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Notes from the Service Shop A fictional series by John Reinicke Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 Chapter 2 It was nice to be out on the first sunny Saturday in spring. Spring this year has been late, and the sun and the warmth felt good. My friend Marc had loaned me the keys to the building in Centerburg and the opportunity to explore among old electronics gear again beckoned. I was happy to have a day warm enough to have the window down as I drove to Centerburg. The trees and bushes were just starting to appear fuzzy and there was a hint of green in the grass and weeds at the side of the road. Daffodils and crocus were showing bloom next to the farm houses. The smell of wet, freshly turned dirt indicated the farmers were out tilling the land, also taking advantage of the nice day. Occasionally the robust odor of manure indicated that the fields were being prepared for the season. The highway runs parallel to a rail line, and at one point does a sharp left turn over the tracks and then a sharp right to again run parallel on the opposite side. As I negotiated the jog and passed the Carter Lumber, I knew I was but a few miles from town. When I arrived in Centerburg I found the local auto dealers and the local implement dealers were having a show around the town square. The side of the square next to the Curtis Hotel was dedicated to the tractors and gardening equipment and the other side was covered with new autos. I thought this might be fun to walk around on a nice day. Because of the show and the resultant gathering, I parked behind the building and walked down the alley for a walk around the square. After a tour of the square and a lunch on South High, just off the square, I returned to the building. This time I entered off the alley and, after locking the door behind me, proceeded directly up the steps to the second floor. When I opened the door to the attic, I noted our steps from the last visit were still recorded in the dust. In the attic, I noticed the sun on the roof was making the room quite warm so I started by opening the window at the back of the building. The window opened in with a tug and to my surprise the screen on the outside was intact. No doubt the hardware cloth behind had served to support the screen in addition to keeping out the pigeons and bats. A shelving unit runs from the wall with the window to the top of the steps. As I turned to walk to the front I noticed a chassis, with a service tag. What caught my attention was the chassis was covered in colored dust and chunks of colored material and the dust was much deeper than on the rest of the shelf. I examined the dust and chassis. The chassis was an Emerson and as I looked closer I realized the dust was the remains of the case! The case had housed a "Patriot". As I examined the scene I deduced the probable cause of the demise of the case. The radio had been taken in for repairs in November of 41. I suppose the owner's plans changed that December and the radio was forgotten. Old Clem had probably put it up here in the spring of 42. High street runs north and south, so the window faces west and the afternoon sun would be on the radio for most of the afternoon. I surmised that 50 years of sun and summer heat had done in the cabinet. The moral for the collector is to keep your Catlin pieces in a cool dry place away from sunlight. The attic is certainly dry, but far from cool. As I walked to the front, I thought the summer heat has not done any of the electrolytic capacitors any good either. At least the paper caps would probably be dry if the wax had not all melted out. At the front I also opened the window and the resultant cross breeze made the attic quite comfortable. The spring smell of the air mingled with the odor of old electronic gear and wood. I again marveled at the office in the attic. With the windows overlooking high street, this must have been quite a comfortable place to do the books. I made a mental note to ask Marc if Clem had lived in one of the apartments on the second floor. I removed a folder from the filing cabinet and discovered notes on a topic that had crossed my mind just a few minutes before. Clem was a very thorough technician and would take the time to "form" an electrolytic capacitor before installing it. Over the years, he had devised a number of means to accomplish the task and the particular folder listed a few of them. I am sure this procedure saved him considerable effort by discovering a bad capacitor before it was installed. This, a particularly a good idea since these capacitors can be a pain to install and can cause other damage if they short. For those who are not familiar with the term, "form", an electrolytic capacitor depends upon a chemical reaction between the aluminum foil plate and a chemical electrolyte. The reaction forms a very thin insulating layer which acts as the dielectric for the capacitor. The electrolyte acts as the opposite plate of the capacitor and the resulting very thin dielectric permits a fairly large capacitor in fairly small space. Some early capacitors actually had liquid electrolyte. Most have the electrolyte soaked into paper or as a paste. So, dry electrolytics are not really dry. Capacitors depend on a seal to prevent evaporation of the electrolyte. In older units the seal will fail and the capacitor will literally dry out. This construction makes the capacitor polarity sensitive. If the polarity is reversed the chemical reaction that formed the dielectric will be reversed, no longer be a dielectric and the capacitor will pass considerable current. This will cause the electrolyte to heat, turn to steam, and cause a build up in internal pressure. The result can be a quite