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.



7D01 Teardown



As I’ve previously mentioned, I’m in possession of no less then three, complete, working 7D01+ Display Formatter pairs (2 DF01s & 1DF02).  I got my original one working, keeping one as a maintenance spare / parts unit, and tearing down & stripping the final one.

It did take a moment of inward reflection before I decided to sacrifice a Tektronix product, but I decided:

  • It’s not particularly rare;  You’ll always see one or two on eBay.  Though they’re asking price is usually upwards of $200 (or sometimes more, which is just silly), they really only sell for about $50 – $100.  They were made from 1977 to 1985, though I don’t know how many were actually made.
  • It’s been an interesting circuit to get to know, and further poking will be educational.
  • It’s a trove of useful parts, including a beautiful ceramic & gold MC6800 CPU in the DF01, 20+ really nice toggle switches, tons of discrete logic, a dual concentric encoder, and some dual concentric pots.  I’ve already made a rudimentary 1Mhz clock circuit using some of it’s parts.
  • It’ll be fun, and I’ll take some pictures for here and the wiki.
  • Did I mention I have three of them?

The front panel circuit board is called out as the recognizer & trigger selector.  It also has the differential line receivers (Motorola MC10216) which are the input buffers for the P6451 probes.  The probes connect via 2 25-pin micro sub-D connectors.



Tektronix 564 CRT Replacement

I figured it was time to break this off into it’s own post.  After some initial troubleshooting, I’d determined that the CRT was the source of the intensity and focus flickering I was experiencing.
I ordered a replacement I found on eBay, and it arrived yesterday, safely and extremely well packed:  Here it is after unbubblewrapping:

I proceeded to carefully disconnect and remove the old CRT, leaving a gaping hole in the business end of the scope:
Note the electrode connectors that press against pads on the neck of the CRT.  in front of that is the trace rotation coil, wrapped around a black plastic bobbin.

I compared the two to make sure they were in fact identical.  Spoiler alert:  They’re not.  The part# is the same, but there are other markings that differ.

OK though, same part number, we should be good, right?  Hmm.. one of these things is not like the other: DSC_0236
There are 6 pads around the old CRT, and 6 corresponding contacts on the scope.  The new screen is missing the 2nd (or 5th?) pad.  Additionally, it looks like there’s a conductive trace coming out of the left of the 2nd pad on the old CRT that’s attached to the 3rd trace on the new one.  I needed to do a little more investigation before I was comfortable just shoving in the new screen and hoping for the best.

These connections are all a part of the storage circuitry, as seen in the schematic below.


After a bit of cold metering in the scope, I was able to determine which connections were which on the CRT, and I marked them with a grease pencil:

So pins 1 & 6 (counting counterclockwise from the bottom in the scope) are STR1 & STR2, the upper and lower storage targets, while 2 – 5 are CE1 – CE5, the collimation electrodes.  I’ve marked the path of the conductive traces so they’re more visible,  you can see that they each terminate at the connection to their respective grids.

So the new replacement is just missing the last collimation electrode.  That doesn’t seem like such a big deal.  Maybe they realized it just wasn’t needed it in subsequent revisions?   The original has the ‘T5640-200’ in it’s markings, which matches the description in the manual from BAMA (©1964), but the replacement does not.  The replacement has what appears to be a lower serial number, but I’d be surprised if it was the older one;  It seems more likely that they removed an electrode in later models than leaving the space and adding one later.  Build techniques also differ.  The traces and connections on the replacement are finer and cleaner, but I don’t know if that equates to a earlier, smaller, more hand-built batch, or a later, more refined manufacturing process.  Any insight would be greatly appreciated.


Feeling confident enough, I carefully installed the new CRT and brought it up slowly on the variac.  I got a beautifully sharp trace that didn’t flinch, even with gentle taps on the tube:

As you can see, intensified mode works great.

One of my other recent purchases was a Heathkit TT-1 Tube tester.  I started out the previous version diagnosing a smoking resistor in the storage section power supply.  Now I had a way to check the tube.  Both sides of the 12AT7 checked out OK on mutual conductance and shorts, but one side failed the grid current test (exhibiting lots of it).  AH HA!!

I suspect the grid current was causing excessive draw on the 475v supply, which is why the resistor was overheating.  I replaced the tube and slowly brought it up on the variac –  low and behold the resistor remained cool.
Wait a minute,  a 12AT7?  The schematic (and nicely silkscreened chassis) calls for a 12AU7.  Huh,  I didn’t realize that upon first inspection.  The 12AT7 has a much higher gain then the 12AU7, and I wonder if that led to the premature failure of the tube.  It now has the correct tube.

Next on the hitlist:

  • Documenting the storage functionality.  It sort of works, but needs some attention
  • Understanding the grid current test on the TT-1
  • Sourcing a replacement handle.  I actually don’t mind going off-brand here, and would dig a custom replacement as long as it made use of the original mounting hardware.



