04 November 2012

Scope Sunday 41

This post is Part Two of my California trip report. Last week I wrote about shopping at Excess Solutions, HSC, and the Pacificon Swap Meet. This week, I finish the story with my experiences at WeirdStuff.

In short, disappointing. I was last at WeirdStuff back in January 2012, where I saw a Tektronix 502 for $160 and a broken 7904 without plug-ins for $250. Both of these scopes were still in stock, at the same prices. The 502 is now missing four knobs.

The broken 7904 ("No power") is still on the shelf as well. I admit that I don't know anything about running a surplus store like WeirdStuff, but if your inventory isn't selling (and it's been a year for the 502, and at least 18 months for the 7904), shouldn't you lower your prices? Wouldn't inventory turnover be more important than waiting for that one buyer who's going to pay top dollar? Especially if people are stealing the knobs in the meantime?

A few other over-priced items also caught my eye. There was a Type D plug-in unit, missing all of its knobs and tubes, for $10. (I might pay $10 for a complete one, but then again, I already have a few of them.)

There was a Tektronix 529 TV Waveform Monitor (also missing a few knobs) for $100. Way too much (the $30 price tag was for the cart, not the monitor).

Seriously, though, I hope that they catch the degenerate low-life that's stealing all the knobs off these scopes and pull out his fingernails. Slowly.

I'll close here with a last picture from the Pacificon Swap Meet. Instead of paying $250 for a broken 7904 without plug-ins at WeirdStuff, you could have bought a working 7904 with plug-ins and cart at the Swap Meet for $300. I still think this price is too much, but only by a factor of two, instead of a factor of ten!

28 October 2012

Scope Sunday 40

Two weeks ago (second weekend in October), I was in California, and I scheduled some time to do some junk shopping. I stopped by some of the surplus stores that Jim introduced me to, including Excess Solutions in Milpitas, Weirdstuff in Sunnyvale, and HSC in Santa Clara.

In addition to some surplus junk, Excess Solutions has rows and rows of components. I bought some of the capacitors that I used in my Capacitor Quiz last week.

I also stopped at HSC Electronic Supply, where they have a new banner sign with an unnecessary apostrophe.

They did have some interesting Tektronix hardware, but most of it was too rich for my blood, like this Type G plug-in unit, which was missing all its knobs and tubes, priced at $25.

However, this 3S1 dual-trace sampler was just too good to resist. It seems to be complete, but there were at least two broken components on the B channel (the vertical-position pot and a trimmer capacitor near the sampling bridge, C433). I don't actually have any 560-series mainframes (well, I have one), but I couldn't pass it up. Due to the damage, I talked the manager down to $25, and packed it into my carry-on bag for the flight home.

Saturday morning (being the second Saturday of the month) should have been the Electronics Flea Market at De Anza, but it was canceled due to a conflict with Pacificon. However, there was a swap meet scheduled for Saturday morning at Pacificon, so I went to that. It was hopping well before dawn (unfortunately I didn't have a flashlight). Here's a picture I took while it was still pitch black out.

There was some nice Tektronix gear there, including a rack-mount 545B, a nice 465B, some 7000 gear, and this 491 spectrum analyzer.

Of course, The Fates taunted me. Having just bought the 3S1 last night, I was tempted by a whole lot of 560-series equipment, including three 561 mainframes, 17 assorted plug-in units, and a cart. Of course, there was no way to get it home, or arrange to get it home (my plane departed a few hours later, early that afternoon), and I am confident that I can find these scopes closer to home. Still, funny.

Eagle-eyed observers will notice a pristine copy of Stan Griffiths' book, Oscilloscopes: Selecting and Restoring a Classic, on top of the 561B. Unfortunately, the seller would not sell it alone; it was part of the whole giant 560-series package (which was wise). You can buy a scan of it from the VintageTEK museum store. I hope somebody bought the whole lot of scopes (I probably would have, if it wasn't 3000 miles from home).

The trip home was uneventful. I carried the 3S1 sampler in my carry-on briefcase, but I had no problems at the airport. The TSA agents did test it for explosive residue, but they didn't even ask me what it was. Having breezed through security, I really regret not buying the 465B scope that I saw.

