Repairing My Maplin Audio Waveform Generator

I picked this generator at the Kempton Park rally in London many years ago.  I use it quite a bit, not only for circuit testing but also as an easy way to get my homebrew BITX rigs to send a signal so that I can adjust my antenna.  I just plug this thing into the mic jack, crank in a small amount of 1 kHz audio, and I am ready to minimize SWR. 

It never gave me any real problems until last week.  I opened it up and examined the circuitry for the first time.  Lots of mystery chips in there. Fortunately they are all socketed.  Thank you Maplin. 

Even without really knowing how each of the ICs work, it was easy to troubleshoot. See the schematic in the article that begins on page 21 in this .pdf: 

https://worldradiohistory.com/UK/Mapelin/Maplin-Projects-Book-48.pdf

My Rigol scope showed a good signal going into and coming out of IC8, the LF351 op-amp.  From there the signal goes to IC5 a 4066BE.  But nothing was coming out of IC5.  Thus IC5 was my lead suspect.  I put in a quick order to Digikey.  A couple of days and a few bucks later the chips (as always, I ordered several) were on my doorstep.  Within a few minutes IC5 was replaced and the generator was as good as new. 

One annoying problem with this generator is that it has no automatic shut-off circuitry.  So if I forget to turn it off, I soon have to buy two 9V batteries. Tony G4WIF suggested a simple timer circuit.  I may do that soon.  Or, given that we are already on the IC dark side with this project, there is this: https://www.youtube.com/watch?v=C4C2H_3BU3o&feature=youtu.be 

I recently find myself replacing a lot of chips in various rigs and devices in my shack.  It seems that chips go bad more frequently than discrete transistors or even tubes.  But maybe I'm just using older gear with older, more fragile chips.   What do you folks think? 

Hyderabad Field Day

Hams in Hyderabad, India held a Field Day this week.  That looks like a uBITX next to that HT.  FB! 

More info here: 

http://telanganatoday.com/hyderabad-hams-organise-field-day

SolderSmoke Podcast #228

Soldersmoke Podcast #228 is available: 

http://soldersmoke.com/soldersmoke228.mp3

Of course, no travel.  But vaccines are here so maybe soon we can leave our shacks.

In the meantime:

I’ve been playing chess against AI bots on chess.com.

Netflix recommendation:  The Bureau.   From France. A review from NPR: 

https://www.npr.org/2020/06/22/881642358/addictively-suspenseful-thriller-series-the-bureau-will-keep-you-on-edge

A reading from "Conquering the Electron."  Germanium vs. Silicon.

Bill’s Bench:

The KLH Model Twenty-one II.  Acoustical Suspension.  First receiver WITH A PILLOW!  Bad speaker? Blown AF amp finals.  Hot heat sink.  VBE Multiplier. Desitin.

Tony Fishpool’s recommended LM386 boards.  10 for 11 bucks.  Nice.  They work.  Pictured in the Amazon ad at the upper right of the SolderSmoke blog page.

Putting a digital display on the Lafayette HA-600A

Test gear trouble.  My Radio Shack multimeter getting flaky.  I many need something better.  Auto ranging? My beloved Maplin AF generator died – will have to fix. I need that thing.  Probably a bad chip.  Good thing they are socketed.

I almost forgot about SKN!  But I remembered and I made one contact with the HT-37 and Drake 2-B.    

Pete’s Bench:

Presentation to RSGB on Homebrew.

TenTek Troubleshoot.

Swan 240?  Looking nice.

SDR adventures.

MAILBAG

Bill N8ET sent me some really nice Showa 9 MHz 8 pole crystal filters. 

Kevin AA7YQ Smoke jumper!  Building a hybrid SDR.HDR rig.  Launched blog. FB

Nick M0NTV working on similar HDR/SDR project.  Great video.

Grayson KJ7UM Hollow State Design – Launched a new blog.  Very FB!

Thomas K4SWL of SWL Post blog.  Kearsarge Mountain Transmission system.  And recent events.

Peter VK2EMU Poetry.  CW poetry.

