Is this off-line flyback? B+ switch, Startup troubleshooting steps?

[gmunee]

Fairly new to this. I've got a schematic for a DVD supply Im trying to fix for a friend. The schematic is available publicly so Im not concerned about post it. 90% sure its an offline flyback topology, but need some confirmation from those in the know. The unit blows the fuse on application of 110VAC which might be shorted transistors. What should I check first? I think Q912 & Q14 are the input transistors?
What is the layman definition of the "B+ switch"?
I've got a variac and scope so I could bring up slow if needed, just dont really know what to check
Thanks for any responses.

 

[barrymagrec]

Most likely problem is IC 901 gone short. Check the bridge rectifier and all the other diodes in the mains input area, also the fusible resitors in the feed to IC901.
If you want to look at it with a scope you either need an isolating transformer or a scope which is not earthed. This is an iffy practice not reccomended if you are not experienced.

Barry

 

[gmunee]

Thanks for the response. I pulled the main control chip IC901. Fuse no longer blows with AC on. Not sure how to check that the input section is working. Should there be clean DC across the fusible resistors R905 and R909? Im measuring 40V on the DC scale of a fluke 179 with on R905 and R906? ANy help is appreciated? Thanks,

 

[barrymagrec]

That would appear to sugest IC 901 is faulty. To prove it you should be able to measure a short circuit across the D and Gnd pins of the IC you have removed. I`m a little worried that you could have over-volted C914 by running with the main chip removed, but hopefully not. If you just check all the diodes and resistors in the input section with your Fluke it should be safe to put in a new chip and switch on - hope you can source the part.

Barry