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Selenium rectifiers: failures and restoring tips

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Forum » Technique, Repair, Restoration, Home construction ** » Repair and restoration: Tips and Tricks » Selenium rectifiers: failures and restoring tips
           
Emilio Ciardiello
Emilio Ciardiello
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04.Jun.20 21:24

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Too often, when powering an old radio or an electronic set using selenium rectifiers, these components fail. Symptoms are a quite fast overheating of the rectifiers themselves, a considerable reduction of the AC and DC voltages in the set, with dimming of the light bulbs, and an overheat of the power transformer. If a fuse is present in the primary circuit, usually it blows. If the fuse is replaced with one of greater current capacity, the power transformer burns and in many cases smoke and a disgusting smell fill the room.

Many improvised technicians conclude that selenium rectifiers are unreliable, their life being quite limited, then they must be always replaced. According to some sites, the direct voltage drop increase with age, causing overheating. Nothing further from the truth! Should the DC voltage drop which can be observed in case of defective rectifier be caused by increased forward resistance, its effects on overheating should be less noticeable and the power transformer should run even cooler. If we assume 50 V drop across a 220 V bridge operating at 100 mA, this should result in about 4 W power dissipation increase. Actually, in case of rectifier failure, both the rectifier itself and the power transformer become hot and other secondary voltages are significantly low. Then the failure mechanism must be different. Selenium rectifiers were rated for almost infinite continuous service life, over than 60.000 hours according to the 1951 GE ad below.

In comparison life expectancies for high-rel vacuum tubes varied between 5.000 and 10.000 hours. Failures in selenium rectifiers therefore do not depend upon their age. In my collection I have many old radios and electronic equipment still working fine with their 65-year old original selenium bridges.

Selenium rectifiers were made depositing a tin layer of selenium between steel or aluminum plates and counter electrodes, sometimes spring-loaded. Reverse voltage rating of each cell could vary from 18 to about 30 volts, depending upon the type. Typical forward drop was about 1 volt per cell. Manufacturers offered high-voltage rectifiers, up to 100 kV, connecting in series enough cells to withstand the required reverse voltage. Rectified current rating was in the order of 0.35 amp per square inch or even higher when forced-air or oil cooling was used. From the typical curves below, we see that the typical reverse current was fairly low, about one hundredth of the direct current.

 

Fig. 2 - Typical curves of forward and reverse current-to-voltage characteristics at different temperatures.

Selenium cells might suffer aging in the first year of operating or idle life, with subsequent increase of forward drop and decrease of reverse resistance. Forward drop increase, if any, was not greater than 7% of the initial value. Therefore we can assume a forward drop always under 1.2 volts per cell. Now let us consider the typical application of a bridge rectifier in the power supply section of a German radio. We can assume an input voltage of 220 volts RMS and a DC current of about 100 mA. Five to six series-connected cells are required for each arm of the Graetz bridge, giving a total forward voltage drop not exceeding 15 volts. At 100 mA the total power dissipation of the bridge will be under a couple of watts, definitely too low to cause overheating and failures as believed by those people who maniacally replace any selenium bridges.

Then selenium rectifiers are reliable, nevertheless in many cases they fail: why? There are two possible reasons. The first one is an oxidation and a subsequent corrosion of the metal plates, steel or aluminum, due to improper storage. In this case the corrosion is visibly and of course the rectifier must be replaced, unless it is a hermetic oil-filled type sometimes found in military sets. The second and by far the more common reason is due to improper handling of the set at its wake-up after long storage. In this case damages are caused by people who rashly powers that old radio found in the basement of the grandma house just to see if it can be sold as working on e-bay.

Looking at the curves of fig. 2 we see that the typical reverse leakage current is between 2.5 and 7.5 mA at a reverse voltage of about 25 volts per cell, depending upon the temperature. Leakage currents increase with the storage time, as it happens even in electrolytic capacitors. After years of storage, reverse leakage current could be very high. Let us assume that 50 mA reverse current flows at the power-on. This means 11 W additional power dissipation in the rectifier which causes a fast temperature rise-up, a further increase of reverse current and so on, up to the complete burnout of the rectifier. The abnormal leakage also overloads the same power transformer, causing overheating and burnout.

