R5 Burnt Out in P48 Circuit

[cleverdick]

Hello Ted,

I hope you are well.

In one of the 828s that I look after, R5 has burnt out and gone o/c, momentarily (so I understand) taking the whole supply down with it. The device IC1 itself is not (at least now) s/c between pins 14 and 11/3, and on-board measurements (still with device removed) show that TR2 and 3 are still intact (i.e. not s/c C-E).

Is this something you’ve seen before?

With the device being obsolete, I’m proposing to dispense with P48 altogether, but have yet to get approval on that. I was just wondering if this is a known issue with this DC-DC convertor design.

Thanks,

Rich

[Ted Fletcher]

No, I have never seen this problem before.

That power supply is normally, in fact always, reliable, so I can’t comment.

Of course, it’s an old design, but it really has stood the test of time, so I suggest replace the parts and carry on using it!

[cleverdick]

Thanks Ted,

Let’s hope this doesn’t represent the back end of the bathtub curve! (Managed to procure a replacement device, anyway. Despite its obsolescence it is fortunately not rare, it seems.)

Best,

R

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