Why we have "BW Limit" Switches on our Oscilloscopes

Dean KK4DAS asked me why we have these switches on our 'scopes.  I didn't know.  Dean asked Alan Wolke W2AEW.  Alan knew: 

Alan Wolke wrote: 

Interesting background on the 20MHz vertical BW selection. This feature has existed on the vast majority of all oscilloscopes since the 50s or 60s (both analog & digital). When I explored the history of this, I spoke to some of the folks at VintageTek.org, and wound up having a chat with THE engineer that did it first!  Tt was John Addis, designer at Tektronix. 

At the time, Tektronix was located in Portland Oregon. While working on a wideband vertical preamp for a new scope (the 7A11 vertical plugin for the 7000 series scope), John Addis was plagued with interference from the local television broadcast station in the 50MHz band. So, he popped in a 20MHz low pass filter that he could switch in/out so that he could complete the work on the preamp. Since it was deemed useful, it was left in the design. 

 

And, since Tektronix added a switchable 20MHz low pass filter  in their scope, and Tek was the leader in oscilloscope technology, other manufacturers followed suit, and this feature has "stuck" as a staple in vertical setting controls. 

 

The main reasons you'd use this filter would be to improve the signal to noise ratio (SNR) for signals when their frequency content is below 20MHz.   You've probably noticed that, even without any signal connected, the thickness of the trace is thinner when you engage the 20MHz filter. 

---------------------

Alan sent a link to a Wiki page about the 7A11 that John Addis was designing.  Alan says this places the initial inclusion of the 20 MHz BW Limit filter to the mid 1960s: 

https://w140.com/tekwiki/wiki/7A11. 

Thanks Dean, thanks Alan!  

A Pretty Good Troubleshoot -- Fixing the Transmitter in my 75/20 meter Mythbuster Transceiver -- Mind the Gap!

Bidirectional Termination Insensitive Amplifier by W7ZOI and K3NHI

All of a sudden the transmitter in my 75 & 20 meter dual band homebrew Mythbuster transceiver stopped working -- there was no output at all.  I went into troubleshooting mode.

The first clue was that the receiver was working fine. This meant that many stages of the rig were taken out of suspicion:  It probably wasn't the VFO, the first mixer, the BFO/Carrier Oscillator,  the second mixer, or the bandpass filters. Nor was it any of the receive sections in the bidirectional amplifiers I was using.  

Suspicion fell on the power amplifiers and on the transmit sections of the bilateral amplifiers.  

With the output going to a dummy load,  I put the rig into transmit mode and put a bit of audio into the mic jack.  Then with the 'scope I started to work my way back from the antenna jack.  I wasn't seeing anything.  Then I got back to the transmit side of the TIA amplifier between the crystal filter and the mixer.   There was a strong signal at the input, but nothing at the output.  Bingo!  I had found the faulty stage.  But where, exactly, had this stage gone wrong? 

There are three transistors on each side of a TIA amp (see schematic above) -- I just started from the input of the first one with my scope probe and moved through the circuit.  Finally, at the output of the last of the three amplifiers, the signal stopped.  I knew I was very close to the problem.  

Looking at the components, suddenly I could see the problem:  At the output there is a 47 ohm resistor (R2 in the circuit diagram above) and a .1uF cap in series.  The cap went to a Manhattan pad.  But when I looked at it closely, the lead was kind of floating above the pad.  See it? 

Mind the Gap 

And when I moved it, the connection between the 47 ohm resistor and its pad seemed quite flimsy. 

I quickly replaced both components and was back on the air. 

I don't really know how or why the lead to that capacitor broke.  Maybe I had bent it repeatedly, to the point of weakness, and, over time, it just let go.   

Whatever the cause,  I found this to be a satisfying troubleshoot and repair.  It required me to think a bit about what could be wrong, and to use some test gear to zero on on the faulty component.  

Out with the old...

The TEK 465 'Scope Used to Create Pong and the Apple II

Wow, this is almost enough to make me take another shot at repairing my TEK 465.  

 

Apex Surplus in Los Angeles

The Polaroid camera for the Tek 'scope was pretty cool.  And the comments about the Simpson 260 made me feel good about finding one at a recent hamfest, but I don't think mine is an extremely rare Model 2.   

CuriousMarc Powers up (and Explains) Old Cathode Ray Tubes

Wow, really cool video from CuriousMarc and the guy who wrote the "Open Circuits" book.  But fellows, you really need to be more careful with the high voltage. Remember poor Ross Hull.  One hand behind the back would help.  Volts jolt, but mills kill.    

Mr. Carlson's New Lab. And his Amazing Tek Collection

So Many Wonderful Things on W7ZOI's Site

 

There he is.  Wes Hayward, W7ZOI in 1957.  I had never seen this picture before.  I found it on Wes's recently updated "shackviews" web page: http://w7zoi.net/shackviews.html . 

