Tektronix 181 Time Mark Generator repair

I’d been in need of a Time Mark Generator for a while, and finally found one that was the right mix of price and form-factor.  With a few exceptions, I’m loathe to spend over $100 on a piece of old test gear, and the only units under that were either RM500 series units which I’ve been avoiding, or the 180 which is huge.  Speaking of the 180, Richard Sears has a nice write-up on both the 180 & 181.


This is clearly an early unit, based on the 3 digit serial number, and the style of the enclosure.  Later models have the familiar textured aluminum side panels and rounded corners, while this bears a resemblance to the earliest of Textronix scopes like the 511, 514, etc.. I think this is also the first piece of tek gear I own thats the ‘wrong’ color.

After replacing the fuse holder, which required some gentle modification of the chassis (hooray drill press), I brought it up on the TU-75 variac, and got a trace.  After being on for 20 minutes or so, the slower markers became unstable and impossible to calibrate, and it would get worse with time.

I checked the power supply and found that the -150V supply was low, at around -139V (or, um, high, absolutely speaking).  The thing is, it would regulate this incorrect voltage like a champ.  Bringing it up on a variac, that -139V was stable from ~80V up to my uncomfortably hot line voltage of 126V.  But, over 20 minutes or so, that would slowly fall to -137V, which is where things would get unstable.

During our semi-annual encounter where he enables my oscilloscope addiction, Kurt suggested I suspect the caps around the feedback loop of the regulator, and he was dead right.  Those .01uF bumblebees were leaky over 50v.  They’re still good below that, so I could probably sell them to some audiot, insisting that the leakage is a part of their distinct warmth and tone.

Screen Shot 2015-11-14 at 7.42.43 PM

Lifted one side to testIMG_5171.JPG

Their tiny replacements

And here’s the trace on my 453IMG_5105.JPG

This is the highest output, 10Mhz.  It’s a frequency multiplier, and I think that modulation is fixable in calibration.


Here’s a pic of with the ‘check count’ switch enabled, showing all of the pulses.  If you notice, there’s 9 small pulses in between every major pulse, not counting the ‘half pulse’ right before the next major pulse.  That means the divide by 10 multi-vibrator is working properly.  This is on my Tek 551 before calibration, which is why it appears to be running fast.



Here’s the 100µS output on my 7D20.  Spot on.


A quick overview of how it works:

At the heart of the unit is crystal controlled oscillator.  Mine has the oven controlled option, which means there’s actually a little container with the crystal, a heater, and a bimetallic thermostat to keep the constant temperature, which minimizes frequency drift.

Here’s a few shots of the crystal oven:


With the outer shell removedIMG_5097.JPG

With the top off, revealing the 1Mhz crystal.  IMG_5098.JPG

After the oscillator, and the 1Mhz pulse shaper, the signal is applied to a series of cascaded 10:1 frequency dividers, which give the 10µS – 10mS pulse outputs.  In more modern times, you would do this by counting the pulses, letting every 10th through.  These are actually re-triggerable multi-vibrators, with a hold-off time adjusted to only let every 10th pulse through, so it’s not actually counting, it’s waiting.  You can see this when you adjust the trim settings on the front panel.  If it’s incorrectly set, it will hold off for too few or too many pulses, so for example on the 10µS output, you’ll end up with 9µS or 11µS pulse spacing.  Using that ‘check count’ switch, this is easy to observe.

More info and pictures on the wiki