New Rig: SDR-2017

It's been almost a month since my last post.  It's not that I haven't been busy, but because I've been doing some high-intensity noodling and trying, with limited success, to get a handle on coding in Python.  But, I'm making progress and finally at the point where I can cut/bend some metal and melt some solder!

In previous posts, I mentioned that I'd be doing a "Software Defined" rig as my next project, but I didn't want to simply regurgitate what others have done. No, I want my stink all over this one.

I started experimenting with SDRs in the mid 2000s when I stumbled across Tony Parks, KB9YIG's early Softrock kits.  These are really incredible little radios, and lots of fun to build - so much so that I built way several, including a couple that I never really intended to use, one of which I modified and re-used in this project.

As nice as Tony's kits are, once the novelty wore off, I lost interest in the operating experience, which is, to me, more like operating a computer than a radio.  This is by no means a criticism, just that it wasn't for me - I prefer a radio that "feels" like a radio.  So, I shelved the SDRs and went back to using more traditional radios at the same time that SDRs started becoming insanely popular with the rest of the amateur fraternity. 

Anyway, I started getting ideas for this project earlier this year when I came across a 40m Softrock receiver that I'd forgotten about, hooked it up and marveled at how competent a receiver it is.  Then, while rummaging through my desk at work, found one of my "surplus" Softrock transceivers hiding in one of the drawers.  That got the wheels turning: Why not build a "real" radio around it?  So, here we are...

The Softrock serves as the modulator/demodulator stages in the new rig, and is controlled by an embedded Raspberry PI 3B running N2ADR's Quisk software. The Softrock has been adapted so that it's frequency is controlled by one clock of the venerable Si5351 in much the same way that others and I have used it to provide the BFO signal in conventional rigs.  In other words, the Softrock is tuned to the IF signal.

The Si5351 is controlled by an Arduino Pro-Mini, that's also used to select the appropriate band and low-pass filters and handle all of the rig's switching duties.  The Arduino and Pi talk to each other via their USB ports: The Pi tells the Arduino when the frequency or band is changed in Quisk, and the Arduino tunes the Si5351 and switches the filters.  Conversely, when the frequency is changed using the rotary encoder, the Arduino tells the Pi about it and Quisk updates the operating frequency.  Cool!  A big "Muchas Gracias" to Pavel, CO2WT, for his fantastic library that makes the Arduino look like an FT-857D, allowing me to use the standard Hamlib libraries to make this all tick.

Referring to the above photo, the pot at the lower left is the AF gain.  I'm using an LA4425A amplifier to bump the soundcard's line-level output up to a few watts of glorious, room-filling audio.  In the upper left corner is a momentary switch used to power the rig on and shut it down gracefully; I've got the first part working, but need to debug the shut-down code (Damned Python.)

The display is a 7" touch-screen, which is about the perfect size to control Quisk in small-screen mode.  The Pi is attached to the rear of the display using a homebrew bracket/enclosure.  I found that the Pi would overheat after running for several hours, so I combated that by installing a small cooling fan on the Pi's enclosure.  I can now leave it running for days without it going into melt-down.

Left of the display is a 100 PPR encoder that I got ($20) from Marlin P. Jones. I love the resolution of the encoder, but am not wild about the clicky detents and intend to see if I can defeat that feature.

So, at this point, I have a working SDR operating at 9 MHz - which deserves mention:  9 MHz is a good single-conversion IF on all HF bands except 17m, where it's a pain to filter the 2nd harmonic of the BFO from the transmitter's output.  But, since there are no fixed IF filters being used and the 2nd LO is programmable, I can change to a 17m friendly frequency with a few lines of code.  Excellent!

That's where I'll leave it for now.  Until next time,

73 de Steve N8NM

Huffing Lacquer

After building up the preamp for my Crosley 86CR, I decided that it was time to finish that radio completely.  You see, I repaired, stripped, grain filled and prepped the thing for lacquer over two years ago, and have using it in a partially assembled state ever since.

I like working with lacquer because it gives me lots of somewhat valid excuses for procrastination:  It's too hot, too cold, too humid, etc.  Since we've had temps in the upper 60s to low 70s, with low humidity, for about a week, I just couldn't put off breaking out the HVLP sprayer and going to town.

Unfortunately, I didn't photo-document the process... I never intended to write it up... but here we are.  I'll do my best to paint a word picture.

Here's what I started with in 2015:

It was actually in decent shape, though there was some veneer damage to the dial panel and a few dents from having things dropped on or banged into it.

To repair the veneer, I removed the dial panel and removed a section of veneer from a location that is hidden behind the side panel and surgically grafted it into the spot where the exposed veneer was damaged.  It took a few hours, but turned out well - the repair is all but invisible.

To work out the dents, I employed the XYL's iron and steamed them out.  This was easy - just lay a moist cloth over the dent, heat it with the iron and the fibers of the wood expand and the dent disappears.

After these and a few other minor repairs, I completely stripped the cabinet using Citru-Strip.  I like this stuff because it doesn't stink, so I'm able to do it indoors.  The trick to using it is to get a feel for how long to leave it on before scraping it off; too soon or late and you make extra work for yourself, but find the sweet spot and you'll have naked wood in no time.

Once stripped, it's time to sand.  This set has some fairly beefy veneer, so I was able to get quite aggressive with it in some spots.  It's not rocket science, start with a coarse grit and keep working it and progressing to finer grits until it's smooth.

But, even when it's smooth, you still have to fill the pores, which is my least favorite part of the job.  Ever see a refurbished antique that, when viewed from different angles, looks like it's had glitter splashed on it?  They skipped this step.  Again, it's not rocket science, the stuff goes on sort of like a slurry and you rub it into the grain, then scrape off the excess by dragging something like a credit card sideways across the grain.  The more you're able to scrape off, the less sanding you'll have to do in the next phase.

More sanding, this time with 400 and then 600 grit until the surface is as smooth as possible.  Light should reflect from it when it's done.

Then it's out to the garage for sprayin'.  Wear a respirator, the stuff ain't good to breath.  Before doing any color coats, I spray a very thin "spit coat" of lacquer over the entire cabinet.  The purpose is to seal the grain and all that grain filler so that it stays put.  If you don't, it'll all come out when you apply masking tape - ask me how I know...

Once that dried (overnight), I taped off the field and shot the trim with a couple of coats of heavily tinted lacquer. I had to play with the amount of dye to get the color right.  It's good to have lots of scrap boards from different species to experiment with.  Lacquer dries to the touch fairly quickly, and I removed the masking tape as soon as I was able to do so.  I've had bad experiences from leaving it on longer than necessary, so once I'm able to handle the piece, it's gone.

Then came the color coats, where the whole cabinet was shot with several coats of dyed lacquer - again, the correct color and shade was determined by experimenting on scrap, and then the "formula" noted so that I'm able to duplicate the mix.  I wasn't counting, but I'd guess I shot 6 color coats over two days before being satisfied that I had enough coverage.  I should mention that I use gloss lacquer for these coats; I think it covers better, but that's just me.

Once the color coats have cured (24 hours or so), I wet sanded the cabinet, first with 400 and then 600, before shooting the top coats.  I did dye these, but not as dark as the color coats.  As before, I wasn't keeping track, but I probably shot 5 or 6 coats of satin lacquer, then let it cure for a couple days before bringing it indoors for assembly.

Which is where we are now.  I apologize for the poor quality picture, the lighting in the shop is terrible for photographs.

 Yet to be installed is the speaker box (more on this at another time), grille cloth and decorative grill.  It's close to midnight, so I'm done for today!