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Legacy Homebrew Challenges

legacy homebrew challenges
Guest Article (2 of 2) by Scott McDonald, KA9P

Building a receiver from an old ARRL Handbook presents legacy homebrew challenges, particularly in the parts department. More on my Junior Miser’s Dream receiver.

My old chassis was stripped, and the holes naively filled with the JMD design.  I couldn’t find a 7360 tube, so I borrowed a 6U8 mixer circuit from the same Handbook chapter.

The IF crystals were unobtanium, but I found an old Heathkit filter near the required IF.  And I wound some coils on whatever cores I could find.  If those coils didn’t work, I’d try some plug-in coils from the Handbook’s DCS-500. And I’d keep shopping eBay, or redesign the front end.  I’d be done in a few weeks.

But I wasn’t.  In hindsight, the difficulties were obvious. The ARRL Lab picked the unobtanium RF coils because they had a very high Q.  This high Q enabled the RF Q-multiplier to work properly.  I only got to the bottom of that issue by finding an old J.W. Miller catalogue and reviewing the coils specs.  And 9 months after starting, eBay coughed up three of the required coils.

And of course the 6U8 mixer was enough different that it wasn’t a drop in replacement for the 7360.  Between the coils and the mixer, I lost about 20 dB.

The Heathkit filter provided the ecstasy of victory and the agony of defeat.  The radio had great selectivity with steep skirts.  But the single stage IF gain measured low. I later did a better job of impedance matching between the filter and the amp and gained several dB.  But that was all trial and error – mostly error.

Legacy Homebrew Challenges – Lessons Learned

And to this day, the AGC is marginal.  I expected too much from a receiver with a single gain control stage, and I can’t be sure the transformer I picked is a proper substitute for the one spec’d in the Handbook.  But I can live with turning the gain down a bit from time to time.

The final lesson learned was – as the real estate folks are fond of saying – location, location, location.  My clever plan to use the existing chassis holes of the HR-10 was a recipe for disaster.  Or in this case a recipe for regeneration.  The radio howled like a banshee.  Careful relocation and shielding fixed this.  I should have known better.

I had one heck of a time building this receiver in 2022.  Could I have built this receiver as a novice?  Maybe.  If I had used the Handbook as it was intended.  Do exactly as they say – layout and parts. No deviation.  Is this why the Handbook had drilling templates, and why radios worked?

As a builder I’m a product of the solid state era. I’ve got quite a few Wes Hayward-inspired daily driver receivers, with 2N5109 pre-amps, H-mode mixers, synthesizers, great RF and IF selectivity, and killer AGC.  But I’m spoiled.  Those radios were all built with solid state 50 ohm blocks, using self-shielding toroids.  My radios were kid stuff to build.

Real women and men back in the day built vacuum tube receivers.  They followed directions to the letter where they could, and they helped each other avoid problems they’d already solved. Design changes came from experience, not hope.  A real tribe, with real tribal knowledge, to quote the Soldersmoke crew.

So John, maybe you could have built that receiver.  I’ve worked a lot of great Q’s with mine.  But my guess is, our radio early years may have worked out for the best.  More homebrew on my blog.

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