Pete WB9FLW looking at DSB rigs…

Drew N7DA  Feels not like a real ham because he hasn’t built a quad from bamboo. Which type of landscape bamboo is best for antennas?

Ryan Flowers of MiscDotGeek.Com blog is also watching the Tally Ho YouTube videos of Leo Sampson. Wants to put a WSPR beacon on the Tally Ho. 

Joe KF5OWY  Working with diode ring mixers, trying to see the mixer action on his ‘scope. 1 and -1!

Jim AB9CN sent a cool idea about how to do a 20/17 Moxon.

Roy GM4VKI – I thanked him for his article in SPRAT about putting a 2n3904 on the output of an NE602 10P mod.  Brilliant.

Roger Hayward Told him that I really liked his Dad’s recent web site updates.

Farhan – Jokingly cursed me for showing him the Oscillodyne regen of Hugo Gernsback and Jean Shepherd.  “Now I will have to build this!”

VBE Multiplier Makes KLH Receiver "Cool Running"

Yesterday I turned to the SolderSmoke wizards for advice on how to fix my KLH Model Twenty-one II FM receiver.   I had finally gotten the thing working -- it wasn't the speaker, it was the AF amp, probably one of the final transistors was blown.  I replaced the finals and the driver. For the finals I used a TIP29C and a TIP30C.  For the driver a 2N3906.    With this fix the receiver was sounding good, but the heat sink on the AF amplifiers was way too hot. 

If you look at the comments in yesterday's post, you will see some great suggestions on how to fix this problem.  The comments and Google led me to Alan W2AEW's YouTube channel and his video on a circuit called the VBE multiplier.  Voltage Base-Emitter multiplier.  I'd never used this circuit before.  It allows you to adjust the bias on the bases of the two transistors in a push-pull amplifier.  

This morning I built the circuit on a small piece of PC board.  There were just two components: a 10k trimmer pot and a 2N3904 transistor.  

With the little board installed, I adjusted the pot for a 1.2 volt difference between the bases of Q6 and Q7. I ended up with base voltage values almost identical to those called for in the KLH schematic. 

The receiver sounds very nice now, and is no longer on the verge of bursting into flames.  I even made up my own version of the pillow that KLH claimed was necessary for proper acoustic suspension.  

Sometimes it is nice to be able to listen to something other than the chatter on the ham bands. And it is fun to do so with a receiver that you have worked on.  

I even used some Desitin as a substitute for heat sink compound. 

Thanks to Rogier for the receiver, to ZL2DEX, K0EET, W2AEW and David McNeill for the good advice. And to Dale K9NN who sent me a box of parts from which emerged the 10k pot I used in this project.  Thanks guys. 73  

KLH Model Twenty-One II AF Amplifiers Too Hot. Why?

I've been working on this nice old FM receiver that Rogier PA1ZZ sent me.  When I first tried it, it sounded terrible. I thought it might have been the speaker, but the speaker is fine. There was clearly something wrong in the AF amplifier.  Schematic above. Click on it for a better view. 

I ended up replacing the complementary pair of output transistors (Q6 and Q7) . The original had house brand designations -- I wasn't sure what to replace them with, so I just used a TIP29C and a TIP30C. With these transistors in there, the receiver sounds good. But the heat sink on the transistors is getting way too hot. I think the AF amplifier is now pulling about 1.4 amps, which is too much.

Another thing I did: I thought Q4 and Q5 might have been bad, so I replaced them with a 2n3904 and a 2n3906.

Why do you guys think the heat sink is getting so hot? What should I do? The supply on this receiver is 25V DC.

