Tektronix 514: restoration update

A bag of parts from Newark arrived and I went to work replacing capacitors in the power supply.  First, I finished re-stuffing one of the cans:
IMG_5582.JPG IMG_5585.JPG IMG_5583.JPG IMG_5584.JPG

I’m really happy with how these came out.

Than went to work replacing the old bumblebee caps – here’s a few shots before I soldered and clipped the leads.  C102 is the re-stuffed can freshly installed.



I was conflicted about just ripping out old components and replacing them with new ones, but (and I’m paraphrasing his words from a conversation I had with Kurt): Tektronix used the best components they had available to them at the time.  If they had these newer, more reliable components, they would have used them instead.  You shouldn’t feel bad as long as the restoration is done in the spirit of the original design and keeps up with their level of craftsmanship.  I’m happy with my work so far and think I’m on the right track.

I decided to power this up on the variac.  It was almost midnight, so I didn’t bother with photos or videos, but:

I started to hear the whine of the high voltage power supply around 60 VAC on the input (frequency is around 4kHZ, wonder if this is right?).  Eventually I saw a fuzzy green dot on the screen around 80VAC, that quickly drifted off.  If I turn the intensity control all the way up, I see the familiar glow of on the screen you get when the beam is deflected way off the face.  All other controls are unresponsive.

The +1,500V supply is around 1,000V, the -1,500V supply is around -800V.  I would like to get those missing 1,200V back…  The -140V supply seems to regulate albeit somewhat loosely.  The 225V, not so much, and the 225V adjust control doesn’t do anything.  I’m suspecting I have to replace the reminder of the .01μF bumblebee caps in the supply.  Maybe I should just rename this hobby “replacing old capacitors”.

It’s a start!

514 Status update

A gaggle of capacitors is en route from Newark / Element14, so until they arrive, the 514 is off the bench.  I got enough to replace all bad caps in the power supplies, including the untested HV cans, just in case.  ‘High Voltage Problems’ is a common utterance when speaking about these old beasts, so I want to be prepared to replace anything.  I’ve been staring at the HV power supply schematic for the last week, and I think I’m ready to tackle it.  I got a enough 630v film capacitors in .1, .01 & .047μF to deal with bad bumblebee caps as necessary, as well as 22μF 450v electrolitics to restuff some cans with.

The plan is to replace all the bad caps I’ve identified, replace the rest of the missing tubes (which I have), then slowly bring it up on the variac.  I’m not expecting a trace, but I would like to see a spot, which would indicate that the power supplies are somewhat working.  after that:

  1. Check of all power supplies.
  2. Check the vertical signal path
  3. Get the sweep generator working
  4. Get it to trigger
  5. A thorough cleaning & make a new graticule

Tektronix 514: When are you from?

Here’s some of my notes about trying to understand a little bit more about the 514, in particular, mine (S/N 948)

The usual questions are:

  • When were they made and for how long?
  • how many different variants were there, and which were visibly different vs which ones were just internal revisions.
  • When is mine from, and which are the correct schematics?  Usually the schematics will indicate a range of serial numbers for which a particular schematic sheet is valid for.

Some of the things I observed in the manuals:

  • the schematics in the BAMA manual for this manual aren’t just raw scans, they’ve been re-composited.  It’s evident elsewhere in the manual as well.  Generally whoever put together the PDF manual did a nice job, but it appears they cropped out the serial number info when assembling the PDF schematic pages.  d’oh.
  • There’s a few pictures in the BAMA manual:
                          SERIES “A”
      With the “serial” field on the bottom left of the unit.  Mine like this, except without the ‘SERIES “A”’ text. (no S/N shown next to “Serial” callout)
      with the serial directly under that. (S/N #5181)
  • Comparing the BAMA schematics to the paper one I bought, I can confirm that the BAMA manual schematics are for S/N 3150 – 3408.
  • There’s a note in my manual that says:
    “Major circuit changes occurred in the Type 514AD oscilloscope at S/N 3409. Numbers in parenthesis indicate the earlier values in the Type 514D oscilloscope.”
    So the ‘A’ version is anything after 3409.  OK great.
  • But, looking at the versions of the vertical amplifier schematic, there is:
    1. “Type 514 / 514D Oscilloscope” S/N 101 – 3149.
    2. “Type 514 Series A Cathode Ray Oscilloscope”, S/N 3150-3408 (same one from BAMA) dated March ’53,
    3. “Type 514 D Cathode Ray Oscilloscope” S/N 3409+, dated March ’56, which has notes about changes in S/N 3650+
  • So it seems like there’s the 514, the 514 series A, and the 514A, all available with or without the ‘D’ for delay line.  The ‘Series A’ seems like it might be the precursor to the plain-old ‘A’ suffix, common of later models, but there’s some inconsistencies in naming conventions across the versions of schematics, so it’s difficult to be certain.  These are the earliest years of the company, so it’s not surprising to see evolution in the documentation.

Observations looking through the catalogs:

  • October ’50 514D is listed for $950.
  • August ’51 514D is listed for $950.  no ‘D’ in silkscreen.
  • March ’52: 514D is listed for $950.  Underside photo shows PS caps identical to mine, but ‘D’ looks like a part of the silkscreen.
  • March ‘53: catalog says there was a 514-D, but there’s no page for it.
  • August ’54: there was -AD only
  • August ’55: gone.