26 October 2012

EE Prototyping 4

Design of the "EE Prototyping" course moves steadily forward.  Unfortunately, there hasn't been a lot to blog about because the topics we've been discussing have been mostly formalities:
  1. How often should the course meet? Twice a week? Three times a week? What is the balance between lecture time and laboratory time for this course? Should we use afternoon lectures and evening labs (like some other courses at Olin)? How long should the class/lab sessions be?
  2. Should the course satisfy "Graduation Requirement A" or "Graduation Requirement B" or both? What are the necessary attributes of a course that satisfies "Graduation Requirement B"?
  3. What topics should be covered in lecture?
  4. How should the reading assignments be ordered and structured?
Some of these issues are settled, and some are still in flux.  We did come to a good, workable decision on the class schedule, inspired by other lab classes and the sophomore design course.  One of the standard time slots at Olin is twice-a-week at 3:20pm to 5:00pm, but some courses use an extended slot of 3:20pm to 6:00pm. This course will be scheduled for the extended slot.  On lecture days, we'll wrap up by 5pm, on laboratory days, the students can stay until 6pm if they need to. Several times during the term, we'll have in-class team design reviews, and we'll use the full period those weeks.

We've also been brainstorming about lecture topics for a few weeks.  Some of the suggested topics include:
  1. Schematic dos and don'ts
  2. Grounding: analog versus digital, ground planes, stars, and the chassis
  3. What's so special about 50 ohms?
  4. Noise and non-noise
  5. Proper use of oscilloscopes and probes
  6. Op-amp applications and nonidealities
  7. Care and feeding of A/D converters
  8. The wonderful (horrible) world of capacitors
  9. Heat sinks and thermal problems
  10. Power supply design
There are many other topics that have been suggested. This list just includes the "bite-sized" ones.  Some of the other suggestions could be entire courses in their own right (like analog filter design, phase-lock loops, power converters, and motor drivers). We plan to start narrowing this list down to a reasonable syllabus in the next few weeks.

The seminar did work on a little class project that I'll talk about next week, but here's a hint:

The reading assignment last week was two chapters in Jim Williams' 1995 book: Chapter 1, "The importance of fixing" (that's another hint!) and Chapter 17, "There's no place like home".  This week, the reading assignment is to start reading Bob Pease's book, Troubleshooting Analog Circuits.

17 October 2012

Capacitor Quiz

Pop quiz!

Examine the markings on the following capacitors and determine their values. To assist you, I've captioned each capacitor with a copy of its text. Click on the picture to get a larger version.

Hint: Here's a link to an explanation of capacitor markings on Wikipedia. Hint hint: You'd probably have better luck just guessing.

The answers are below, contained in the first comment.  No peeking!

15 October 2012

Scope Sunday 39

Currently I am teaching an industry seminar on analog-to-digital converter technology. I was doing a little "pre-assignment" reading (reading text before assigning it to my students) of Chapter 5 in the Analog Devices Data Conversion Handbook. Normally, I wouldn't draw attention to an error in another author's work (I have plenty of my own errors to worry about!), but this one was particularly egregious. On page 5.15 I came across the following discussion of measuring settling time and oscilloscope overdrive:
Modern digital storage scopes (DSOs) and digital phosphor scopes (DPOs) are popular and offer an excellent solution for performing settling time measurements as well as many other waveform analysis functions... These scopes offer real-time sampling rates of several GHz and are much less sensitive to overdrive than older analog scopes or traditional sampling scopes. [pg. 5.15]
Unfortunately, this statement is backwards. Traditional sampling scopes are virtually immune to overdrive (see part three of my series "Vintage scopes are better"), but modern DSOs and DPOs can be just as sensitive to overdrive as older analog scopes.

The text continues:
From a historical perspective, older analog oscilloscopes were sensitive to overdrive and could not be used to make accurate step function settling time without adding additional circuitry. Quite a bit of work was done during the 1980s on circuits to cancel out portions of the step function using Schottky diodes, current sources, etc. [pp. 5.15-16]
This statement seems to be an indictment of Jim's careful settling-time-measurement work in application notes such as App Note 74, but the objection is erroneous. The authors qualify their statement in the first sentence of the following paragraph:
Even with modern DSOs and DPOs, overdrive should still be checked by changing the scope sensitivity by a known factor and making sure that all portions of the waveform change proportionally. Measuring the mid-scale settling time can also subject the scope to considerable overdrive if there is a large glitch. The sensitivity of the scope should be sufficient to measure the desired error band. A sensitivity of 1-mV/division allows the measurement of a 0.25-mV error band if care is taken (one major vertical division is usually divided into five smaller ones, corresponding to 0.2 mV/small division). [pg. 5.16]
The first sentence is good advice (ALWAYS verify your measurement chain!), but the last sentence is terrible advice. You cannot measure the fine settling time of a DAC by simply cranking up the vertical sensitivity. It doesn't matter if your oscilloscope is vintage analog or a modern DSO, if ANY part of the waveform is off the screen, you can't trust the results.