Jenny List's Favo(u)rite Things

Over on Hack-A-Day Jenny List (G7CKF)  has a really nice article about ham radio and homebrewing.  She truly has The Knack:  She got her start in radio electronics at age 9 when her parents gave her George Dobbs's Ladybird book.   

https://hackaday.com/2021/01/21/a-few-of-my-favorite-things-amateur-radio/

One of her paragraphs really seemed to capture the SDR-HDR conflict that we so often joke about: 

The age of the homebrew RF tinkerer may be at a close, at least in the manner in which I started it. Nobody at the cutting edge of radio is likely to be messing around with discrete transistor circuits in the 2020s, unless perhaps they are working with extremely exotic devices up in the millimetre wavelengths. It’s all software-defined radios, opaque black plastic boxes that deliver a useful radio experience on a computer but that’s it. No more homebrew, no more tinkering. 

Whew, good thing I'm not on the cutting edge.  It sounds kind of sad.  Oh well, that leaves more discrete transistors for us to tinker with.  

Jenny's Profile on Hack-A-Day: 

[Jenny List]: Contributing Editor and European Correspondent

Jenny List trained as an electronic engineer but spent twenty years in the publishing industry working on everything from computer games to
dictionaries before breaking out and returning to her roots.

She grew up around her parents’ small farm and blacksmith business in rural England, so making (and breaking) things is in her blood. Countless projects have crossed her bench over the years, though these days you’ll find her working with electronics and in particular radio, textiles for clothing and costume, decrepit classic cars, and real cider from first principles.

When she’s not writing for Hackaday she works on language corpus analysis software, designs and sells amateur radio kits, sits on the board of Oxford Hackspace, and is a freelance electronic design engineer and programmer.

• Hackaday Articles
• Hackaday Profile: Jenny List
• Twitter: @Jenny_Alto
• Personal Website: Language Spy

Thanks Jenny! 

Another Poem about Radio: Radar Man

 

http://www.alphalpha.org/radar/poem/story.html

Thanks to Allan WA9IRS for sharing this with us. 

Guilt Trip: Video on the Heath QF-1 Q Multiplier

Hack-A-Day had a nice post about this piece of gear: 

https://hackaday.com/2021/01/18/q-multiplier-er-multiplies-q/

My radio emotions were swinging wildly as I watched this video. 

Readers may recall that over the years I have brutally cannibalized several QF-1s.   I was enticed into doing this precisely by the tuning cap that the videographer so alluringly describes.  It has a  built in 7:1 reduction drive!  How could I resist?  These wonderful caps live on in several of my homebrew rigs.  

I also put the conveniently sized metal cabinets to good use -- one holds frequency counters for my AM station, the other houses an Si5351 VFO/BFO that can be used with many rigs. 

After extracting the cap and putting the boxes to good use, I was left with the remainder of the circuitry.  I recently put even this stuff to use by using the coils to make a triple LC circuit filter for 455 kHz.  This may someday be used in a receiver.  So you see, I've not been wasteful. 

And the thing only cost 9 bucks back in the day...  So I didn't really do anything bad.  And besides, adding a regen circuit to a superhet is kind of backwards, right? 

But then the video producer started talking about how nice his QF-1 looks, even after more than 60 years.  And about how much it improved the performance of his AR-1.  And then, the kicker:  He said the QF-1s are now "relatively rare." 

I hang my head in shame.  I am a serial QF-1 killer.  And I don't know if I can stop. 

"The Transistor" 1953 Video from The Bell System

Interesting video. 

-- Good discussion of the transition from tubes.  

-- Go Arlington, Virginia! 

-- Nice video of Bardeen, Brattain and Shockley at work (Shockley does seem a bit off to the side). 

Kevin AA7YQ (Montana Smoke Jumper) Launches his SDR/HDR Build Blog

 

Earlier this month I posted a note from Kevin AA7YQ describing his effort to design and build a nice transceiver that mixes the best of SDR and HDR.   Kevin has launched a blog in which he will describe the project in more detail.  He is looking for constructive feedback and suggestions.  Check it out: 

http://aa7yq.blogspot.com/.

Thanks Kevin. 

STOP. LISTEN. Shep on Building a Shortwave Receiver

Oh man, how could I have possibly missed this one?  Perhaps I didn't, but even if this one has been on the blog before, it is so good that it is worth repeating.  

Shep really captures the frustrations and joys of a teenage radio builder.  I could really identify with this.  It all reminded me of my heartbreaking effort to build the Herring Aid 5 receiver. 

