Variac on Steroids: Tektronix TU-75B

IMG_4651.JPGEvery 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. IMG_4657.JPG

There’s also a fuse holder, that appears to be a mod of some sort:IMG_4666.JPG

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.

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


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.

2 thoughts on “Variac on Steroids: Tektronix TU-75B

  1. Pingback: TU-75B Wattmeter repair | Buttons, Switches, Knobs & Lights

  2. Pingback: Tektronix 181 Time Mark Generator repair | Buttons, Switches, Knobs & Lights

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