone these intermittent radios that refuse to 'intermit,'" he fumed. The owners don't give them any kind of a check. The first time the set refuses to come on when they turn the switch, they grab it up under their arm and gallop down here and tell us it's dead, We put it on the bench, and it plays perfectly for hour after hour in spite of our doing everything we can to make it cut out. Then we don't know if the plug simply wasn't in the wall socket firmly, or if there really is something wrong with the set that refuses to show up here but will likely rear its ugly head immediately the set is back home." "My heart bleeds for you," Mac said with mock sympathy. "Intermittents waste more of the service technician's time than any other radio or TV fault. Anything that helps him cope with this familiar demon fattens his pocketbook, preserves his temper, aids his digestion, and doubtless improves his chances of eventual salvation. Suppose you put that thing on about 125 line volts while we sort of review what we know about intermittents and their cure." Barney plugged the radio into the isolation transformer and adjusted the transformer for 125-volt output. "Let's start by defining an intermittent as a radio or TV set that intermittently performs poorly, with intervals of normal operation in between," Mac began. "The poor performance can take the form of either partial or complete loss of sound or picture, of distortion of sound or picture, or of noise appearing in the sound or picture. The 'distortion' in a radio could well be a crackling, whistling, or motorboating, while in a TV set it could take the form of a loss of sync." "Your definition is broad enough," Barney said impatiently as he glowered at the perfectly playing little radio; "let's get down to cases. "A very common type is the mechanical intermittent," Mac said unhurriedly, pausing to light his pipe. "This is the one that can be made to reveal itself by jarring the set. The jarring may be caused by walking across the floor, by vibration of certain notes coming from the speaker, or by pounding the top of the cabinet in anger with the clenched fist. The cause is usually a loose connection, but search for the intermittent connection cannot be confined to poor solder joints. In addition, one may be found in such unsuspected places as connections inside the tube, at the contact of a tube pin with the socket, where a capacitor lead connects to the foil, and so on. "Finding one of these can be quite tricky, but skillful, gentle tapping of suspected tubes, capacitors, or wires with a pencil or similar instrument is the best method to use. The accent is on 'gentle' for two reasons: first, too strong a blow will vibrate other parts near the one struck and produce confusing indications; second, a heavy jar will bend capacitor leads or otherwise jam the parts of the poor connection together and produce a temporary cure that will make quick location of the trouble impossible. "The thermal intermittent is just about as common. This is the intermittent condition that only shows up when the radio or TV chassis is in a certain temperature range. Sometimes the set plays perfectly until it warms up; then it starts misbehaving. In other instances the set will perform erratically until it becomes thoroughly warm, after which operation is completely normal until the set is turned off. A really nasty version is the set that performs normally when cool or warm, but during the warming-up process it passes through a comparatively narrow temperature range in which the intermittent condition is evident. By the way, turn that set off now and let it cool down. Set the transformer for about 100-volt output." Barney did as he was told and then said, "I think I know the methods we use to handle thermal intermittents. What we try to do is keep the defective part in its critical temperature range until we can locate it. Sometimes this means covering the set up with a heavy cloth so that it gets hot in a hurry, Again we heat individual suspected parts by holding the tip of a soldering iron close to them or shining an infrared spotlight on them. If we think they have passed through the critical heat range, we cool them off rapidly by spraying them with pressurized refrigeration gas, such as 'Circuit Cooler.' Metering and signal tracing are often used in conjunction with this temperature manipulation so that we can isolate the temperature-sensitive component when we force it to tip its hand." "Let's not forget that hybrid, the thermo-mechanical intermittent," Mac chimed in, "This is actually a mechanical intermittent that only shows up when the affected part is in a particular temperature range. Tubes and capacitors are notorious offenders in this group. You can tap them all you want when they are not in that heat range, and you will get no cutting out, noise, oscillation, or what have you. By the same token they will not cause trouble unless they are subjected to vibration while in that range. That's why these sets will often play perfectly at a low volume level but will misbehave frightfully when the volume is boosted. To ferret out these nasty little cross-breeds, we simply combine the