So much cool stuff in this 1963 recording: 

-- The wonderful smell of radio service shops. 

-- The terrible shirt and tie choices of radio service guys. 

-- The truly dire consequences of mistakes in published schematic diagrams. 

-- The AGONY of not being able to get a homebrew radio to work. 

-- The JOY when you finally do get it to work. Shep's "whole life changed" when that happened. 

-- Hugo Gernsback, Lee DeForest and "unscientific scientists."

As the YouTube video plays, they show several covers of old Short Wave Craft magazines. At one point they show some homebrew phone rigs.  I think they look like my wooden box BITX rigs.  And the front panels are clearly Juliano Blue.  TRGHS. 

Here is the 1933 Oscillodyne article that launched Shep's effort: 

https://schematiclounge.weebly.com/uploads/6/4/5/7/6457679/oscillodyne.pdf_0001.pdf?fbclid=IwAR1SKvnmHuAW3eh5MKJH2-2vgoE0jZAdwrBy4-IBiEaZosbXnPPZJtDieSU

EXCELSIOR! 

Nick M0NTV's Really Useful SDR Transceiver (video)

Even though I am more of an HDR guy, I really liked Nick's SDR rig.  I like the modular approach, with all the modules inside a transparent plastic box (see below).  Don't worry about the shielding Nick -- I had the same concern about my BITXs in wooden boxes, but they worked fine. 

Nick really did a great job on the video.  The bloc diagram was especially useful, both on the hardware and on the software.  Very cool.   It is nice to use this phasing approach,  with the digital magic happening at audio frequencies.   I fear that soon FPGAs and direct digital sampling will take the hardware fun out of these rigs.  We already have a bit of that with the RTL-SDR dongles. 

Very cool how the Teensy takes care of the 90 degree audio shift.   I had to do that with chips in my phasing receiver. When I first saw Nick's bloc diagram, I was looking for the audio phase shift network -- then he explained that that was in software, in the Teensy. 

Nicks arrangement for switching the filters is very nice.  

Thanks Nick!  

 

A Poem about Shacks and Rigs and Ham Radio

The Little Black Box of jewels and rocks

with lanterns that flicker and glow,

make lighter the gloom in my little back-room

where often I hasten to go.

An anthem it peals of whistles and squeals

and voices so ghostly and dear

that you'd never decry, should you chance to pass by-

what a brotherhood foregathers here!

Each separate tone has a soul of its own;

each voice is the voice of a friend.

United through space in this gathering-place

at the radiant signal's end.

Reverberant sounds ride the wave that rebounds,

like the waves of the sea from afar,

reporting the doings, the comings and goings

of brothers... wherever they are.

A curious band, spread over the land,

yet joined from equator to poles

disperses the gloom in the little back-rooms

by this magic communion of souls.

I could part with a lot of the things that I've got,

but I'll carry my love to the tomb,

of that little black box and the joys it unlocks

... when I enter that little back room.

(published in QST magazine sometime 1965.)

Thanks to Jeff Murray for alerting us to this. I had not seen it before. It really got me -- I am working in my little back room on a box with jewels (jeweled movements!) and rocks.

A Wizard Near Blackpool: M6AGC's Wonderful Homebrew Rigs

https://www.qrz.com/db/M6AGC

Alan's YouTube Channel:  https://www.youtube.com/channel/UCISxWaAc5xW6oFElxGN4xWg

One of Alan's recent projects (spotted on Facebook): 

Pete N6QW and Steve G0FUW Talk to RSGB About Homebrew (Video)

Wow, what an unexpected treat!  Here we can watch Pete N6QW and Steve G0FUW talk to the Radio Society of Great Britain about homebrew radio.  Steve talks about kits and scratch-built rigs.  I really liked seeing his early rigs and his description of how building these rigs helped him become a more advanced homebrewer.  I also liked his mention of George Dobbs as a guiding light in the QRP and homebrew world. 