spectacular explosion. (This is why modern caps have the triangle shaped cuts in the top -- controlled pressure relief.) Clem realized that as the capacitors sat on the shelf, the dielectric layer would tend to go back into solution with the electrolyte. The result would be a thinner layer of dielectric and a much lower voltage rating. Therefore it is necessary to apply a lower voltage at first to establish the chemical reaction to replace the dielectric. As the reaction proceeds, the voltage may be raised slowly to the rating of the capacitor. This process forms the dielectric, and is, therefore, called "forming the capacitor". If a capacitor has lost it's form and has full voltage applied, it will often fail shorted. Clem's later device for forming a capacitor was a variable DC supply with a 7 watt lamp bulb in series to limit current. He would start at about 10% rating and increase the voltage in 10% increments at about half hour intervals. He would use the lamp to indicate current. If the cap failed at any time, the lamp would light brightly and not damage the power supply. He would watch the lamp, and if he could increase the voltage without the lamp staying lit, he would run up the voltage more quickly. Up to about 200 volts, he would use the lamp to check the capacitor. Clem would simply discharge the capacitor through the lamp and judge capacity by the brightness and duration of the resulting wink. He would repeat this process a few times to see if the cap was improving with applied voltage. Clem noted after about an hour, most would improve. As a final check, Clem would discharge the capacitor completely with the lamp and then wait a few seconds. After waiting he would check the capacitor for voltage with a VTVM. If the capacitor shows voltage, it indicates a phenomenon known a dielectric absorption. A cap with this condition will permit hum even though it is otherwise OK. This technique should be of particular interest for the collector who usually has to deal with New (very)Old Stock capacitors. All NOS capacitors should be formed before placing them in service. The secret for forming is to increase the voltage slowly and limit the current in the process so as to not boil the electrolyte. Clem's first device consisted of a battery eliminator with three lamps in series. He would advance the voltage by moving up the taps on the eliminator. At one time he used a surplus variac with a power transformer. He used a separate transformer for the filaments of an 80 as a rectifier. The separate transformer kept the 80 lit. Again, he used the 7 watt night light lamp in series as an indicator and fuse. All very useful information I thought as I replaced the folder and took out another. The next folder was labeled "Portable Radios" and as a collector of these things I took some interest. I opened to what proved to be a mystery for Clem. Early in '41 he had taken in one of the new 3 way portables. These are the units that run on AC, DC or batteries, Hence the term, 3 way. The set was dead and Clem had, as it turns out, correctly identified the problem as an open filament in one of the tubes. Since he did not have the batteries installed, he was testing with AC power. He chose the technique of tube substitution to find the problem. In this technique one tries a new tube in each location. After substituting each tube, he still found no life, so he removed all the tubes and checked the filaments with an ohm meter. He found not one, but two open filaments. Odd, he thought, as he plugged two new tubes in the appropriate sockets. Still, with all the tubes replaced, there was no life. At this point Clem was really puzzled. Again he removed the tubes and again he found an open filament, this tine in one of the new tubes! Clem then unplugged the radio, and rounded up a new tube. This time he checked each filament before inserting the tube. As he inserted the last tube he thought he saw a small flash in one of the tubes. He then checked that tube and found the filament open even though the radio was unplugged!! At this point, he rounded up the schematic and solved the mystery of the open filaments. His lesson will prove interesting to the modern collector who has a far smaller supply of spare tubes. If you look at the schematic, you will see the filament supply comes from a rectifier directly on the line. A large dropping resistor reduces the 120 volts to the required 6 volts. This resistor usually consists of two sections with a filter capacitor on each leg. In this set, the manufacturer had included a capacitor on the 6 volt side of the resistor. As long as the filaments were intact, all worked well. However, when a filament is open, the capacitor will charge to the full B+ voltage, usually about 120 volts. So here is the cause of the problem. When he plugged in the tubes, the 120 volts was present and the capacitor would hold the charge. 120 volts in place of 6 will certainly open filaments! The capacitor will hold a charge for some time, so it is possible to blow the filament even if the set is unplugged. Clem's solution is quite simple; be sure the capacitor is discharged before installing the tubes. When he discharged the capacitor, and replaced the tubes, all worked well. Clem's solution should be heeded by the modern collector as well. Newer sets include a resistor in parallel with the tubes or eliminated the capacitor at the 6 volt side to reduce the incidence of filament failures. Even with the newer sets, be sure to discharge the capacitor. Having learned a couple of valuable tips, and having discovered one sad case, I replaced the folder and proceeded to explore the attic. As I explored I began to wonder if the Curtis had a spare room. ---
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