fuse blows after diode swap in Toshiba SD-3950 PSU - diyAudio
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Old 17th August 2004, 09:03 PM   #1
pburke is offline pburke  Germany
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Join Date: Nov 2003
Location: Madison Wisconsin
Default fuse blows after diode swap in Toshiba SD-3950 PSU

I've been doing the Toshiba mods in stages. Last night I decided that it was time to do something about the diodes in the power supply. I figured it worked great in my Cambridge Audio D500, then it can't be bad for the Toshiba.

So what I did is swap Diodes D801, 802, 803, 804 and D 828, 838 for some 100V schottky diodes I got from digikey.. yes, I know - I didn't pay attention that in this power supply these diodes are on 110V AC... so I blew two of those along with the fuse. Next I got smart and went back to my parts bin, grabbed a 4-pack of Cree Schottky diodes that are good for 600V and 1amp - put those in the D801 through 804 positions, put a new fuse in, plugged in the power and - you guessed it - poof - there goes the next fuse (and this time a nice ceramic one...)

So before I blow up more ceramic fuses, does anyone know if my problem could still related to those diodes in the main rectifier? I didn't check if one of the Cree schottkys is blown, but I have a hunch that it may be.

Basic question to the people who know more about this stuff than I do: if the player has a 1.6amp fuse, are the 1amp diodes too small, or should I look for a problem elsewhere? The parts replaced are labeled as 1N4005S and according to the datasheet, they are rated at 1amp 600V - we're talking a $0.02 part here - there is a reason why the 3950 is so cheap.

So I am at a loss - did I short something out by accident? did I break something with the first set of under-rated schottkys that blew the first fuse and now it'll blow every time until I isolate that broken part?

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Old 18th August 2004, 06:08 AM   #2
HDTVman is offline HDTVman  United States
diyAudio Member
Join Date: Aug 2001
Location: Chicago area
I don't know and you didn't say, is this a switch mode power supply? If it is the most likely cause of the fuses blowing now is a failed switching transistor / FET and / or shorted switcher control IC. If the diodes on the AC line failed they would send AC to these components and damage them. My guess would be that all of the silicon on the AC side of the switcher transformer is blown.

Good luck

What ever makes the tunes flow
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Old 18th August 2004, 01:10 PM   #3
pburke is offline pburke  Germany
diyAudio Member
Join Date: Nov 2003
Location: Madison Wisconsin
yeah, it is a switching power supply and your logic makes sense. The good news is that I can get a part-donor 3950 for about what I paid for those Schottky diodes

I'll better get a few of them, just in case

thanks for the explanantion

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