The Tektronix R 556 got pulled out of the rack for inspection yesterday. The biggest mystery so far has been how a fuse blew when both legs of the AC mains should have been open at the switch.


Side panel open to show mains conditioner and primary fuses; no dead animals in here, removing one potential cause off my list. I'm also not convinced that the fuse selection was correct, the ten amp fuse being a 3AG 10A 32V.


Revisiting the fuses just now has brought my attention to a fact that has been eluding me. The 10 amp fuse is for 125 volt mains, and the 5 amp fuse is for 230 volt mains, explaining why F1402 (the open 5A fuse) is depicted as an isolated self contained circuit. At THIS point I am proceeding as if the fuse is in fact a red herring, the condition of the failed fuse is not what one would expect after an audible arc with an olfactory aftermath. Besides, the open fuse shouldn't even be in circuit!

The simplest explanation is that a tech harvested the 5A fuse, knowing the oscilloscope wasn't connected to 230 volts to use in something that required a 5A fuse. The dummy non functioning part was then inserted to load the twist-lock cap so it didn't wander off somewhere or rattle while the fan is in operation.


I can't bank on that theory naturally; it is, however, feasible enough to wipe my mind of preconceived notions that could otherwise mask the real problem. The above shot is the rectifier section, and for the moment it's not suspect either (thankfully).


An operational power switch shouldn't be a source of heat. I find this suspect, though each pole of the switch is either fully open or fully closed according to my multimeter (which isn't lab grade, so there may be a small fraction of an ohm there). I'll revisit the switch later on, what it's not doing is sticking closed, so my concern of stray voltage feeding the circuit while the unit should be powered off is, for the moment, satisfied.

Now I'll focus my attention on the components that are on the hot side of the switch, the FL1400 filter and the 0.1pf cap that is strapped across the mains.


On a lighter note, digging through Tektronix equipment is inspirational when it comes to lead dress. I'll strive to achieve a similar level of beautiful geometry when I start in on point to point modular builds.