Every bench needs a a good Variac.  Until I found this piece, my daily use variac was a 60’s era Radio Shack unit cobbled onto some ply with a JBox on the count of missing the original cord and socket.

It works fine, though only rated for 5 amps and gets a little warm with loads well below that.

The Tektronix TU-75B was apparently designed and built primarily as an in-house device to aid in final testing & adjustment of scopes on their way out the door.  I’ve yet to find documentation on it, and the only other photos I’ve seen show similarly low serial numbers, so I suspect these weren’t made in large numbers.  When I saw it on eBay, I couldn’t resist.

The simplest of Variacs just have a knob to adjust the voltage.  Some of the nicer ones also have a voltmeter and/or ammeter.  This unit has a voltmeter, a wattmeter, and selectable ballasts, in the form of 3 light bulbs (more on this later)

There were a few problems noted in the listing, and a few that weren’t.  One meter is missing it’s glass, and the other’s glass is loose.  I’ve got tape on the wattmeter’s glass face to pull it forward and keep it from interfering with the needle.  I’m going to pick up some Permetex glass sealer for the job, which is also apparently what you should use to re-seat the glass envelope of tubes & CRTs in their socket bases.  I’m also going to laser cut some acrylic to replace the missing face.

The voltmeter’s needle is a little bent up, but it seems to move OK.

The other glaring error was that the brush on the variac wasn’t making contact with the winding.  Super simple fix:  I just had to loosen the rotor and move it closer on the shaft until the brush made contact.  Here it is after the fix.

There’s also a fuse holder, that appears to be a mod of some sort:

I like the spare fuse holder.  Note that the tektronix part numbers written in.  The knob that the fuses are connected is the wattmeter multiplier, which appears to be a very low resistance rheostat.  With this mod the knob not only controls the range of the meter, but which fuse is in-line.

I’m a little fuzzy on how the transformer is wired.  Perhaps it’s set up as a series of shunts for the wattmeter?  I’ll have to draw this all out if I can’t find a schematic.

Speaking of the wattmeter,  it’s a 4 terminal device, with separate connections for voltage & amperage, which I guess makes sense; if the voltage was constant, you could get away with just measuring the current, but since the voltage is variable, you have to take both into account.  So how does this meter work?  Does it mechanically multiply the two?  A gentleman on this post confirms my suspicion:

“If this is basically an analog meter using two coils to perform the required instantaneous multiplication, then the voltage coil is connected to the needle, moves, is a high resistance, and probably has a series resistor to scale the voltage, and the fixed coil that the moving coil interacts with is the current coil. If no external current transformer is used, then there is a basic current rating of the current coil relating to the full scale power rating of the meter.

See:
“Basic Electrical Measurements”, by Melville B Stout, 1950, Prentice-Hall.
Look up Indicating instruments – electrodynamometer movement – 418 – 423, and 442 – 450.
”

Interesting…

Looks like there was an… electrical incident at some point; the leads on the power switch have felt the wrath of some poor fellow’s mistake.  The connections appear to be solid, so I may just slowly back away.   I’d hate to disturb that beautiful wire-loom.

When I snugged brush back up against the winding, the inner contacts (which is a brass pad pushing against a brass plate) make a horrible squeak.  I tried everything I could to eliminate this sound to no avail.  The squeak happens right around 90 – 110 volts, so I’ve rationalized this as an audible alarm that you’re approaching full line-voltage, which is actually quite handy.  Go Tektronix.

I’ll cover the ballasts in another post, but short answer: They’re light-bulbs that you can switch in in series to limit the current.  It’s an old trick, but nice to have it all built in one nice enclosure.

Today I finally got schooled on how to use the laser in the model shop. 

As mentioned in a previous post, I drew up the graticule for my Tektronix 519 as a DWG for this laser intro.

The basic workflow is:

1. make a DWG, using colors to represent different laser attributes (power, speed)
2. Power up the laser, and use bed Z control and spacer stick to focus
3. open the file in AutoCAD Trueview(?)
4. Print to laser driver, which opens up driver app
5. load the medium
6. assign colors to laser attributes
7. drag the print file onto the print area
8. a few other things
9. hit go

We used 1/8″ clear cast acrylic.  I don’t remember what the cutting power was, but the engraving setting was power of 8, speed of 1.  Keeping the power the same & halving the speed would result in a deeper cut.

We made two, just for kicks, and they turned out pretty well

With the paper removed

Compared to the one from my 551

Closeup

the 551 graticule stacked on top of the freshly cut 519.  Just different enough to be annoying.

Here it is, installed

Not bad for my first attempt.  I’m going to build a solid black aperture plate next.

I purchased and subsequently scanned the manual for the Waveforms 510-B Oscillator.  I’ve uploaded it up BAMA, and included it here as well.
Wavefrorms type 510 oscillator

Enjoy!

‘Boat Anchor’ is an affectionate term used to refer to old tube gear, primarily radios and test equipment.

This guy has a great site dedicated to his collection & adventures in restoration.  I didn’t see any Tektronix gear, but hey, no one’s perfect.

Speaking of boat anchors, I stumbled across this site that attempts to humorously quantify the relative beauty of these old relics:
BABE (boat anchor beauty evaluation)

My 519 is generally in immaculate condition, however it’s missing it’s original bezel.  It came with what appears to be a standard 500 series bezel, and some aluminum knurled knobs keeping it loosely attached.

