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then multiply it by 1.20 {this allows a £0 percent margin for error and variation). Next, multiply the WVDC rating of the existing capacitor by 0.8Û (also allowing for a 20 percent margin). If the voltage limes 1.2 is near {or higher than) the WVDC rating of the capacitor, then opt for the next higher value.

I've seen this problem several times where 350 WVDC capacitors were used in a receiver that should have used 450 WVDC. I recall a medicaÊ oscilloscope used for bedside monitoring in hospitals a couple of decades ago. It had a 270-volt OG power supply that used 60 (iF/350 WVDC capacitors in the two actes. They were typicaliy found bulged ouï after a year or so in service. Note that 1,2 x 270 = 324 volts, and that 0.60 x 350 WVDC = 280 volts. Get the point: Under the worst caseH which apparently occurred from time to time, we have a 280-voJt capacitor in a 324-volt circuits With 20 or so of those 'scopes in our system, we were replacing electrolytics on a fairly regufar basis. While this scenario is a bit extreme, it does happen. We replaced the capacitors with 60 nF/450 WVDC units and never had another failure.

Some receivers use a multisection electrolylic capacitor These capacitors sit upright on the chassis, and may have either two, three or four electrolytic capacitors in a single metal can (although it might have a paper insulating cover over it). All sections share a single common ground connection, which is the case of the can. Some old-timers would replace an open-circuited section (a fairly common failure mechanism) by shunting a tubular electrolytic across the bad section. This saves the replacement job, and is cheaper than buying the entire multisection electrolytic capacitor. Like many such "solutions,11 however, it is simple, elegant, easy and utterly stupid. In other words, it's wrong. Why? Because those electrolytics tend to fail later by shorting out. I've seen that happen so many times that I tend to get curmudgeonly and short-tempered when I see it. Besides, in those years when 1 worked on medical instruments I saw one case where such a repair might have cost a patient's life. A contractor (not one of us in the hospital lab, thank God) had pulled that trick on a defibrillator, and then the main capacitor shorted. When the patient went Into ventricular Fibrillation, the docs tried to use the instrument, and it failed. Fortunately, there was one nearby on another unit and it was brought to the scene post haste . , , but time was wasted. We'll never know whether proper repair procedures would've saved that life, but I know Til never be party to such a shabby repair. Such are the lessons of life.

Another thing to check is the condition of the switches and potentiometers in the set. Use switch contact or TV tuner clearer to spray down each switch, especially rotary types, and each potentiometer. Get the stuff inside the component, and then mechanically operate the switch/pot vigorously eight to 10 times to thoroughly clean the insides. IVe found antique bandswttches so corroded that only gentle rubbing with a pencil eraser did the trick. Be careful, especially on bandswltches, to not disturb surrounding circuitry.

On receivers of old you will find a massive main tuning capacitor, a multi-sectioned marvel of an air dielectric variable. Wonderful sight, those were! How ever, they are also a pain in the neck after 10 or 20 years of unuse. The front and rear bearings supporting the rotor plates are lubricated. The lubricant dries out with time. In addition, there is a ground spring that connects the rotor plates to the capacitor frame. These (usually brass] springs are the electrical connection. If the receiver intermittently howls and screams when tuned, or is excessively microphonic (howls or screams when touched or vibrated}, then suspect either the lubricant or the spring.

The lubricant can be cleaned out with spray cleaner, but be sure no! to spray the plates. Be gentle with the spritzer button! Replace the lubricant with white petroleum grease such as LubriPlate, or something similar. Some people claim that silicone grease works as well, but I haven't used it so I canTl corroborate the claim. Use a toothpick to apply the grease, and then run the capacitor through its entire range several times,

The ground spring needs to be cleaned, especially under the tip that rests on the capacitor mounting plate. Gently raise it—but not too far—and burnish the metal underneath it and on its bottom side. This trick should reground the capacitor and make the radio work again.

Of course, there may be other problems, and they will require normal troubleshooting procedures to find. Those mentioned above are often due to old age or the abuse of nonuse.

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ter capacitors are often bad after storage, even when they were good when the rig was first placed in storage-

Photo C. Capacitor bearing and rolor ground are often sites of problems in Older receivers.

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