Pete focuses on SDR and provides a really great description of this approach to homebrew.  I was struck with how great it is that, after a lifetime of HDR building, Pete is willing to embrace this new technology. He talks about it as part of "a learning journey."  As always, he sets the example for us all.   

Thanks to Pete, Steve, and RSGB. 

KLH Model Twenty-One II -- Is My Speaker Dried Out?

A few years back Rogier PA1ZZ very kindly sent me a box of electronics parts.  Included was an FM table-top radio with a nice walnut case.  Thanks Rogier! 

I hadn't looked at the receiver in years, but this week I dusted it off and looked it up on the internet.  Turns out that it is kind of famous.  It was produced by the KLH company.  The K stood for Henry Kloss, one of the giants of Hi-Fi audio gear.  Henry appears in the picture below. 

I got the receiver working, but it sounds awful.  It sounds much better with an external speaker, which is disappointing because the internal speaker was the main attraction of this receiver. It even has a little badge on the front panel trumpeting(!) its "Acoustic Suspension Loudspeaker."

I'm wondering if the problem is in fact the speaker.  The cone looks intact, but it seems very dried out.  It has been more than 50 years...  What do you guys think?  Picture above.  Any other suggestions on what to do with this thing, or how to make it sound better? 

Some KLH history:  

https://klhaudio.com/history

https://antiqueradio.org/KLHModelTwentyOne21FMRadio.htm

KLH receiver with pillow

The Dream of a Shortwave Fiend

 

Thanks to Mark Rowley and Jeff Murray for posting this magazine cover on Facebook.

A Parachute that Flies Home Autonomously

 

Too often ham radio bloggers and podcasters tend to focus their efforts on the projects of, well, older guys like us.  I think it is a good idea to direct attention toward young innovators, the next generations of people who are working on interesting new projects using new technology.  

Yohan Hadji is definitely one of these young innovators.  He is 16 years-old and is working to develop a system that would guide the parachutes of descending balloon payloads to designated safe landing areas.  Having spent a lot of time chasing the parachutes of Estes rockets, and after having to PERSONALLY guide my own parachute to a safe landing area (sometimes without success),  Yohan's project caught my attention.  

The videos above describe the project.  

A Hack-A-Day article provides good background: 

https://hackaday.com/2021/01/07/gps-guided-parachutes-for-high-altitude-balloons/#more-454705

And finally, if you want to support Yohan's work, he has a GoFundMe site: 

https://gofund.me/c0ae8d1f

The Amazing Rigs of WA3TFS

There are a lot of great ideas and circuits on WA3TFS's page.   He spans the SDR-HDR divide.   He makes great use of simple SDR dongles.  Check out his QRZ page and the web site that describes his projects in more detail: 

https://www.qrz.com/db/WA3TFS

https://044640f.netsolhost.com/

An End to the HDR - SDR Conflict? Kevin AA7YQ Combines the Best of Both Worlds

We've had some pretty amazing contact with Kevin AA7YQ over the years.  Kevin and I originally bonded due to our common experience with parachutes  (he was smoke-jumper, I jumped while in the army).  Kevin once used a parachute to insulate a QRSS beacon.   And one day, while thinking about SolderSmoke during a drive through Montana, Kevin turned on his rig only to hear... ME!  He caught one of my infrequent CW contacts. TRGHS.  

Now we hear that OM Kevin may be poised to end the HDR-SDR civil war that has for so long been dividing our great podcast.  Can Kevin's new rig heal our wounds and allow us to enjoy the beauty of SDR waterfalls while not forsaking the joy of hardware defined rigs?   Kevin will soon launch a blog describing his effort at rig-building. See below for a preview.  Stay tuned.  