I think there were two versions of the original bezel assembly, one with a camera attachment, one without.  Tim Koeth’s appears to have a the camera attachment (chronicled here), while the one I saw at Skycraft did not.  Check out this shot of his collection – he just may have me out oscilloscope’d.  Here’s the shot of the one at Skycraft:

What’s interesting about this one is that the entries for serial, sensitivity, and risetime appear to be blank.  A few things to note about the 519 and it’s CRT:

• There is no vertical amplifier.  The signal is passed directly to the distributed vertical deflection plates.
• The CRT & shield are typically paired, in part to help protect the CRT.
• The vertical sensitivity and risetime is individually characterized for each CRT at the factory, and is marked on the shield and faceplate.  They’re typically on the order of 10v per cm and 350ps, respectively.
• The sensitivity and risetime are marked on the backlit faceplate such that it’s incorporated into photographs, like this
• Because of the extreme speeds that this device operates on, the usable area of the CRT is comparatively tiny; 6cm x 2cm.  Compare that with other scopes of the time, which had an active areas of 10cm x 4cm, or 10cm x 6cm.

A little more on that last point.  In order to capture extremely fast events in real time,  you have to sweep the beam across the screen very quickly.  The quicker the beam scans across the screen, the less time the beam spends at any given point, and thus the less energy is imparted onto the phosphor.  To get enough energy on to the phosphor to make it glow visibly at these speeds requires tremendous accelerating voltages; 24,000 volts(!) in the case of this scope.  The trouble is, the more energy an electron beam has, the harder it is to move it around.  This is why the 519 ended up being a 100 lb beast, with sweep generator is driven by a forced-air cooled RF transmitter tube, a relatively dreadful vertical sensitivity, and a minuscule viewable area.   Another thing to keep in mind is that while repetitive signals keep sweeping across the CRT face, continuously refreshing the image, this was made to capture single shot events on camera (such as nuclear events), so the beam had to be bright enough to register on film on a single pass lasting less than 50 billionths of a second.

Back to my scope.  Here’s what it looks like with the bezel removed

For kicks, I tried installing the graticule from my 551.

That’s not just parallax.  The graticule illumination lamps and the notches on the plexi sheet don’t line up.

Remember, this was bespoke machine, only loosely based on the 500 series standards of the time.

Upon closer examination of the CRT shield, I noticed mine is sadly lacking the sticker that proclaims the sensitivity and risetime.

Other examples chronicled have a second sticker above this one with hand marked sensitivity & risetime.  I really don’t have the courage to pull the CRT from it’s shield to hunt for any other markings.

I went ahead and drew up a graticule that should fit:

This’ll make it’s way to the laser next week when I’m back in town. Stay tuned!

On my way home from InfoComm, I had a chance to visit what I would highly recommend as a pilgrimage to any geek,  Skycraft Surplus.

I didn’t snap too many photos, since I thought it seemed rude, but here’s a few:

Would you believe a Tektronix 519 was staring me down as I walked in the door?

Sadly, not for sale (on second thought, maybe that’s for the best).  Serial # 000729.

Who wouldn’t want one of these?!?

Isles and isles of hardware, fasteners, fittings, switches, passive & active components.  Drawers full of any little bolt, nut, washer, or standoff you can imagine, in a variety of materials.

I need one of these for my Meth Lab.

Don’t forget to look up

Somehow, I walked out of here spending less than $50, probably because none of these would fit in my carry-on.

Need wire?

This is about a third of their stock.  I ended up picking up a few 50′ coils of some nice silicone jacketed hook-up wire, great for patch-cords.

And finally, I snapped a few close-ups of the 519’s bezel, since mine doesn’t have one, and I’m attempting to recreate it

More on that effort in another post.

The idea of designing a programmable, high voltage power supply came to me after hobbling together all of the gak that I needed for reforming old electrolytic capacitors.  There are a few good examples circuits out there for home-built reformers,  but none of them were exactly what I was looking for.  I’m into sure how digital or programmable I want this to be, but my thought was that if there was at least some logging capabilities, I could easily plot curves of the process and maybe tease out some interesting trends.

Without thinking too much, I’m going to spitball a list of possible features:

• 0 – 600v DC
• 10-20mA max, with current limiting.  20mA is probably a lot for reforming, but maybe this would also see use as a general lab supply
• Current & voltage meters
• Logging current, voltage setting, and voltage across terminals, with timestamps (rtc needed?)
• Discharge resistor (variable ?)
• Ability to set a maximum voltage setting, and automatic incrementing of voltage when current drops below a threshold
• Voltage regulation?  This probably won’t have the effect I’ll usually want when reforming (i.e. I wouldn’t want the supply chase to match a target level),  so perhaps it wants to be disable-able,  if that’s even possible.

Some questions:

• Form factor – rack mount gear is cool,  but often impractical 
• UI – keypad entry? Menus? 
• Meters?  Ideally analog, if possible, with switchable ranges.
  
• Tube or solid state?

Research:

Heathkit PS-3 Power supply schematic – I feel like there may actually be one of these up at the “upstate home for wayward oscilloscopes” (aka, Mom’s basement)  as a side note, I also have an unbuilt Heathkit PS-4 waiting for a nice rainy day (or my retirement) </brag>

This guy built a beast of a high voltage power supply

Repair log of a Heathkit IP-17
Antiqueradios thread on a Heathkit IP-17 repair

Side note – I’m trying to sort out the difference between blog posts, pages(or posts) about actual projects, and pages(or posts) about specific pieces of equipment, so there will likely be some organizational turmoil over the coming months.

To be continued….