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 Post subject: B&K 2050 no modulation fix
PostPosted: Feb Sun 17, 2019 2:13 pm 
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Joined: Nov Thu 22, 2007 11:31 pm
Posts: 947
Location: Johnston, Iowa
All, I purchased a B&K 2050 signal generator and discovered that the internal modulation was intermittent. With mine, I could occasionally hear a tone during alignment after the unit had warmed up for 30 or so minutes and I thought was strange for a solid state device. I did a google search and found this video on youtube https://www.youtube.com/watch?v=Ltq5zoAatcU . In this video the author traced the problem with his B&K 2050 to the 2SC458 transistors and he pointed out that these transistors have a horrible reputation. On the audio forums folks treat these as if they were 70 year old electrolytics and replace these whenever they are encountered. Apparently this was especially a problem with the earlier production runs of this transistor. This transistor was widely used in mid-seventies audio gear. After a google search on this transistor I took the video authors advice and replaced all five of the 2SC458’s with KSC1845’s. All of the legs on the 2SC458 were extremely corroded. It took about 30 minutes to swap out the transistors and the modulation now works! I wonder if anyone else has encountered this problem. Are these transistors as bad as the audiophile’s suggest?
Keith


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 Post subject: Re: B&K 2050 no modulation fix
PostPosted: Sep Tue 22, 2020 3:28 pm 
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Joined: May Sat 25, 2013 11:15 pm
Posts: 1491
Location: Des Moines, Iowa, USA
I have replaced at least two of those in as many years. One was in a voltage regulator.

Another one to watch for is a 2SA798. This is an inline five pin IC. It’s just two
matched PNP small signal transistors in the same package with the emitters tied together to the center pin.
It’s most often used as the input long-tailed-pair in audio amp boards. Being in the same case, they share thermal changes. Sansui used them a lot.

They like to slam the amplifier to one rail or the other and blow everything up. They should be replaced with a pair of fairly well matched transistors physically tied together.
https://commons.wikimedia.org/wiki/File ... rs_ENG.svg


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