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Sunday, January 24, 2021

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 often 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  

Saturday, January 23, 2021

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.

Friday, January 22, 2021

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.

Thanks Jenny! 

Thursday, January 21, 2021

Wednesday, January 20, 2021

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


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

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. 


Tuesday, January 19, 2021

"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). 

Monday, January 18, 2021

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. 


Sunday, January 17, 2021

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: 


EXCELSIOR! 

Saturday, January 16, 2021

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!  




 

Thursday, January 14, 2021

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.
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