Tektronix 564


Picked this up as a part of a larger lot, and was the first I attempted to power up.  I did so stupidly, without a Variac.  Quickly shut down after smelling smoke from R646, a 100Ω 1W resistor that’s a part of the unregulated 475v line.  Metered it after the incident, and it appears to be OK.

Yanked the plugins and grabbed my newly re-wired Variac.  as I rounded 50v, things started to spring to life – around 80v I got a spot on the CRT, and R646 was HOT.  Shut ‘er down.

I think I’m going to yank all the tubes & transistors in the power supply, and check each supply one at a time, starting with the -100v supply.  We start with the -100v because as with most Tek scopes of the era,  the -100v (or -150v) supply was the one that all other supplies were referenced from.  We’ll start with -100v, then +125v, then +300v.  While the 475v supply is unregulated, it comes into close contact with some of the other supplies around the storage circuitry, which is also on the list of suspects.  Just for a chuckle, remember all of these are considered ‘low voltage’ supplies 😉

Pulled the following:
V667     6AS7     +300/150v supply
V674     6AU6     +300v supply
V654     6AU6     +150v supply
V800     6CZ5     -3kv supply
V814     12BH7    -3kv supply
V913     12AU7     storage

With only the tubes in the -100v supply installed, I get -150v on the rail when line voltage is brought up to 115v.  There’s about 90v across the gas regular tube instead of 80v.  I’m not sure if this is because the 125v supply isn’t up?  Furthermore, what’s with the pin 6 connection on the regulator tube?

With V667, V674 & V654 reinstalled, I can get the -100v power supply dialed into -100.0v.

Attempted to dial in the +125v supply but it jumps between 123.7v & 126v as I adjust the pot. 125.9v was as close as I could get it.  Getting the +300v rail dialed in was no problem, same with the -12.2v supply.

420v unregulated reads 427v, 475v unregulated reads 485v with 115v line voltage.

Now I’m suspecting the problem is in the storage circuitry, since that appears to be the only thing powered from the 475v supply.  As a side note, are these tin whiskers?


OK, with V800 & V814 installed, I get a spot on the CRT!  Focus & astig controls change the spot shape, and location on the screen.  When I get it as sharp as I can, the intensity control also makes the beam move around.  Lets try a horizontal plugin.


SUCCESS!  Lets try the vertical plug-in.


It needs a cal and cleaning, but all 4 traces seem to work somewhat.  Even the delayed sweep seems to work:DSC_0015

So the trouble is in the storage circuitry.

After running for a while, the intensity started to jump around, became very bright, and the intensity control no longer functioned.  Stay tuned for more…

10/12 UPDATE

it’s general intermittent intensity issues, stemming somewhere from the HV supply.

C830 & C832 measured around .0026u which correlates to the parts list, but not the schematic, which calls them out at .0068u

R842 ((2)2.7M+(2)3.3=12M) Measured 13M cold, measured open after about 15min operation, then slowly dropped the meter picked it up around 50M, and it’s almost all the way back down.  Some metering in the early minutes after shutdown leads me to suspect one of the 3.3M resistors.

there’s a strong mechanical component to the problem; lightly pressing on the HV portion of the central chassis would drastically affect the intensity of the trace.

metering across the two neon bulbs I’d see anywhere between 50v & 120v that’s directly affected by the intensity control.  In one flicker scenario, the intensity wavers but this voltage remains consistent.  After being warmed up for about 20 min, the flickering would affect this voltage, causing the neon lights to fire

11/1 UPDATE:
I was wrong about the resistors, they read OK when immediately lifted from the circuit after power-down.
I’ve measured the HV supplies, and they seem consistent during the focus & intensity flickering, though measuring them does affect the trace (which I was surprised would happen with a 75MΩ probe impedance).
Screen voltage of V800 is around 65v for a normal trace at .2ms and can jump as high as 90v during a full bloom event.

As per the recommendation of Albert on the forum, I checked the V800 screen w/o the CRT connected, and it was around 65v.
Gently tapping on the chassis above the HV terminal strip does accentuate the issue, but I can’t find a component (either tube or passive) that responds specifically to some more directed tapping. I’ve re-seated the CRT socket, but it seems those wires are particularly sensitive to giggling. I opened up the the socket, and the pins look good.

It turns out tapping directly on the CRT shield or neck causes the biggest change in intensity, so now I’m fearing it’s a mechanical problem with the CRT.  I can wiggle the CRT or leads to get a good trace, but it quickly goes back to incessant flickering.  This is getting really frusturating.  I think the only fix might be:
1) Sourcing a new CRT
2) Hitting it with a brick until I feel better

UPDATE:  There’s a replacement CRT en route, part # 154-0410-00  (P31 phosphor, same as mine).
CRT replacement chronicled here