I'm preparing to briefly touch upon the ANALOG vs. DIGITAL debate (oh so tired a discourse), guess which side I'm going to take.

So, as determined last time I had a look at the Tek 556, it would behoove me to focus only on circuit elements that are upstream of the power switch, narrowing my attention to the mains filter. Easiest test was to read for continuity across the AC legs of the power jack, at the highest range I watched my DMM read lower and lower resistances. Digital meters send a pulse out one probe and weigh the voltage drop vs current delivered into the completed circuit to determine resistance between the leads. In this case the pulse was charging the capacitor meant to shunt high frequency line noise, so as the cap slowly charged the perceived voltage drop was less and less, delivering the false reading of a lower resistance. Clearly, sometimes easiest is not best, or even good.

Time to start sniffing around for a decent analog meter.


While it has been over a week, this hole retains traces of the odor of failure.


The inlet cavity of the Sprague filter also appears to have seen heat in the past. While this could be patina that evolved over the past 45 years, I don't trust this part, so it will go.


I highly doubt I can find another one of these with ease, and if I did I'm betting it would be of roughly the same age, meaning it would be "vintage" and there'd likely be some sort of premium on top of the decrepit part. No thanks.

Seems the easiest path before me is to build in a modern power conditioner in the space previously occupied by the large can of the Sprague. I toyed, for some fraction of a moment, with the idea of building the circuit inside the Sprague can, but the PITA factor coupled with not knowing what's inside the can (1966 was within the polychlorinated biphenyl era) sobered me up.


Not reusing the Sprague component will demand I change my hardware a little in order to mount the voltage badge, I'm thinking nuts.


Spot the high voltage wire!

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.

How do you blow a five amp fuse while something is off, and has been powered off for a while?

I don't know, hopefully I'll find out. Otherwise I'm sort of dead in the water. But yeah, getting a little ahead of myself here. I tore down the rack as built since it was sort of tip happy with the bulk of the 556 bolted in.


Flowers anyone?


Not as nice as having it bolted directly to the floor, but stable enough to freestand the populated rack.


Repositioning everything in order to bolt in a rack that can carry the weight of the 556 during installation (and removal).


Alignment has become a bit off, three screws in.

Anyway, I fired the scope up, pulled up four traces and, having nothing to measure powered it back down again. Half hour later the lights in the house dim briefly while the distinguished sound of arcing occurs. One of the mains fuses has opened on the scope.

Of course, I can't look at it while it's bolted in (without contortion, anyway), so out it will come, again. Thinking about a decent lightweight solid state scope at the moment.

Winter, with its cooler temperatures, has chased me away from the uninsulated hell hole that is supposed to be my bench. I picked up an open frame server rack earlier this week, allowing vertical orientation of junk in the utility room (a welcome change).


With the inclusion of my scope, I can proceed with a bit more power in some of these piled up projects. However, with something in the neighborhood of 100 pounds hanging 24 inches out the ass end of this piece of equipment, my attempt at an installation pictured here has become little more than a mock-up. I need to tear this down and fortify the base (which is being helped on the balancing act by tube amps and bungee cords) in order to reach a comfortable level of structural integrity.

Atop the dual beam scope (each "side" being loaded with dual trace preamps for four simultaneous traces on the screen) are a General Radio 1192 and a Heathkit IB-1101 frequency counters. I look forward to deploying real tools again, and hope this will fertilize these chronicles.