Here's a quick example, using the same TDS3012B (and the same four-volt square-wave oscillator) from my post on aliasing last month. The falling edge of the square wave exhibits a little undershoot. At 500 mV/div, the undershoot appears to be about 800 mV (at the bottom of the screen). Note that some of the four-volt waveform is off the top of the screen.

However, if we move the trace up to the top of the screen, now the undershoot appears to be about 1400 mV (note that the vertical scale is still 500 mV/div).

The only change between these two screenshots is a small rotation of the vertical position knob. Clearly, oscilloscope overdrive is a concern, even in a modern DSO.

03 October 2012

EE Prototyping 3

There are several major projects that we're considering for the course.  Two of my favorite ideas involve taking stuff apart.  The first idea is just to collect a bunch of random electronic products and disassemble them to explore a variety of construction methods.
  • Printers and scanners
  • WiFi routers and ethernet switches
  • Cable TV boxes
  • AM/FM radio receivers
  • Laptops and cell phones (modern and ancient)
  • Heavy-duty adjustable power supplies
  • Function generators, oscilloscopes, and other lab equipments
The second idea is a focused effort in reverse engineering. The class will be split up into an even number of teams.  Each team gets a product that they have to reverse engineer and create a complete documentation package for, including schematic, bill of materials, and maybe even a working simulation (but no board layout and no pictures). Then, each team trades their documentation package with another team, and they have to build a working copy of the object just from the documentation (without ever seeing the other team's original).

My thought is to use cheapo guitar pedals for this project. The circuits are relatively simple, most use commodity parts, and they're fun.  Plus, when you're troubleshooting an audio project, in addition to looking at the waveforms on an oscilloscope, you can listen to it (and tell that something is wrong from the sound it makes or doesn't make). To get an idea of the complexity of this project, I went to my local Music Go Round shop, and bought all the pedals that I could find for less than $10.  Here they are, the lambs for the slaughter (with a couple others... the Boss DS-1 and Joyo JF-06 were a little more than $10).

I had the students in the seminar take them all apart and look at the circuit boards and the chips used. The Danelectro BLT Slap Echo has two boards, a switch board and a sound board.  The sound board has a PT2399 echo chip on it, which is no surprise.

The Behringer CS400 Compressor/Sustainer uses a lot of tiny surface-mount parts.

The Ibanez PL5 Powerlead uses all through-hole components on a single-sided board.

The Danelectro D-6 Fab Flange also has two boards inside.  The switch and amplifier board has some CMOS 4053 switches and some TL072 dual op amps on it.

The sound board uses a pair of BL3207 bucket-brigade chip (with a BL3102 clock driver chip)

The sound board in the Danelectro D-2 Fab Overdrive just uses a TL072 dual op amp and some passive components.

I think these will make great projects.

The reading assignment this week is Sections 9.5, 9.6, and 9.8 (Hardware Design Techniques: Thermal Management, EMI/RFI Considerations, and Breadboard and Prototyping) of the Analog Devices Data Conversion Handbook.

30 September 2012

Vintage scopes are better part 5

Vintage scopes are better. Back in February, I did a four-part series on why vintage oscilloscopes are better that their modern counterparts (see part one, part two, part three, and part four).  Here's another reason that recently bit me.

Reason number 5: Aliasing.

Another reason that digital scopes are inferior to analog scopes is the inherent aliasing that occurs in any sampled-data system.  If the time base is set incorrectly (very incorrectly), aliasing can cause significant confusion and can mislead a novice user.

I have heard people say "Modern digital oscilloscope do not have a problem with aliasing."  This statement is demonstrably false. Here is an example of a Tektronix TDS3012B scope looking at a 8 MHz crystal oscillator.  (Embarrassingly, this example occurred in front of a room full of students. Although I realized what was happening, and quickly fixed it, it still angered and disappointed me.)

Here is the waveform shown at 40 milliseconds per division.  The period appears to be about 32 ms, but the scope is in roll ("strip chart") mode, so I can't get a stationary trace on the screen.  The frequency measurement reports about 31 Hz, which matches the period observed.

