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Rebuilding Antique Collectible Vacuum Tube Radios Page 3

OddMix.com - Radio Technology Note - RTN0801 - Karl Nagy
Picture 1. Good and Defective Vacuum Tube [5 KB]
Picture 1. Good and Defective Vacuum Tube

The shiny mirror like deposit inside a healthy tube - left side of Picture 1. is called the getter. It is usually made of some magnesium compound that the maker places inside the vacuum tube before closing the envelope. After the glass envelope is evacuated as much as practical, the getter is fired by electronic means. The resulting deposit colors the glass, and it improves the tubes vacuum.

Assuming all the tubes are present in all possible sockets, and are all the right type and are nice, dark and shiny, it is time for do a detailed visual inspection on the top of the chassis. At this time the inspection should center on all electrolytic capacitors. They are tubular and many times of shiny aluminum cylinders. Their surface and vicinity on the chassis should look dry on the outside, and display sound physical presence.

Electrolytic capacitors are the weakest link, as they are used the wet process way back when and thus contain, or leaked out all of their electrolyte. Drying up, or leaking away their electrolyte is the most common failure mode for electrolytic capacitors. Lost electrolyte is highly visible as it is corrosive and makes a great mess. A dry capacitor will identify itself after the set is turned on with a high amount of loud sixty-cycle noise that is called a hum.

Unused electrolytic capacitors slowly loose their oxide-layer that gives them the required breakdown voltage, so ideally they need to be reformed. This may sounds complicated, but reforming automatically takes places in most electronic equipment containing electrolytic capacitors if they were not off for a half century or so.
Picture 2. Old Radio Chassis Bottom View [10 KB]
Picture 2. Old Radio Chassis Bottom View

The top side inspection should not take too long as there is not all that many component s are located on top of the chassis. Once it is completed. The radio or the chassis should be turned upside down and a real thorough visual inspection should begin Picture 2. All resistors will show discoloration or burn marks if defective. Capacitors will not reveal much, but still each and every one needs to be looked at for anything unusual. Old capacitors can be shorted out, or open. Old resistors can short out, but seldom do, and most of them open up, that they should not do (so says the appropriate MIL Handbook).

If all looks good, it is time to return all loose and removed pieces in place and reassemble the radio. Just before closing up the set, inspect the speaker for physical damage. Many old speakers are still operable in their ancient condition if there is no obvious visible damage afflicted onto them. If all looks reasonable then put the back panel in place. Take another final detailed look of the line cord and if it looks good, plug it into a live outlet. Take a deep breath, and turn on the radio. Turning on is usually done on most radios by rotating the volume control out of its leftmost off position. If the set is equipped with little lights to illuminate the scale, or as an activity indicator, they should light up at this point.

Most tube sets need some sixty seconds to warm up enough to begin to function. Once a few minutes elapsed, turn the volume control all the way up and slowly rotate the other knob, the one coupled to the rotary capacitor. If all is well, some stations will be heard. If none comes in all across the scale, and the set has a band selector switch, change the position of the switch. Sweep the entire scale again until some station will sound. If the result is still nothing, then turn off the radio, as it is time to do some troubleshooting.


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