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

Fabio Trevisan
 

Hello,
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.

Rgrds,

Fabio

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