The waveform at 4 milliseconds per division. The period still appears to be about 32 ms (eight divisions for the brightest trace), but I can't seem to get a stable trigger. Despite the fact that the oscilloscope says that it's triggering, the waveform is rolling around the screen with several ghost images. The frequency measurement reports about 31 Hz, which still matches the period observed.

The waveform at 400 microseconds per division. Now the waveform looks like a smudge, and the frequency measurement reports 500 kHz, which doesn't make sense. The "Low resolution" label is the first indication that the scope thinks that there might be something wrong.

The waveform at 40 microseconds per division. The waveform still looks like a smudge, but the frequency measurement now reports 8.25 MHz, which is in the right ballpark. At least the smudge is uniform across the screen, which implies the time base is much too slow. The "Low resolution" label is still present.

The waveform at 4 microseconds per division. Now we can see many, many rising and falling transitions, and we know that the time base is set too slow.  The frequency measurement is 8.015 MHz, which is much better.

The waveform at 400 nanoseconds per division. Finally, we get a clear view of the actual waveform, and a frequency measurement accurate to three significant digits.

The waveform at 40 nanoseconds per division. At last, the correct time-base setting. Now we can see the whole waveform, at the right speed, and we get an accurate frequency measurement.

Go back and look at the first two pictures again. Imagine yourself as an undergraduate student, racing to complete a laboratory project by the end of the class period. You don't have a lot of experience with oscilloscopes, and you're just trying to get a frequency measurement. You can see the period on the screen, the automatic frequency measurement matches, and there is no other indication that the scope is lying to you. Would you take the time to get a stable fixed trigger?  Or would you just press the STOP button, make the measurement, and go on the the next step?

I admit that this problem is mostly due to user error (when looking at a new waveform, start at the fastest time scale and gradually slow down the horizontal sweep until you see the signal). However, the conclusion is sound: modern digital scopes DO have a problem with aliasing, whereas vintage analog (non-sampling, of course) scopes do not.

26 September 2012

EE Prototyping 2

Today was the Course Fair, where the professors hold a "trade show", of sorts, and advertise their classes for the Spring term.  I brought along some show-and-tell items to promote the EE Prototyping class.  First off, the following description for the course catalog (written with help from the seminar) was submitted:
Through a series of projects, we will learn to design, build, and debug electronic prototype systems. We will cover multiple aspects of the prototyping process, including circuit and system design, soldering, deadbugging, troubleshooting, component selection, schematic capture, printed-circuit board (PCB) layout, PCB fabrication, PCB assembly, and thermal analysis. We will discuss the tradeoffs among "faster, better, cheaper", and explore examples in the realms of analog, digital, RF, and power. In addition to hands-on reverse engineering and fabrication experience, students will learn technical communication through design documentation.
Secondly, I brought a number of boards to demonstrate the right ways and wrong ways to build circuits (stealing some ideas from Appendix F in App Note 47). First the wrong ways: solderless breadboards and wire wrap (just say no!):

And now the right ways: dead bug on copper clad, as preferred by Jim Williams (described in detail in App Note 47; the board shown is actually a simple oscillator that Jim built for me); and, of course, the good life, a custom PCB (like this Analog Devices eval board):

I also brought along some other examples of interesting PCB boards. Exhibit A: The low-cost circuit board from a floppy disk drive (a single-sided PCB, that includes a "square-wave" trace around the motor for the position encoder).

Exhibit B: The controller board from an inkjet printer, which includes a wide variety of IC packages such as the socketed DIP, several SOICs, and that EPSON ASIC in the middle with a million pins on tiny spacing.

Exhibit C: A ruggedized power supply (you can tell it's rugged from all the epoxy holding the parts in place).

And finally, Exhibit D: The PCB inside a Spectral Synthesis ADDA2218, an 18-bit analog-to-digital audio converter, which uses colored FR4 (the color isn't painted on, it's impregnated in the fiberglass). I thought this was a neat touch, for a circuit board that only one in a thousand customers would ever see (only those willing to void their warranty!).

The reading assignment this week for the seminar was Sections 9.1 and 9.2 (Hardware Design Techniques: Passive Components and PC Board Design Issues) of the Analog Devices Data Conversion Handbook.

Next week, I'll talk about some of the proposed class projects.

24 September 2012

Scope Sunday 38

I forgot to mention: I attended the MIT Flea Market last weekend, but I didn't buy anything oscilloscope related. (I almost bought a Tektronix 7904 with a bad focus problem, but somebody else beat me to it. That's OK; I didn't really need another 7904 project.) I did, however, happen to wander through the Stata Center, which stands on the former site of MIT Building 20.  On one wall in the lobby, there is a display of photographs of famous people from Building 20, including this one of Norbert Wiener with a Tektronix 541 scope in the background.

