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Old 26th March 2011, 08:02 PM   #1
hotscot is offline hotscot  Scotland
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Join Date: Aug 2004
Location: California
Default LM3875 power supply question.

I'm only getting @ 17 V DC when I'm expecting @ 37 V.

Does this suggest one of my diodes has failed?

I have a multimeter but find testing diodes tricky.

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Old 26th March 2011, 08:09 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Plug your transformer in via a bulb tester and tell us what the bulb filament has done/is doing.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 26th March 2011, 08:14 PM   #3
diyAudio Member
Join Date: May 2008
Depending on your multimeter there are two ways of testing. Both require you to test the diode twice, once in forward and once in reverse direction. Some meters come with a diode test facility that shows the forward voltage in one direction and should show overload in the other. If your meter does not have that, use the impedance measurement. You should get overload in one direction and a low value in the other.
Diodes that show continuity (or forward voltage) in both directions or show overload in both directions are broken.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 26th March 2011, 08:27 PM   #4
hotscot is offline hotscot  Scotland
diyAudio Member
Join Date: Aug 2004
Location: California
Default Thanks for the input.

Found a dodgy diode with multimeter.
Now replacing.

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