Tektronix 620 XY monitor repair

This is a wholly unremarkable early 80’s XY CRT monitor.  It’s like, the least interesting thing in my collection, but I’m taking a workshop in vector graphics next weekend, so I figured I’d pull this out of the ‘upstate home for wayward oscilloscopes’ and see if it worked.


It did not.  I got a full intensity spot on the center of the screen, with no ability to move it around or adjust it’s brightness.  I bought the manual from Artek Manuals and off I went.

I actually spent way too much time on this repair; learning how the power supply worked, re-drawing schematics, and making detailed measurements.  I’ll spare you all of that.  It was a bunch of tantalum capacitors.  In gear of this era, it’s a very common mode of failure.  They usually fail shorted, so they’re easy to spot with a resistance measurement in situ.  A shorted capacitor can mean bad news, and tantalums have a reputation for failing violently, spewing their gooey capacitance all over the innards of your gear.  Fortunately, this is Tektronix, so the power supply has a few different layers of protection to minimize the collateral damage caused by single part failure.  The problem manifested as a low voltage coming from the 15v supply.  Under no load, this would float around 19v, but with a modest load, of 100Ω or so, it would happily regulate. The amplifier board had some shorted tantalum capacitors (C397, C398, C401, C402), which brought the load way down to a few ohms.  Fortunately, there’s a clever little current regulator in power supply that drops the voltage if there’s a near-short like this.

Replacing these got the 15v supply working, but uncovered/caused some issues in components elsewhere, notably caps in the -20V & -70V unregulated supplies.  I also inadvertently roached one of the capacitors while testing it, and a probe slip took out a diode.  It was… not my proudest moment.  There’s also a few fuses that saved my stupidity from making even more of a mess.

Here’s some shots of the re-worked area:

With C142 pulledIMG_5267.JPG


Replaced C142, CR142, CR143, C143.  IMG_5268.JPG


I brought it up on the variac and was rewarded with a controllable trace that responded well to inputs, waiting for some vector graphic goodness.  I smell a clock project in my future.

I will say, I’m used to working on the giant old tube gear, which is simply a joy to service.  This on the other hand, was not Tektronix’s brightest moment in industrial design.  Getting this thing apart felt needlessly belabored.  Just getting the boards disconnected from eachother was a giant hassle because of how one of the cables was run and how a shield just overlapped the board enough to get in it’s way.

That’s the power supply on the top, the amplifier board on the bottom, and the connector in the center of the image.  There’s a line voltage cable pinched between the amp board, and a transformer shield, which I had to loosen to get the two apart. To get the amp board out, you had to remove the whole back, which of course the CRT was attached to.

Also, no pilot light?  This was by far my least favorite piece of tek gear to work on thus far.  I’m just glad I got it working and off my bench.

Here’s the compulsory Lissajous pattern.  There’s some DC offset, but I think that’s the Waveforms audio oscillator.