I also took my kids to a small community museum last weekend, where they had a large model train layout (my son lost his mind). In one corner of the museum they had the following sign.

Of course, I left a post-it note that said "vintage Tektronix oscilloscopes". Unfortunately, the display case for these "member collections" was only the size of a small bookcase (about 30 inches square and 8 inches deep), and it was currently filled with porcelain cat figurines... but I did ponder for a minute, "Wouldn't it be cool to put on a temporary museum display of my scope collection?"  I wonder if there are any local museums that would be interested in that?

22 September 2012

EE Prototyping 1

I'm working on a new project-based class this fall, and I wish I could get Jim's advice. I think he would love it...

Here's the background: Olin College has a popular course called "Introduction to Mechanical Prototyping":
Introduction to Mechanical Prototyping is an elective course with no prerequisites that can be taken by any interested Olin student. This course is all about learning to build things. Through a series of accelerated design projects, we will learn to design, build, and debug mechanical systems. We will cover multiple different fabrication approaches including sheet metal, 3D printing, molding rigid and compliant polymers, and long fiber-reinforced plastics. In addition to hands-on fabrication experience, students will learn and master the technical communication of mechanical design through design reports and professional engineering drawing practices.
Students also get hands-on experience with reverse engineering and modelling, as well as practical CAD and fabrication techniques. The resulting projects are really cool (see the web site for more).

This course has caused some long-simmering jealousy among the EE students. "We want something like that, except for us", with requisite pointing, was the demand. Several students asked me (and other faculty members), "Can you teach a class on EE prototyping next Spring?" Well, sure. But what does "EE prototyping" mean? Circuit design? Breadboarding? Soldering? Troubleshooting? Component selection? PCB design? PCB fabrication? PCB assembly? Value/cost engineering? Design for manufacturing?

Of course, a class on practical electrical engineering could be a really useful thing, but what topics should be included? What should be the mix of lecture time versus project time? Should the class cover analog, digital, RF, power, or all four? (Are there enough weeks in the semester for all four?) What are the best references? (Jim's application notes? Bob Pease's book? Howard Johnson's books? The free Analog Devices handbooks?)

To help solve the problem (and get some help), I've convened a seminar this Fall called "How to teach EE prototyping?" Eight students are going to help me brainstorm ideas, develop assignments, and generally design the course. I've promised them that the seminar is going to be equal parts round-table discussion, show-and-tell time, individual research, group activities, application-note club, rumors, innuendos, and lies. We have five goals:
  1. document the needs and wants for this class,
  2. manage expectations and preconceptions,
  3. achieve sufficient buy-in for the course,
  4. plan some of the course content and deliverables, and
  5. spend some time chatting about circuits.
My hope is to create a useful, practical, hands-on course that Jim would approve of. To set the mood, I started the students off with a hefty dose of App Note 47. The first reading assignment was
  • Mr. Murphy’s gallery of high-speed amplifier problems (pp. 7-15),
  • The tutorial section (pp. 15-32), and
  • Appendix F, Additional comments on breadboarding (pp. 98-112).
I'll let you know how it goes.

02 September 2012

Scope Sunday 37

Remember how I mentioned last week that I hadn't bought any scopes this year, and I then I bought two 422 portables at the Boxboro flea market?  Well, funny thing: I found another deal on Craigslist this week...

The listing advertised two 547 scopes for $50 in western Massachusetts.  Good deal (I've paid a little more for a 547, but I've also paid less).  I called and said I'd take them.  Turns out the seller was a high-school science teacher who had gotten the scopes from a neighbor of his parents to give to his school.  Unfortunately, the school didn't want them because they didn't have the space, and now he needed to get rid of them. (Unfortunately, his parents live in Ohio, so my thoughts of meeting this excellent neighbor were quickly dashed.)  So, we made plans to meet up.  The seller was driving into Boston this weekend to help his son move, so we met at a gas station along Route 2.

