Friday, May 3, 2019

Mic Amp/Limiter for uBitx

The past few months have been very busy for me, but I somehow managed to knock-out a Version 5 uBitx.  Pretty incredible little rig, congratulations, Farhan!

As it comes, the uBitx is set-up to use a condenser microphone, but it doesn't quite have enough gain to be used with my beloved Astatic 10D.  Not wanting to have yet another microphone on the desk, I adapted the mic amplifier circuit that I used in the SR-16 for the uBitx. 
The circuit is based around the common LM-324 quad op-amp and will work equally well with pin-compatible chips like the TL-074 or TL-084.  The circuit is installed between the dynamic mic and the input to the uBitx, where the mic bias voltage provides the VCC for the op-amp.  I installed it in the rig, though it could just as well be installed in the base of the microphone, provided space exists. 

The circuit itself is pretty straightforward: The first stage is a typical inverting amplifier with about 20 dB of gain.  This feeds the limiter stage, which clips any positive or negative signal peaks exceeding about .7V.  The limiter is also inverting, so the signal at its output is in-phase with the input signal from the microphone (not that this is important...) 

The limiter is followed by a low-pass filter to remove any high-frequency artifacts created in the limiter.  The "knee" of the filter is around 3KHz. This is followed by a buffer stage to provide a low-impedance output to drive the rig. Resistors R4 and R5 form the voltage divider used to provide virtual ground (1/2 VCC) for the first two stages. 

The output of the buffer probably looks odd, what with the two caps and all.  What I'm doing there is creating the equivalent of a 2.35uF non-polarized cap.  This is probably unnecessary; thinking about it,  C9 could be eliminated completely because the DC voltage at pin 14 will always be lower than the uBitx's mic bias voltage.  But, lets ignore that for now.  The DC bias voltage is routed, through R10 to capacitors C2 and C3, which remove any AC from the DC before applying it to the LM324.  Resistors R4 and R5 form the voltage divider used to provide virtual ground (1/2 VCC) for the amplifier/limiter stages, the filter and buffer are "self biased" to 1/2 VCC by the output of the limiter. 

I optimized the gain to suit my 10D/D104 mics, and as such, really have to eat the microphone before it sounds nasty.  This is born out by the reports of "excellent audio" from the 40m SDR police, so I'm satisified with its performance.  If, however, it doesn't provide an appropriate amount of gain for a different mic, that can be adjusted by changing the value of R3 as the gain is determined by the formula AV = R3/R2.  The reason I say to adjust R3 instead of R2 is because R2 pretty much determines the input impedance of the circuit, so you want to keep it around 500-1K.

Friday, February 8, 2019

Homebrew WAS

Could this be the first FT8 WAS earned using only QRPish (10 Watts) scratch-built homebrew gear?  If not, it's got to be a very small club.
I'm not much into paper chasing, but see the appeal of it and would love to see the ARRL (and other amateur radio organizations) recognize our accomplishments in the realm of homebrewing much the same as they do contest results and DXCC stats. 

After several years and moderate success as a DXer and contester, I was totally burned-out until I added the homebrew dimension.  How many more of us are out there?  Certainly, I can't be the only one.

ARRL, CQ, etc.; Are you listening?

Thursday, December 6, 2018

The Trans-Oceanic Project

If you're reading this, there's a pretty good chance that you've got a soft spot for Commander MacDonald's baby: The Zenith Trans-Oceanic.  I certainly do: When I was about eight years old, my family took holiday at a cabin on Hubbard Lake in rural Northern Michigan.  At that time, there were no TV stations covering that portion of the state, but the cabin came equipped with a glorious model G500 Trans-Oceanic, and the sounds that came from it made enough of an impression on my pre-adolescent psyche that I became obsessed by all things electronic.  Eventually, this hobby became a career in radio from which I recently retired after close to 40 years.  Soft spot indeed.

Earlier this month, while on vacation in Florida with my wife and friends, I came across this very neglected model G500 at an antique festival.  The picture above really doesn't do it justice - this thing was totally trashed; the famed "Black Stag" covering was torn in several places and appeared to have been gnawed on by mice, as had some of the wiring.  The hinges for the rear panel had rusted completely through, the cover being held in place only by the rusted spring clips.  And the smell... Trans-Oceanics have a unique, friendly aroma, but this was a different fragrance: Mildew, rotted wood and mouse pee. 

