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Old 28th December 2010, 05:45 PM   #1
Stabby is offline Stabby  Belgium
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Default Replacement part for 50yo bridge rectifier?

I'm working on the pre-amp of a 50-year-old electric piano and will probably have to replace the bridge rectifier. It's pictured in this photo next to the transformer:

I have no idea what to look for though, since I can't interpret the information printed on the rectifier:

B250 075
KC068 22/8

Also, the rectifier is connected to two 350/395V capacitors.

Could anyone help me out?
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Old 28th December 2010, 06:16 PM   #2
nac134 is offline nac134  Canada
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Google B250 rectifier. Theyre available.
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Old 28th December 2010, 06:20 PM   #3
Mooly is offline Mooly  United Kingdom
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Hard to make out...
I assume it's silicon and not selenium... did selenium work at those voltages ?

You need to measure the AC voltage across the bridge and the DC across any reservoir cap. Does it follow the Vdc = 1.414 times Vac formula or not. Selenium has a larger volt drop. Just with you saying it was 50 years old.

Silicon... loads of choice but measure the voltages first and see what it runs at.
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Old 28th December 2010, 06:37 PM   #4
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You can replace selenium rectifiers with silicon, but sometimes you have to add series resistance at the appropriate wattage and voltage rating to keep the load voltage from getting too high. Note 1/4 W resistors are no longer rated at 400V because they are too short. 1 W resistors are usually rated above 400 V but check your supplier tables. Phenolic headers are handy for mounting discrete components to chassis at these high voltages, attaching to the metal with machine screws. sells them, calling them "terminal strips". I can't find headers at mouser, farnell, any of the usual international sources. With phenolic headers, you can also make a bridge out of 4 each 1n4007 etc. the way most vendors did in that era.
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Last edited by indianajo; 28th December 2010 at 06:42 PM.
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Old 30th December 2010, 08:19 AM   #5
RJM1 is offline RJM1  United States
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Are you sure it's not a B250 C75 bridge? If it is there is some information on replacing the B250 C75 selenium rectifer about two thirds down the page on this site:-

1958-1959 Grundig "2066 PX" AM/FM/SW Radio Restoration

You could use a small silicon bridge rectifier (about 1A 1000V) or four 1N4007 diodes in bridge formation, and a 100 ohm 10W resistor in series with the output to compensate for the voltage drop.

Mooly, 30KV Selenium diodes.
Attached Images
File Type: jpg B250 C75.jpg (34.4 KB, 67 views)

Last edited by RJM1; 30th December 2010 at 08:41 AM.
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Old 30th December 2010, 10:31 AM   #6
Mooly is offline Mooly  United Kingdom
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Originally Posted by RJM1 View Post
Thank you
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