What made this transaction a unexpectedly GREAT deal were the accessories that came with the scopes.  The scopes are nice, but the accessories (some of which are shown above) are fantastic:
  1. A Model 500 scope cart, painted Tektronix grey instead of Tektronix blue.  This cart should match my Tek 511A.  It has a drawer, but no storage for plug-ins (of course, the early Tek scopes didn't have plug-ins, but that big blank panel still seems like a lot of wasted space).
  2. A complete C-12 camera system.  This is my first scope camera, and I'm kind of excited about it. Anybody have a good source for Polaroid film?  (Gotta love that smell.)
  3. A mint-condition vinyl cover, with the Tektronix logo on the side (very nice).
  4. A pair of 1A4 plug-ins, in addition to the two 1A1 plug-ins that came installed in the scopes.
  5. Original bound manuals for 547, 1A1, 1A2, the camera, and a 549 (for some reason).
  6. A photocopied manual for the 1A4 plug-ins.
  7. A 1971 Tektronix catalog.  So cool.
  8. Three extra 154-0568-00 CRTs.  Unfortunately, they're all the same and two of them are labeled "gassy".  When I saw the box of extra CRTs, I had hoped for different phosphors, but, alas, no.
So I actually got all of this for $50 and a scope.  When the seller told me that he was a science teacher, who wanted a scope for his school, I offered him the smallest scope that I had (it was a "modern" 20-MHz, two-channel, no-name scope, but it works, and it's smaller and lighter than a 422 or 453).  I hope it's small enough that he can find room for it.  I wish my high-school science club had had an oscilloscope!

26 August 2012

Scope Sunday 36

I recently came to the unbelievable, shocking realization that I haven't bought any additional scopes since I helped clean out that barn last December.  I felt that it was necessary to rectify the situation... However, despite attending the monthly MIT Swapfests this summer, several Electronic Flea Markets in California, and even the Hamvention in Dayton back in May, I haven't found anything worth dragging home.  (I've bought several plug-ins (stories for another time), but no new scopes or mainframes.)

Luckily, my friend Will reminded me that this was Boxboro weekend, so I drove up to Boxborough, Mass. Saturday morning, and looked around.  There was a dirty 564B for $200 (which is at least 10dB too high, $20 would be about right), a 7834 for $100, and several 7603s around the flea market, but the things that caught my eye were two 422 scopes (at two different sellers).

The 422 is a 15-MHz portable scope that had an optional battery pack.  The one on the left (serial number around 33,000) has the AC/DC power supply and the battery pack, but it doesn't power up.  The one on the right (SN around 12,000) works, but it has one bad channel, a broken handle, and no battery (it's AC only).  I'm hoping that they both can be fixed, but I should be able to get at least one of them working easily.

They aren't great scopes, but they remind me of a missed opportunity...  Several years ago on eBay, there was an auction for a whole pallet of broken 422 scopes.  If I recall correctly, there were 30 or 40 of them.  I thought seriously about buying them all and using them as the basis for great class project: student teams would study, troubleshoot, and fix the broken scopes.  The more scopes you fixed, the higher your grade!

I really should have done that.

05 August 2012

Scope Sunday 35

Last weekend, I finally got a chance to visit the vintageTEK Museum in Portland, Oregon.  I had a great time.  The museum is smaller than I expected; in fact, there are only a few dozen instruments on display.  Here is a picture of the whole museum taken from the front desk.

Despite being small, the museum has an impressive collection of rare instruments, including quite a few that I had never seen before in person.  First up, of course, is Tektronix's original oscilloscope from 1946, the 10-MHz-bandwidth 511. Only about 350 of these scopes were made (I have a 511A, which isn't nearly as rare).

On the other end of the frequency spectrum, they have a 519, the 1-GHz special-purpose monster from 1961. (There's also a 1-GHz 7104 in the museum, of course.)

Next up is the 945, which is the military ruggedized version of 545.  Heavy and heavy duty.

 They also have a 7704A mainframe, complete with the P7001 digitizer.

Of course, I thought the best part of the museum was the storage room and repair lab.  Here's the wall of letter-series plugins (check out the three different colors of type CA plugins in the bottom row; there's also two type O plugins and a type Q here, and a type N just out of frame).  I spent quite a bit of time poking around in the back room.

I actually spent most of the afternoon in the back, hanging around with two volunteers, who were busy sorting spare parts and fixing the horizontal sweep in a 547 (it was a busted tunnel diode, of course, part number 152-0125-00; luckily, there was a 547 parts donor on the shelf).  I don't know if I was a help or hinderance in the process, but I had fun.

After the museum closed, Ed Sinclair invited me to join the TERAC (formerly known as the Tektronix Radio Amateurs Club) for their weekly Friday night dinner at Round Table Pizza in Beaverton.  There I met another great group of (mostly former Tektronix) engineers, including Deane Kidd, and had a great time.