But, the set appeared complete, including the coveted 1L6 converter tube, and the seller was willing to deal, so we agreed on a very low price (a fraction of what the airline charged me to check it as baggage) and the derelict became mine.

Things didn't get any better when I returned home and got the set on the bench and found that someone who should not be allowed anywhere near a soldering iron had attempted to repair the radio.  The underside was covered in soot; the power-supply section had lost its magic smoke in a big way.  My heart sank thinking about the rather delicate 1.5v tube filaments.  Had they also been damaged?    If so, my equity in this set would be in the red.

Fortunately, when I measured across the filament string, I got a nice, low resistance.  Holy crap, had the tubes survived?  I had to know!  So, I started undoing the previous work to see what all was wrong.  

My theory is that someone attacked the set based on "internet knowledge" that says selenium rectifiers are ticking time bombs, but without the understanding of what the rectifier does or how to replace it.  Equipped with this lack of knowledge, they bypassed the selenium with a silicon diode.  That probably went OK, but things went awry when they got to the filter capacitors... I'm not sure exactly what went wrong, but it's obvious that something was dead-shorted to ground at the input to the "candohm" resistor, vaporizing the diode and dropping resistor but not damaging anything else.  So, while it must've gone up in a glorious shower of sparks and smoke, the carnage was surprisingly minor.

Ironically, the selenium rectifier checked good, so the carnage was totally unnecessary.

Carnage removed.

New parts, including 1N4007 diode, mounted to added terminal strip.
Once the damage had been, um, rectified, the rest of the electrical restoration was uneventful.  The electrolytic and "Bumble Bee" capacitors were all replaced, as was the 3V4 audio tube, and the set roared to life.  Success!

But, cosmetically, the radio was a mess, and the only way to deal with it was going to be to strip it of it's upholstery, repair the damage, and recover it with new Tolex.  I wasn't looking forward to this part; I've seen reupholstered TOs before and you could immediately tell that they weren't original by the grain of the Tolex.  So, I scoured the web until I found a pattern that, at least in the photos, looked passable, so I ordered a couple of yards of "Black Bronco" from Parts Express.  When it arrived (in a very large box!), I was stunned - it was nearly a perfect match!

Thus, I set about the task at hand, photo-documenting every flap and fold of the old material as I removed it, and using each piece as a pattern from which the new pieces were cut.  Then, each piece was reinstalled exactly as the original had been by the factory.  Well, all but one - I goofed.  Still, it looks OK so I may leave it.

In addition to replacing the fabric, I polished and relacquered all the brass bits and installed a replacement dial lens (from Mark Palmquist - Excellent!), gave the set a final alignment and buttoned it all up.  

 Was it worth the effort?  Not economically, but I can't put a price on the rewarding feeling I got from bringing this beauty back from the brink. 

I'm still as enamored by these radios as I was back in 1970, they really are fantastic pieces of radio history.  It's unfortunate that so many international broadcasters have abandoned shortwave, but there's still a lot to tune in and nothing better to do it with than a Zenith Trans-Oceanic.

For anyone interested in learning about these sets, I highly recommend Bryant and Cones' book The Zenith Trans-Oceanic: The Royalty of Radios  from Schiffer Publishing.  It may be out of print, I don't know, but used copies are readily available through Amazon.  But please, don't be like the previous owner and attempt to service your TO without understanding what you're doing!

Monday, October 29, 2018

Philco 42-327T: Closure

It's been a couple of busy months, but the old Philco is done.  Once I repaired the damage I inflicted on it by foolishly lifting the cabinet by one of the grille bars, everything went together rather smoothly and it turned out quite nice.

I don't think I mentioned the chassis in any of my earlier posts, but it's rehab was relatively uneventful, mainly because nobody'd tried to fix it before I got my meat hooks on it.  So, it was simply a matter of replacing the electrolytic and paper caps, cleaning and lubricating the controls and tube sockets, and a final touch-up on the alignment.  The only thing that put up any sort of fight was the pushbutton presets, but that was just because they were full of crud and took some extra cleaning before they worked reliably and noise-free.