The punchline:

  • Made from 1950 – 1956
  • Mine is from somewhere between ’50 & ’52.  Had to guess I’d say ’51.
  • There were a few different milestones:
    1. S/N 101 – 3149
    2. S/N 3150 – 3408
    3. S/N 3409 – at least 5181
  • some minor changes within the first few hundred are called out in the schematics.
  • They weren’t advertised after ’53, but available until at least ’56.
  • BAMA schematics are incomplete (I’ll upload mine, promise)
  • I wonder if there were examples where the delay line was added post-sale, and the ‘D’ was hand-stamped in, like mine?

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.


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

Tektronix 551: Adventures in timebase calibration

Relatively speaking, the 551 is one of the less useful scopes in my collection, as specially considering it’s size.  With a separate power supply at 42 lbs, and the mainframe 10 lbs over that, it comes in over 100 lbs once you include plug-ins.  It is the least capable of dual beam scopes in the 500 series, having only a single time-base connected to a single set of horizontal deflection plates, and a lone intensity control shared across both beams.
Nevertheless, I’m in the mood to work on a 500 series, and this was the only ‘normal’ one that’s currently the apartment; frankly I’m still too intimidated to work on the 519 or the 661.

Here it is on workbench B (aka the dining room table). On top of the power supply is my TU-75 variac which I was using to check the power supply on the 181 time mark generator (more on that later). Pardon the underwhelming photo quality, proper glamor shots are on the winter todo list.


The vertical section is the same for both beams, and seems to be working well.  The timebase and triggering are are also in good shape, but the timebase is off by 3% – 8%  across almost all of the ranges.  Here’s output from my 181 timemark generator at 1ms.


Parallax induced by the fact that the graticule is forward of the CRT surface makes it difficult to take an accurate picture, but the in the photo above, the left marker is aligned with the left graticule, and the right most marker is at the 2nd to last small tick; making this about 4% off.  Out of the factory, accuracy was within 1%, and I’d like to see about getting that restored.

The manual doesn’t have a section for calibration, it’s a separate document – both available on the Tek wiki, here. To calibrate the sweep, there’s a trim-pot on the back of the ‘Horizontal Display’ control, visible in the photo below, just behind the red test-lead.

Update: What’s written below is mostly incorrect (though I’m leaving it for historical purposes).  I was able to get the timebase into calibration by following the instructions. Before calibrating the sweep, you’re supposed to calibrate the sweep magnification.  This seems completely counter-intuitive, and I never use the mag, so I didn’t bother with this step.  This turned out to be a giant mistake.  Once I did that, I was able to get the sweep calibrated quite easily, with the exception of .1/.2/.5ms, which are running a little slow.  That points to C180A, perhaps I’ll try to replace it another time.  For now, I’m calling this instrument done.


This pot is already as far as it will go, so no luck there.  Turning it the other way increases the error.  What’s interesting is this adjustment doesn’t actually alter the rate of the sweep ramp per se, it changes the amplitude.  There are trimmer caps for some of the faster sweep rates, but there’s nothing in the ramp generator to alter global timing.

The sweep signal is generated in the miller run-up circuit, given a variable DC offset to adjust the horizontal placement of the trace on the display, and then is turned into a differential signal and amplified before being sent to the horizontal deflection plates.
So what’s wrong with mine?  10ms worth of 1ms markers are being displayed in less than 10ms of divisions (1ms per division).  That means by the time the 10th marker goes off, the beam hasn’t yet gotten to the right spot on the screen.
What do we need to do to correct this? The slope of the ramp is determined by both the RC constant of the ramp generator, and the subsequent amplification stage, so it’s possible (within reason) to adjust for a slow ramp in the amplification stage. In the photo above, that’s what I’ve done:  By putting a 500k resistor in parallel with the 100k resistor that’s in series with the trim-pot, I was able to get the sweep into cal.  Is this cheating?  It feels like it.  I haven’t soldered it in yet, I’m just using a substitution box.  At higher sweep speeds, the capacitance of the leads have a significant impact on the waveform.

Screen Shot 2015-10-23 at 1.14.44 PM

It’s easy to make erroneous conclusions if you don’t understand the effect the controls have on the measurements, or don’t heed & grok all of the notes on the schematics.  While it’s possible to get the voltages listed on the schematics, if you have to deviate too far from the stipulated settings, or adjust other controls to their extremes, there’s probably something else wrong.  For example, I could get a few measurements within spec if I altered the sweep length beyond normal or listed ranges.  In another case, I measured the length of the sweep ramp at the cathode of V173, and found it to always be about 5-10% too long.  At first I thought this was indicative of the ramp running too slow and I’d found the issue, but then I realized that the sweep length control effects this, and it was only a coincidence.

I’ve gone back and forth between suspecting that the issue is in the ramp generator or the amplifier.  Like I said, the important thing is the slope of the ramp, and that can be adjusted (within reason) in either section.  All of the timing resistors & capacitors are within 1%

Here’s a bunch of the measurements.  It’s extremely challenging to find a 5% error among readings that are 10% off.
Screen Shot 2015-10-24 at 11.23.10 AM

This is getting annoying.  I may just solder that damn resistor in place and move on with my life.  More to come.