There are so  many treasures on that page, and on all the other portions of Wes's site.

Some highlights for me: 

-- Wes's description of the station in the above picture. 

-- On his page about Doug DeMaw, Wes mentions that after Doug edited Wes's 1968 article about direct conversion receivers, Doug built some himself, experimenting with different product detector circuits. Having used Doug's mixer circuit in many of my rigs, and having recently experimented with different product detectors for my HA-600A, I kind of felt like Doug was watching over my shoulder, guiding me along as I experimented. 

-- Wes's use of a digital Rigol oscilloscope.  Makes me feel better about giving up on my Tek 465. 

-- The page about Farhan's visit to Wes, and the awesome gathering of homebrew Titans that ensued... 

-- Wes's meeting with Chuck Adams.  

Thanks Wes.  Happy New Year and best of luck in 2021!  

Builds Oscilloscope at age 12! Nick has THE KNACK

Thanks to Grayson Evans for sending us this very encouraging video.  
It is from Tektronix;  I wonder if Alan W2AEW provided some ideas and inspiration? 

More stories like this:  https://www.tek.com/stories  

Discrete Dead-Bug O'scope Pong

Thanks to Bent KD0GLS for alerting us to this excellent example of the awesome power of discrete transistors and dead--bug construction.

Check it out.  There is a video.

https://www.eevblog.com/forum/projects/oscilloscope-pong-for-1-or-2-players/

Oz Tektronix 'Scope Repair (in Juliano Blue)

Rob is a braver ham than I.  When my Tek 465 quit, I tried to fix it, but quickly chickened out.
Very nice that he painted his in Juliano Blue.

Dear Bill and Pete, 

I do enjoy your podcast, and I must present an offering to the "Gods of Homebrew", 

An on-line find of an old Tek 545 oscilloscope presented a chance to enjoy the warmth of 100+ tubes (once repaired)

The outside was heavily scratched, the inside looked like a chicken coup, but no major bits missing or broken. 

Lots of cleaning, testing all tubes,(using the excellent uTracer tube tracer),  replacing the broken 3, remounting the cooling fan, lots of reading about tube oscilloscopes, adjusting the trigger circuit, rebuilding 3 electrolytic power supply capacitors, sandblasting the cabinet and a coat of BLUE paint. 

Voila,  the joy of (visual) oscillation!     (1MHz 2V p-p)

Rob VK5RC

Video: Sweeping a Filter with a FeelTech Signal Generator and a Rigol Oscilloscope

This short video shows how I used my new FeelTech Signal Generator (90 bucks shipped from China) and my trusty Rigol oscilloscope to display the passband of the filter in my Armand HROish receiver.  Feedback on this test procedure would be appreciated.

Alan Wolke Talks 'scopes on "The Workbench" Podcast

George KJ6VU is a long-time supporter of the SolderSmoke podcast.  He has recently teamed up with another ham and launched a podcast called "The Workbench."  This morning I listened to Part I of their interview with the legendary Alan Wolke W2AEW.   It was great.  As I was listening to Alan I was soldering together a crystal filter for my new receiver, and using my RIGOL 'scope to check the results.  

I liked Alan's description of how they made images of 'scope patterns in the days before the advent of Digital Storage Scopes (Polaroid!).  I also liked Alan's scorn for those who use the "Auto" switch on the 'scopes.  The host's reaction to Alan's description of a $300,000 Tektronix 'scope was also fun: "For that price I want to be able to drive a car into it and put a swing set behind it!"  Indeed.

Recommended listening:   
http://hamradio360.com/index.php/2016/08/30/ham-radio-workbench-5-oscilloscopes-part-1/

A Knack Victim's Midlife Crisis

Thanks to Steve WA0PWK for sending us this Jim Williams cartoon.  

Been there, done that. It doesn't mean you are a bad person.

Clocks, CRTs, HV supplies: Eric has The Knack!

Nice interview by Jeri Ellsworth.  This fellow definitely seems to have the Knack.  At the end of the video he shows a high voltage supply that he is WEARING AROUND HIS NECK! 

Here's Eric's site: http://tubetime.us/


Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

BITX 20/40 Build Update #5: It is ALIVE!

I got the BITX 20/40 on the air this morning.  The receiver has been working for a week or so, but as usual it was a bit of a struggle to tame the transmitter.  I got up early this morning and started poking and probing.  I played with the driver and final coils a bit.  I had used the same trifilar toroids that I'd used in the BITX 17, but this rig didn't seem to like them. So I went with FT-50-37 bifilars -- that seemed to work better.  That IRF-510 seems to put out about seven watts on 40.  (I still need to tame the transmitter on 20).  The 40 meter signal looked great on the 'scopes (RIGOL and Tek!).  Shortly before 8 am I worked AD4SY who reported that I was filling his shack with booming audio.  Life is good.  

