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.

IT LIVES!

After hacking away at this thing FAR MORE than I had intended to, the diode test fixture works, again. Having displaced the guts enough to tackle some metalwork, I bored out the power supply connector hole to accept a DB-9 connector that will support all power feeds, though I've retained the banana connectors for the variable supply for convenience.

After plumbing that in, I applied power only to learn that I'd lost a channel. Following a bit of hair pulling (from the front, naturally), I isolated a poor solder joint on one of the three breadboards. Closed it up and discovered that while I'd fixed the channel I was working on another channel took a shit. Traced that to the same board, repaired it.. gently tucked everything away and closed the lid. You'll just have to live without gutshot of the most current incarnation, sorry. Imagine a lot of wires that have been cut and spliced numerous times and three ugly breadboards.

Here's a couple face shots:

These numbers are meaningless in terms of absolute voltage. I have traded accuracy for precision. Here is a shot that demonstrates the sort of swing one gets from a change in temperature, my finger throwing off the readings of channel D (it took about 20-30 seconds to drop that far and it hadn't finished dropping).



That is why all matching of semiconductors takes so long, one must allow the part to thermally stabilise or any readings are essentially useless.

Sporadic updates, I'll try to be a better archivist.. it won't be difficult.

Things are looking good for my diode test rig, only 1.5 years in the making. To recap, it's a comparative machine that tests four at once. I was having a hell of a time calibrating it (across 4 parallel voltage dividing resistances) so I added a bank of MOSFET transistors to decouple each branch from the power supply, and eliminate the pesky crosstalk from wreaking havoc on my calibration process. But, before I go into that, here's photo-documentation of the earliest revisions:

In the event it hasn't occured to you by looking at that pic, this phase was a HUGE pain in the ass to calibrate. It entailed flipping the lid (which required mental inversion of position, since everything you see there is actually mounted to the lid) and gingerly turning the screwdriver slotted into a pot that is suspended by wires. Simply locating the correct pot for display and range was bad enough, but this mock-up proved the theory enough to pave the way to revision 2:
This was a huge step forward, all calibration being routed into the base of the unit allowing for adjustments with the test fixture closed and operating. Now, being able to physically engage in the act of calibration, I could come to the conclusion that from an electrical standpoint calibration was impossible.

In a nutshell, here's what's going on: An external variable voltage is fed through 4 identical load resistances which in turn each feed a diode in a conductive position; from the legs of the DUT is a three tier resistance ladder comprised of precision metal film resistors and trimpots (my basis of calibration) for each step, a millivolt meter is fed from taps along this ladder.

My theory is that the variable internal resistance of the diodes under test was large enough to throw the four parallel loads out of balance, making calibration impossible. So, the easiest (attempt at a) fix I could come up with was to slap some buffers on the variable voltage feed and hope to eliminate or at least reduce to an immeasurable level any cross talk between the test channels.

My first attempt at this was during a beer fuelled ElectronFeed meet, in which I electrically placed the MOSFET buffers feeding the DUT directly, on the wrong side of the load. This resulted in my polarity protection 1N4001 on the 12 volt meter feed acting as a fuse.

After sorting that out the meters wouldn't hold a steady measurement. Fearing them cooked, I disentangled one from the device, only to discover it worked fine. So now, with the inclusion of the buffers in a logical manner my metering has gone to shit. I'm a rat's whisker from gutting the entire thing and starting fresh when on a whim I disconnect the meter supply from common ground. Worked like a charm. So now, since I haven't any DC to DC converters laying about, I'll dedicate three power supplies to this thing. The variable bench supply (now only a reference thanks to the buffers), a 12 volt supply for the buffers sharing ground with the variable, and a 12 volt floating supply for the meters.

This weekend I hope to plunge a larger hole in the case to support the multipin PSU buss so I can escape the web of supply wires in order to... begin calibration.

All in the name of ring modulation.