methods used to locate mechanical or thermal intermittents. "We can get a lot of help in running down any kind of thermal intermittent, if we will just use it, from timing. After a set is turned on, different parts heat up in predictable sequence. First the tubes get hot; then the current-carrying parts, such as resistors, coils, and filter capacitors warm up. Finally other parts, such as bypass and coupling capacitors, are warmed by the heat radiated from the tubes and resistors or conducted through the chassis. "Keeping this in mind can be a great help in knowing where to look for a temperature-sensitive part. If trouble starts almost immediately the set is turned on, look for trouble in the tubes. If the intermittent shows up a few minutes later, look for a defective coil winding or a bad current-carrying resistor. When the difficulty only makes itself known after the set has been on for an hour or so, look for a defective coupling or bypass capacitor, especially those considerably removed from any heat-radiating component." "I believe the worst of the lot are the shock-triggered intermittents,' Barney said thoughtfully. "You know the ones I mean: the sets that can be thrown in and out of the intermittent condition by snapping a light switch anywhere in the house, by the refrigerator's cutting on or off, or simply by touching a test lead to any part of the circuit. The shock of the tiny pulse of signal that occurs when any of these events take place is all that is needed to throw the delicately balanced condition one way or the other. "That sensitivity to being touched with a test lead is one of the things that make solving these intermittents so difficult. If the volume drops and you try to use the signal tracer to see where the signal is lost or to bridge a suspected capacitor with a good one, the instant you touch any part of the circuit with probe or lead, the volume hops back up and stays there. About all you can do is connect meters and signal tracers to different parts of the circuit and then wait for the intermittent condition to take place. When it does, if your signal indicating instruments are connected to the right places, you will get an indication that will pinpoint the defective part, or at least locate it in a comparatively small area." "There are a few intermittents that are so comparatively easy to locate you might almost call them good intermittents," Mac said with smile. "I'm thinking about the worn volume control that causes the volume to hop up and clown erratically or the set even to go into oscillation at certain settings. Then, too, there is the intermittent noise that can be easily traced to a rectifier mounted near the loop antenna. The slightest jarring of this tube will radiate noise into the antenna. A new tube, of course, is the cure. "Metallic particles between tuning capacitor plates can also cause intermittent noise at certain dial settings," Barney threw in; "and sets that go completely dead shortly after they are turned on are usually caused by converter tubes or second detector tubes that short out internally as they warm up. Tapping these tubes will usually bring the signal back up for a few seconds. Tapping them again will make the sound disappear." Mac reached over and felt the tubes of the little radio. Finding them completely cold, he turned on the switch. The dial lamp came on, flickered fitfully, and then went out. Barney did not need to be told what to do. "Working with cautious haste, he eased the set over on its back, attached the common clip of the v.t.v.m. to one side of the power switch, and used the a.c, probe to explore the voltages present at the tube filament connections on the sockets. When he reached the 50C5, 100 volts was present at one filament connection and zero voltage at the other. Quickly he transferred the common lead clip to the dead socket connection and placed the probe on the other. A hundred volts was still indicated, but at that moment the dial lamp flickered on and the voltage fell to around fifty volts. "Well, it was an intermittent," he admitted as he reached for a new tube. "How come it showed up for you and not for me?" "Candidly, I don't know why operating a set with an intermittent filament at an elevated voltage for a few minutes and then allowing it to cool down and starting it up again at a reduced voltage will often make these stubborn cases 'intermit,' " Mac admitted; "but by sheer, blundering fool luck I've found out it does. Undoubtedly it has something to do with the expansion and contraction of the fractured filament and breaking down the temporary tiny weld that is holding the broken ends together and keeping the set going when you want it to quit. I always use this method as a last resort before returning an alleged 'dead' radio to a customer and telling him I just can't find anything wrong with it." "I wonder how long I'll have to work at this business before some of your 'sheer, blundering fool luck' rubs off on me," Barney said wistfully as he put the little receiver back in its case.
Posted November 11, 2022 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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