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Digi 'scope, Analog 'scope

Carl (M0ICR ex 2E0TEC) saved the day with my Tek 465 -- he sent the replacement HV multiplier that put the 'scope back in action.  (Ace Tek repairman Tad WA1FQO made the repair.  Tad sells Tek scopes on E-bay -- check out his products: Search for fqo63 on ebay.)

Carl notes a number of similarities between our workbenches and recent projects.  All he needs now is a Drake 2-B!

Alan Wolke said he'd be interested in my reactions to having both a digi scope and an analog scope on the bench.  I can already see areas where one might be a bit better than the other.  The Tek is better at watching a fast moving audio or SSB waveform.  But the Rigol doea all kinds of math tricks.  Check out the FFT function shown on Carl's page:  http://iqrp.net/

Bill,

I am very pleased that you finally got it working with the help of brothers from the fraternity.  Having listened to your latest Podcast I think that must be your alter ego!  I have A Tek 465, Rigol 100MHz DS1102E and have built 2 Bitx, one for 20m and the other for right bands- using a DDS VFO  (3 if you include the 80m version by Steve Drury G6ALU - look up MKARS80) ...I also have 2 copies of SSD!

My website is here:  www.iqrp.net  where you can see both scopes in my shack and images of my Bitx and my Hans Summers WSPR beacon!

I have just returned from my local radio club social meeting (the Radio Society of Harrow) and took great pleasure in listening to Soldersmoke 158 whilst driving there ... half on the way there and the other half on the way back.

Very best wishes from London (where it is cold, wet and miserable !)

72/73 de Carl M0ICR (ex 2E0TEC from Nov 13)
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Santa Arrives at SolderSmoke HQ! Rigol and Black&Decker

Santa Claus obviously heard my distress call when the TEK 465 gave up the ghost.  This morning it was replaced by a RIGOL digital scope.   I'm going through the tutorials.  Wow, lots of features.  A Black&Decker jig saw also arrived -- this will help in my cabinetry efforts. 

Last night just before dinner I talked to Kiyo, JH1MDJ, in Tokyo on 17 meter SSB with the 5 watt BITX.   I kind of broke through a small pile-up and we then had one of those "please repeat your power level... I can't believe that is 5 watts to a dipole" conversations.  

Happy Holidays to all!

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

SolderSmoke Podcast #155 BITX17, TEK465

Podcast # 155 is available for download (or you can listen by clicking on the button above):
http://soldersmoke.com/soldersmoke155.mp3

October 14, 2013  (Delayed Back-to-School edition)

Topics covered:

-- BITX Beauty -- Building a BITX17

-- VFO woes, and the wisdom of Doug DeMaw's love for crystals

-- Why does a 55 year old man NEED clear nail polish varnish?

-- How the VFO KILLED the Tek 465

-- Trying to fix the Tek 465 (not for the faint of heart)
 
-- New products:  Justin Beiber Duct Tape, bifocal safety glasses,
magnetic reading glasses, Ward Silver's new book

-- Backwards British Bike Brakes

-- MAILBAG 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Tek Troubleshooting Triumph

Thanks to the manual provided by Jim and the HV test gear provided by Alan Wolke, I was able to finish the troubleshoot on the broken Tek 465.   It is the HV multiplier, U1432.   The cathode voltage is supposed to be -2450.   It initially tested at -1000.    The manual calls for a jumper to be removed to take the HV Multiplier out of the circuit.  As soon as I did that, the cathode voltage went to -2500.   Bingo.

 

Now I have to get a replacement for U1432.    Any carcasses laying around?

 

Thanks guys. 

 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Inside a Tek 465

The TEK 465 'scope is as they'd say back in the Bronx "a piece of work."  It has real character.  Perhaps a bit too much.  If you look closely, you'll see transistors in sockets.   I had kind of gotten my head around that, when I noticed that many of the transistors were not in sockets.  They looked like they had been soldered to into the holes in the PC boards.  But NOOOOO!  They are snugly in the holes, but they are NOT soldered in!  You can pull them out, then (if you are lucky!) slide them back in.  There is even quite a bit of "ugly construction." with connections floating in the air.  You can really see the influence of tube era thinking in this instrument.

I think I have narrowed the problem down to the HV supply.  Alan Wolke is lending me a high voltage probe.  I hope to be able to get it fixed this weekend.  I need to get it working because my BITX 17 is nearing completion and I'll need the 'scope for the lineup and de-bugging.

The picture above is from:  http://www.eevblog.com/forum/projects/tektronix-465-repair-and-restoration/   More pictures of the scope are available there.  Also see http://www.spurtikus.de/basteln/repair/tek465/index.html for more scenic views of this instrument.


Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20