7844 power supply

Peter Gottlieb
 

A couple of weeks ago I mentioned a 7844 which instantly blew the line fuse but which had previously worked.

It was suggested I look at the line rectifier and bulk caps.

I finally finished the project on my bench and put the scope there. The problem turned out to be that the two main switching transistors were shorted.  After replacing them all was not well though.  The supply kept trying to come up, making a weak rattling sound.  I could see it was trying.  Then ever once in a while it would come up, fan run, and scope start, but then within a few seconds go back to rattling.  Then in the midst of considering where to look next it just stopped.  Now, nothing.  HV is on the main caps and the neon is rapidly flashing, but nothing else.

These supplies are infamous for being a pain in the neck.  Has anyone seen this and have any hints for what I should look for?

Peter

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