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After quite a long period of good work, my beloved 5403-D40 scope died.
Out of any doubt, the fault is in the high-voltage section and was announced by some unexpected blows of the fuse F410. I replaced it two or three times, fearing for some overload in the HT circuitry, but during this period the image on the CRT was perfect, that means voltages were O.K.
After the last fuse-blow, the scope definitively turned out and I noted the 3,3 ohms resistor R410 to be cooked.
-Easy, you will answer, the tantalum condenser C410 is shorted or leaky...
Well, I immediately replaced it.
I also replaced the HV osc. Q410, who for some reason burned in the meantime.
But what is strange is that no-voltage was present at the primary winding of the HT transformer T410.
I have replaced the 3,3 Ohms R410 resistor FIVE times with a new-one. Well, the unbelivable is that immediately after I have switched-on the scope, R410 becomes interrupted. (I have said interrupted, i.e. its resistance from 3,3 ohms jumps to infinity).
Now, since the dawn of time when a resistor is crossed by an excess of current will produce some preliminary sympthoms like overheating, smoke, smell etc. Nothing of this. The wilt is immediate and resistor remains apparently like new.
The question is: whats the hell that can produce a so strange sympthom ??
If anybody was affected by the same problem, or was noticed about-it, I will grateful for an answer…
p.s.) I've been unable to attach the *.jpg of the HT schematic, but I'm confident you may download or already have it...
All the best from pizza’s land,
Gianni, i4-MGA in Bologna, Italy
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