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 Post subject: What are the odds, bias diodes Altec Lansing 911
PostPosted: Nov Wed 08, 2017 8:43 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1363
Location: Hutchinson KS
Customer brings me one of these heavy stereo receiver/turntable units with the complaint blows fuses. Found a shorted output transistor, replaced both on the bad channel, fired it up slowly on the variac, found it was drawing way too much current. Found an open bias diode, replaced, gave it another try, works perfect!. Lubed bias and balance controls and adjusted both values to factory specs. Set played fine all day. Put cabinet back on, it blew the fuse again. The other bias diode in series with the first one had opened also!! Fortunately it blew the fuse before doing any more damage.


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 Post subject: Re: What are the odds, bias diodes Altec Lansing 911
PostPosted: Nov Wed 08, 2017 9:20 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20691
Location: Annapolis, MD
I would say that there was a high probability of what happened. All designs have weak spots...either parts that have minimal margins, or are "fragile" in some way.

In one recent job, I found a Ge diode bad. I checked another one, and it was marginal. All Ge diodes removed and replaced.

I do not normally "shotgun" with Silicon semiconductors, but if something like a power transistor failed, I'd probably replace all like.

What were the diodes that failed?

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: What are the odds, bias diodes Altec Lansing 911
PostPosted: Nov Thu 09, 2017 5:03 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1363
Location: Hutchinson KS
The schematic says the diodes are HV26X6, 2 in series used as temperature sensors mounted next to the output transistor heatsink. I just stuck in a regular silicone diode. And adjusted the bias current. Someone had worked on it before and turned all the bias pots to minimum!


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