Kevin wrote: 

I am currently working on a new rig design.  It is a hybrid HDR(Hardware defined)/SDR radio that incorporates some classic superhet design along with some of the more useful features of SDR.  I have found that pure SDR is really not that enjoyable for me.  I love using GNU radio to mock up and test design concepts, but SDR basically dilutes the “magic” of radio to nothing more than software and touchscreens, stuff we use every day all day.  Its not the Ham Radio I grew up with as a kid and was fascinated by.  On the other hand, I have always fought temperature drift, large variable capacitors minimal tuning range, and associated with classic VFO and VXO designs.  In fact, in 1997, for my senior capstone design in EE at Montana State University, I designed a 20m superhet that used a DDS LO.  At the time DDS was cutting edge technology I used an AD7008JP50.  I had to beg and plead with ADI to get a couple samples for my design, since they exceeded my self-funded college student project budget. 😊  But that’s another story.  SDR has made me grow extremely fond of the waterfall display.  I love having the visual “situational awareness”  of what is going on in a moderate bandwidth outside of the spot I am tuned to. I also am a big fan of digital filtering and modification-ability that comes with boot-loadable microcontroller designs.  So this design includes most of the real highlights of SDR but does not take the fun out of designing,  building, and operating a HDR.

Anyhow, this design is a big goal of mine to complete and build in 2021.  I am not retired yet so I still have to balance, work, family, and tinkering time, but I am very excited about this project.  I have “noodled” this design to the point of what I have achieved full-on “analysis paralysis”.  That is, I keep designing and redesigning, optimizing, and figuring to the point where after months of thought, I have nothing to show for it 😊.  So my New Years goal for 2021 is to make “good enough” rather than “perfect” design decisions and move forward.  I will keep you posted on the design and possibly start a blog so I can get some peer review input from the greater RF Design/Homebrew community on my project.  I’ll keep you informed on my progress.

Glowing Numerals for the Lafayette HA-600A (With Jeweled Movements)

 

I really like this receiver.  I have strong sentimental ties: it was my first SW receiver.  But the frequency readout situation was kind of rough -- depending on where you put the Main Tuning cap, your Band Spread dial could be WAY off.  

China to the rescue!  Specifically the very nice San Jian PLJ-6 frequency counter boards.  I have used these in several projects.  I like them a lot. I get mine on e-bay.  They are very cheap.  Here is the manual with specs: 

https://www.mpja.com/download/35057tebasic%20manual.pdf

As I did with my BITX20, I put mine in an Altoids-sized box.  I got to use my Goxawee rotary tool with circular metal blade to cut the rectangular hole.  Hopefully future efforts will yield neater results, but the flying sparks were fun;  they made me feel like one of those car-part  "fabricators" on cable TV. 

To tap the VFO frequency, I just put a bit of small coax at the point where the 10 pf cap from the VFO circuit enters the first mixer.  I ran this cable to the unused "Tape Recorder" jack on the back of the Lafayette -- this connects to the input of the counter.  I attached 11 volts from the power supply to an unused terminal on the accessory jack of the Lafayette -- this powers the counter.

Having a counter on the VFO proved very illuminating -- in more ways than one.  I measured the Center Frequency (CF) of my IF to be at 456 kHz.  I set the PLJ-6 to display the VFO frequency MINUS 465 kHz.   For AM broadcast signals, this worked fine:  I'd tune the signal for peak S-meter reading.  This meant that the carried was right at the CF.  

For SSB, things were a bit different.  I set the BFO knob  to be RIGHT AT 465 kHz when the dot is in the center position.  With the BFO there, I could tune in SSB signals.  The suppressed carrier would be right at the center of the IF passband, with the audio information above or below the suppressed  carrier frequency.  But it didn't sound good this way -- it sounded better if I would tune an LSB signal 2 kHz down from the center, then adjust the BFO down about 2 kHz.  This put most of the the audio in the peak portion of the IF filter(s) curve.  Doing it this way means that I have to remember that the number displayed on the PLJ-6 is 2 kHz down from the actual suppressed carrier frequency of the transmitting station.  I can live with that.  

I am going to leave the Lafayette on the corner of my workbench so that I can easily tune in hams  and SW broadcast stations.  Having modified the product detector and added the digital frequency readout makes listening to this receiver even more pleasing.  The jeweled movements are as smooth as ever. 

So 2021 is off to a good start on my workbench.  HNY to all!