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

Fabio Trevisan

Hi Guys,
An introduction...
I was still busy with my new 7A13, of which I already replaced all the relays (will write another topic on that and post pictures... soon!), when I got to the point of adjusting the overload thermal balance adjustment.
The requirement is that after overloading the input, when setting the input back to 0V (GND) the trace must return to the center line (within 0.5mV) in less than a second.
Piece of cake I thought, but when changing the adjustment, it doesn't cause much of a difference on the time the trace takes to recover from overload, so I thought about registering the settling time with the storage mode of the 7623A (so that I could make an objective comparison between different settings).
When I tried to engage the store mode, the trace became dim, thick (out of focus, almost 1 div thick) and, upon applying a signal, I realized the deflection was exaggerated on both H and V directions... a clear signal of collapsed Hi Voltage.
Searched for the usual culprits on the visible area of the scope to no avail... so the problem had to be inside the H.V. inverter cage.
I opened the Hi-Voltage cage and found the H.V. inverter decoupling capacitor (47uF) blown...(really blown! Rubber cap completely out of the can)
Minor setback I thought... Searched and found a suitable replacement on my parts bin and replaced the capacitor... Took the opportunity to check all other electrolytics inside the H.V cage and all were were fine.
End of intro..

After putting the H.V. cage back, putting back the Z axis board and Readout board (what a mess of wires!), I turned the scope back on and voila... A beautiful trace was back, and I could engage again the storage mode and the trace would remain bright and focused... As Dave (from EEVBlog) would say, Bob's your uncle!

But then, after about 10 minutes it was on... all of a sudden, I heard a "pop", and all lights of the scope went dark (including the Power Light) and I had to leave (didn't have chance to open it).

Besides the usual and general advice, of looking for blown up electrolytic capacitors, or burnt / discolored bead tantalum capacitors... Is there any specific advice to this model, that I must follow to prevent making matters worse?

Since the last place that I touched was the H.V. inverter, I thought about disconnecting the fuse that feeds the H.V. inverter and trying to reapply power to the scope (and see if all the LV power supplies come back to life.

Any other ideas?

Thanks for any recommendation.



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