Re: Tek 7623A started blowing mains fuse - Any specific tip or known failure mode?

Fabio Trevisan

Thanks Brad, Craig, Tim and Rogers for your heads up and sorry for the long delay in answering. Past was a busy and crazy week.

In the end, I only wasted the bandwidth and your time because it wasn't the mains fuse what was blowing.
I only assumed it, because even the power pilot light wouldn't turn on, but I realized only later that the power light on this scope is powered from the +5V.

The net result however, was almost like if the mains fuse had blown, because the scope ***almost*** didn't have any power... Only the -50V was there... all the others were collapsed.

I found a blown 150mA fuse on the 130V P.S.
I found the resistance of the +130V to ground to be only about 20 Ohms, something that couldn't be right for a 130V rail that's limited to 150mA (tops).
After some chasing, I foundthree 2N5551 short-circuited, all three of them belonging to the +50V supply and all were being either fed by the +130V or referenced to it and, being shorted, they were blowing the +130V fuse.
After a thorough check of all components around those three trannies, to rule-out they could have been blown by something else, and having found nothing else blown or even drifted, I simply replaced the three trannies and the fuse and scope is now back on track.

Curious fact is that, although almost all LVPS of this scope are over-current protected, the +130V is the only one that is protected by a fuse (I don't count the H.V. power supply fuse, because that is on the unregulated side), and almost all the other supplies rely directly or indirectly on the +130V rail to operate so, a +130V rail fuse blown brings the whole scope down.

That's the closing for this topic.
Thanks again for those who took the time to answer... This report goes for you.



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