As in the case of electrolytic capacitors, leakage currents can readily drop to their normal values after a low voltage reforming. The simplest way is to power on the set at one half the nominal supply voltage for ten to fifteen minutes. A Variac, if available, is the best tool, to slowly raise the primary voltage while monitoring the power drain. As alternative, a step-down transformer or even a series connected resistor or a lamp can be used to limit the supply voltage during the reforming. The same process must also be performed each time an already restored radio was left idle for years.

Often anyway we buy a radio or another set already damaged by the seller. What can be done if the selenium rectifier is dead and must be replaced? In this case we can try to repair the old rectifier using silicon diodes, as in the example of fig. 3 below.

Fig- 3 - Starting from left: A) Original selenium rectifier. B) ½ inch fiber washers for flexible water hoses are approximately the same size of the metal washers used between the plates in the stacks. C) Once mounted the insulating washers into the selenium stacks, silicon diodes are soldered to the terminal lugs. D) After a coating with nail lacquer, selenium rectifiers are back again in this Tektronix 515A oscilloscope, fully retaining their original charming look. Click on the image to enlarge.

Otherwise we can use a silicon bridge, preferably a type with a mounting hole, which can be screwed or riveted to the chassis. Use of silicon replacements will cause a little DC voltage increase, due to the fewer forward junction drops. To return the rectified voltage to its nominal value, the simple solution in the insertion of a power resistor between the positive terminal of the silicon bridge and the positive terminal of the filter capacitor.

 

Omer Suleimanagich
Omer Suleimanagich
 
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04.Jun.20 22:54

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There are some good threads on how to replace a selenium rectifier to silicon bridge rectifier , with four snubbing capacitors, and a dropping resistor (possibly with a capacitor instead). The color television of the 1960's owes its presence to the use of the 1N4007 diode. By then the majority of electronics manufacturers abandoned selenium rectifiers because of their failure rates and damage to power transformers

The Lionel train transformers actually were designed to adjust voltage to compensate for their degrading rectifiers

Emilio Ciardiello
Emilio Ciardiello
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05.Jun.20 08:12

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When I speak of improvised technicians, I am referring to the many sites that recommend replacing selenium rectifiers anyway, stating that they are unreliable. It may also be that companies like GE or Westinghouse exaggerated in their advertisements, but this is not true. Those sites would do better to recommend proper wake-up procedures for old electronic devices. The electrolytic filter capacitors themselves would benefit from such advice.
I have almost a hundred German radios in my collection, phonographs, tape recorders and instruments that use selenium rectifiers. All were put back into operation, proudly leaving the original rectifiers, as the speakers, tubes, wooden cabinets and all the other original components remained in place. The only exception is represented by six of them, in which the rectifiers had already arrived irreversibly damaged by ignorant people who had switched on the apparatus without carrying out the procedure for reforming the rectifiers and electrolytic capacitors.

Of course I always perform a quick reforming every time I switch-on again, after a prolonged storage, an already restored radio.

Omer Suleimanagich
Omer Suleimanagich
 
USA  Articles: 424
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05.Jun.20 08:53

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I believe you about your experiences with selenium rectifiers, In fact they are still manufactured!

Our experiences in North America are a bit different, probably due to poor storage of equipment that had an impact on their components due to in-house climate conditions in garages, attics, and basements 

Owners that kept radios inside their homes, in better climate conditions, probably only need a variac to very slowly power their radio on

What is interesting, radios that were maintained and kept running for sixty to seventy years, only have seen some vacuum tubes replaced

Personally, most of my radios were purchased with catastrophic failures. The selenium rectifiers were damaged probably when the radio was  taken out of storage and just plugged in and turned on

I would like to know other member experiences on this forum for changing components 

The variac is one of the most important tools we have and turning an old radio on that sat idle for many decades, will take time to very slowly power on

Last week , a friend added a battery iliminator to a big farm radio that is still using capacitors , resistors,and vacuum tubes from 1937!

 

 

This article was edited 05.Jun.20 08:55 by Omer Suleimanagich .

  
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