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.

I've been really dragging my feet about burning in the caps on the RCA oscilloscope, seeing as how they are dated late 1964 for manufacture and I'm not doing the recap, rather hoisting it off on someone else. So I've been tiptoeing around the machine a little bit, keeping my heavy hand to myself.

In pondering the guts, two things are obvious. A) this machine has been subject to an impact of sorts at one point in the past 46 years:


B) the wire which broke free was essentially under tension and I'm willing to embrace the theory that the wire gave out when the impact occurred. You can see visual signs of oil staining on the wafer at which the wire was bent. I believe such leeching is inherent in the chemistry of the plastics, which in and of itself can be considered alarming.


Sticky wires sort of comes with the territory though, I do minimize contact with them when they are in this state, typically by leaving them be. Unless they've collected organic matter on the sticky surface I've not seen first hand evidence of voltage insulation failure, so... it's filed in the leave well enough alone category.

The fix for this unit entailed simply stretching the wire, which, due to the impossibility of directly applying that concept to actual wire I ran a leg off the terminal using a lead clipped from another component.


I'm happy to report that at a little over 80% wall voltage the scope is showing trace!


The hawkeye's amongst you will notice that the horizontal is far from true, I'll adjust clamp positioning once it's up to full voltage.

Yesterday afternoon I returned to the local electronics recycler with intent to buy an inexpensive dusty old oscilloscope for a friend who had displayed interest in it. Behold:


An old RCA WO 91B, I don't think RCA really made a huge splash in the scope market, dominated as it was by Tektronix, HP and so forth; still this feels as if it has a bit more capability than an EICO or Heathkit of the same era.. but I realize that may just be the conclusion I draw after being hypnotized by the RCA meatball.

I love this logo.

Scanning of the guts revealed no erupting capacitors, though, if this thing works it'll be a good preventative maintenance maneuver to replace all those paper bound caps. I love it when stuff is easy to spot and access.


I did spot one hanging wire in the horizontal refresh range that could have rendered this unit inoperable


It appears to have been connected to this lug:


So I'll solder it there prior to the application of electricity, what's the worst that could happen?

The lug itself slightly puzzles me, it looks like a section of solder has reflowed and has what could be an arc crater at the right edge. This could be visual artifacts from the photography, I'll give it a closer look when I'm in there. My current theory is that a resistive joint may have been heating up and the thermal cycling triggered a fatigue failure at the wire, which could have arced on its way out. Which, in my mind points to a tube failure, as that sort of current probably shouldn't be occurring at a ground reference for an oscillator to begin with.