Performance wise, it's competent but not spectacular, as one would expect from a mid-low end radio.  The internal loop antenna works OK for the local stations, but it really comes alive when connected to an outdoor wire antenna, particularly on the shortwave bands. 

So, was it worth the effort?  Probably not from the financial perspective, though I don't have but maybe $50 invested in the thing and shouldn't have much trouble breaking even.  From the standpoint of personal entertainment, though, it was more than worth it.  Breathing life into a radio that's been neglected and dormant for decades is almost as much fun as building one from scratch!

Friday, August 17, 2018

One more time...

I made a stupid, bone headed move shortly after the last entry in this saga:  I was cleaning up and, rather than make multiple trips, carrying stuff from the garage to my basement workshop, tried to take it all at once.  So, I had my arms pretty well full, and grabbed the newly finished, nearly perfect cabinet by one of the grill bars (removed in the above photo) and it broke off.  Of course, it couldn't have broken off cleanly at a glue joint.  No - the glue bond holding that bar to the veneer was stronger than the veneer was bonded to its substrate, so about a three inch section of veneer peeled away and remained attached to the bar.  Crap!

I made a couple of failed attempts at "spot repair" before stripping it down to what you see above.  The good news is that the veneer patch turned out great - probably the best I've ever done.  The bad news is that I'm back to ground zero and into the second week of a project that should've taken just a couple days.  Oh well.

Haste makes waste.  The good news is that, this time, my lazieness only cost me some time; the damage is repairable and it's not a total loss.  The bad news is that, once this project is a memory, I'll probably make the same mistake again.  That's how I roll.

Thursday, August 9, 2018

Almost Instant Gratification

Once the grain filler has cured, I give it a wet sanding with #400 grit.  After cleanup, the cabinet is nearly perfectly smooth.  If this were a more valuable set, I'd probably repeat this step until it was absolutely perfect, but this is a pretty common radio and the law of diminishing returns applies.

This set had what I'd call a 2 1/2 tone finish; the dial escutcheon and grille bars look to be neary black, while the trim along the bottom was a very dark brown.  I don't know if that was intentional or the result of nearly 80 years, but I'm going to stay as true as I can to that.  So, after sanding, I mask all of the cabinet that isn't going to be black or dark brown:

Then, before I apply the "color", I give everything a quick blast with clear.  I discovered, by accident, that this will seal the masking lines and helps avoid the color leaking in to where I don't want it.  

Being a low-budget operation, I don't use any sort of computer to match colors, I just have a variety of dyes in shades of brown, red, yellow and black, and match by experimenting, noting the how many drops of which go in to how many ounces of lacquer and the ratio of the lacquer to thinner.   Once I get a test spray in the tone I want, I note the "formula" for the next time, but I have to admit that I don't often refer to my notes anymore; I've done enough to where I "kinda" know what'll work.

Since this has the two dark colors, I first mix six ounces in the dark brown and spray away, two coats gets me to the color I want.  That barely uses an ounce, so not being one to waste material, I add black dye to the mix until it's, well, black, and spray the escutcheon only, being careful not to spray the black on to the fresh dark brown. 

Lacquer dries quickly, so it isn't long before I'm able to remove the masking and start adding color to the rest of the cabinet.  I mix the dye with the lacquer as before, but not adding so much dye that it becomes opaque, I go for a sort of weak coffee looking mix that takes a few coats to get to the final color.  This is where the instant gratification sets in, because as soon as that first coat goes on, the thing starts looking more like an old radio and less like a pile of junk.

And that's enough for tonight; clean the tools and put 'em away.  

It's amazing, with these projects, how much time you spend waiting for things to dry.  Actual work time spent so far?  Maybe a couple hours, but it's spread out over five or six days.  Anyway, I suppose I'm commited deep enough into this radio that I HAVE to fix the chassis... That'll be another installment in this saga.

