grundig: 80U (80 U); Rectifier / Gleichrichter
What kind of solid state rectifier is in the Grundig 80U?
The part number in the original Grundig schematic is E 250 C50N. The schematic shows a single diode.
The shape is unusual:
The model 88 has a full wave bridge rectifier type B250C75. The Sams schematic classifies the bridge as Selenium:
The Grundig model 85 has a single diode rectifier of the type E 250 C85:
If these are all selenium rectifiers, are they more reliable than the kind where the diode plates are exposed?
this is a selenium rectifier. E stands for 'Einweg' what means one way or single diode.
Opposite you find B for Brücke what means bridge.
all the above rectifiers are medium current selenium types. The different shapes depend upon their manufacturers: AEG made can shaped rectifiers, while Siemens built flat types. I do not have data about AEG production, may be that somebody from Germany knows more. The Siemens design, where the small piles of elementary cells inside were distributed very close to the metal chassis, seems to have more efficient conductive heat dissipation. The AEG types radiated part of heat through to their dark surface external: they were bulky and, as far as I can remember, became warm to hot in their operation.
The disk or plate types were convection cooled by air flowing through the plates. The active surface of the rectifier was usually smaller than the entire disk, the outer surface being used just as heat radiator.
Regardless of the construction, the forward voltage drop of each cell decreases with the temperature increase, while the leakage current shows considerable increase. The maximum junction temperature should be anyway limited around 75ºC, well under the melting point of the alloy of the soft counterelectrode. As far as the temperature is kept within the said limit, the selenium rectifiers are very reliable, also in presence of moderate overloads. Life expectancies of 25.000 to over 90.000 hours were claimed by US major manufacturers, as GE, Westinghouse and IR, in their ads.
The factor that may shorten the life of these devices, other than excessive temperature increase, is a prolonged stand-by. When full power is first applied after a long storage, selenium rectifiers show an increased leakage current that can cause dangerous thermal rise. This type of rectifier should be allowed to operate at reduced a-c voltage for a short while before full power is applied.
The documentation on AEG rectifiers kindly loaded by Mr. Knoll gives an idea of their size. By the way, the based models may be considered forerunners of the silicon solid-state tube replacements from the late ‘950s.
The AEG 250B100, 250Vrms, 100mA, bridge circuit, measures 30mm dia. by 80mm high. Its equivalent by Siemens was a small brick, measuring about 45 by 35 by 7.5 mm. Of course the Siemens B250C100 could be operated at its nominal 100mA only when mounted on the chassis (infinite heatsink); in free air its current rating was halved. But, since radios in the fifties had a metal chassis, the Siemens design appears to be very efficient, granting compactness and excellent heat dissipation.
A lot of people may disagree, but I think that selenium rectifiers really were reliable components, when used within their design limits.
Dear Hans and Emilio,
Thank you for the excellent information. I also followed the excellent article threads that Hans shared.
This will be useful as I attempt a restoration on Grundig 1045W, which has a full wave bridge. I am currently on vacation in the Azores. Last year I offered a neighbor in the Azores to restore his 1045w. I have a couple of pictures that I could contribute to the model, but the radio is still quite dirty.
One point stands out on Emilio's recommendations, and that is that a selenium rectifier may need some reforming if it has not been used for a long time. This long term degradation suggests to me that the rectifier is always in some state of flux. Perhaps occasional use keeps it in good shape, as is the case with electrolytic caps.
Another point is that many natural processes, that involve particle flows, tend to double every 10oC. This brings forth the importance of keeping a low temperature to stabilize the chemistry of the selenium rectifier.
I have replaced only two selenium rectifiers in several Grundig AM-FM three tube radios that I own, and I use them regularly, without any trouble. So reliability seems good, but don't exceed the limits. One of the radios that I replaced the selenium rectifier was the Grundig 80. The other rectifier was perhaps in a model 85 with the thin chassis mounted kind shown above.