Can I correctly test these diodes in circuit?

Thread Starter

Chris Wilson

Joined Dec 2, 2011
I have a device which died the other day, it's been dry stored for years, powered up OK, then died on soak test after three hours with part of the front panel LED's and other stuff not working. I am now getting 5.9 volts on the 7 V + line with an accurate DMM. I tested the diodes circled in green in circuit and some showed bad (conductive both ways the top two), and one on another rail seemed bad, but forget that for now. Am I right in thinking this circuit should allow on board testing? I intend to replace the IN4001 's with similar or higher rated, and change all the electrolytics, but was wondering if I am right in thinking this circuit would allow accurate on board testing of the rectifiers circled? Cheers.



Joined Jan 28, 2011
For a quick check, testing the diodes in circuit may be OK, but also may be misleading. It shouldn't be difficult to unsolder them, or to clip one of the leads. (They are cheap to replace.)

Unless you have somehow exceeded the current or voltage rating on the diodes, if they are no longer diodes (either open or shorted), the 1N4004 should be OK for this circuit. If you decide to replace them, there are a couple of inexpensive diodes with the same PIV rating and a 3 amp capability.
1N5408 leaps to mind.


Joined Feb 28, 2009
Do you have an oscilloscope? If so, check the voltage for excessive ripple at the filter caps. I had a 12V 5A wall-wart that had low output voltage and it turned out that the filter caps were shot and the output was all ripple.