Sunday, August 5, 2018

More Cabinet Work

Work on the Zenith continues, and I'll detail that in the future.  For now, I thought I'd take a diversion and go through the steps of refinishing something less ambitious: A derilict Philco 42-327T that's been in my way since the spring.

Unfortunately, I didn't start the project with the intention of "blogging" about it, so I didn't take any "before" pics, nor any of it being stripped.  I think I can describe that, though:

The finish on the cabinet was quite rough, appearing as though it had been left out in the rain and then dragged, face down, across concrete.  Fortunately, the grille bars took most of the damage and the veneer was largely spared; other than a few chips and a bubble or two, it was pretty much intact.

So, to start, I carefully removed the grille bars, stripped them, then sanded the damaged sections while trying to maintain their original radius.  At the same time, I chemically stripped the cabinet, patched the damaged veneer and glued-up a couple of loose joints, clamping everything and allowing it to dry overnight.

This morning, I gave everything a "scuff" with 220 grit paper, then took it out to the garage and shot it with sanding sealer.  The pic below shows the cabinet and grille bars with the sealer just starting to dry.  The white portions are where I shot the sealer thicker than I should have, some of which was intentional.  These will become transparent as the sealer cures.

I'll let this dry overnight.  Tomorrow I'll tackle the fun-filled process of filling the pores in the wood and prepping for it's lacquer finish, which, once completed, will be as near to glass-smooth as I can make it.

Part II: Filling the grain

Well, normally I'd have let the sealer dry overnight, but with the temperature in the garage hovering around 100 degrees F, it dried very quickly and was ready for the next step in just a few hours.

At this point, the grain and pores have been sealed, which basically means that the wood isn't going to drink the lacquer that I'll eventually be applying, but the wood still is far from smooth, and if the top coat were applied at this point, each pore would "sparkle" as light reflected from the tiny pond of lacquer within it, making the surface appear rougher than it actually is.  The easiest way to avoid this is to use a commercial grain filler.

Now, grain filler is NOT the same as the wood filler sold at the big box stores - that stuff is to patch gouges and larger blemishes.  Grain filler is a bit different, it has a consistency that's a bit thicker than pancake batter, and is actually rubbed into the surface of the wood rather than brushed on to it.  Generally, you apply it against the grain with a plastic scraper, I find old credit cards work great for this.  Some people will rub it in with something like burlap.  I don't know about you, but I'm not exactly swimming in burlap, but have a seemingly endless supply of expired charge cards.  Like they say in amateur racing: Run what'cha brung.

Before I apply the grain filler, though, I sand the cabinet again using 220 grit to remove any excess sealer.  Since we're ONLY sanding the sealer and not the wood, I usually go across the grain at this point; I think it works better, but your mileage may vary.

After sanding across the grain with 220 grit.
After sanding, I scoop a glop of the grain filler from the container and push it across the grain with the aforementioned credit card.  In this case, I used a Discover that expired in 2005, but I don't think that matters.  What does matter is making sure that you actually press the filler into the pores rather than dragging it across them.  Think of buttering toast, where you're not so much spreading it across the surface as filling the voids with artery-clogging goodness.  Same basic idea, but with one difference: You don't want to leave any excess - the stuff dries like iron - so the motion is  a push and a scrape... Hard to describe, easy to do.

After filling the grain.  Notice that the surface has become reflective, it'll be more so after I wet sand it later.  The tub in the foreground is the grain filler, and, of course, my long expired Discover card.
A couple of comments about grain filler:  There are basically two types: Oil based and water based.  Some folks say that oil based is easier to work with due to it's longer drying time, but I've used both and haven't noticed a difference.  Expirement.  Also, I'm using a transparent filler here (Acqua Coat), but I've also used tinted and tinted my own to suit the type of wood I'm working with.  Again, expirement and see what works for you.

Also, I can't mention enough that you want to scrape off as much excess as possible before the filler dries. We're not going to be using any more coarse sandpaper, and you can develop tendonitis trying to sand smooth filler boogers with 400 grit...

So, now we're back to the exciting practice of waiting for stuff to dry.  Don't rush this step, else you'll have a mess on your hands when you start wet sanding, which is the next step.  Riveting stuff.  Stay tuned!