The solder has melted! SW40 begun

Audio amplifier portion of board and scope probe

Audio amplifier portion of board and scope probe

It’s been almost a year before any real projects have been undertaken.  Finally, I dug out the SW-40 board about 2 weeks ago and began to play.

To refresh, this is a kit I bought back in the early 2000s from Small Wonder Labs, Dave Benson’s now shuttered kit company.  I’d already built a Rockmite 40 and an SW-20 and had a blast.  I’d just begun to understand the stages of the circuitry in these kinds of kits.  But I ran out of motivational gas and time and the 40 got shelved.  For a very long time!

I’m following Dave’s installation order so far, and installed the 78L08 8 V voltage regulator first.  I had to remember where all my hookup cables were but I managed to fire it up and measure spot-on 8V for a 12 V input.

Distorted 10 V p-p 660 Hz output signal from 1 V p-p input to op amp

The audio amp was next.  Dave used an NE5532 op amp chip for this purpose.  I injected 1 V p-p @ 660 Hz into pins 2 and 3 of this chip, in a haphazard manner, first passing through the installed capacitor and resistor network on Dave’s board.  There was a lot of AC hum and you can see the waveform reflects distortion.  I discovered, to my dismay, that it was coming through the power supply leads.  This is a problem I’ve had before with this supply.  But I’m also getting it from the leads coming out of my signal generator.  I put the circuit on batteries and used my iPhone as an audio source and the output of the amp cleared up nicely.

I recall having more trouble with AC hum pickup from Dave’s other kits as well.  The more recently completed Bitx-20 kit does not have this problem.  I may need to investigate a way to clean this up.  I know the hum isn’t there with my more pricey Astron supply.  But for now, at least I know the amp is working, and giving me 10x voltage gain (20 dB).

I tried to determine the theoretical gain from the circuit as drawn but my knowledge of op amps is more than rusty.  I suspect it’s a very simple ratio, but the app notes and info on the web only show very complicated circuits for audio equalizers and mixers and various audio equipment.  Maybe I’ll dig through my textbooks and figure it out.  Or, more likely, I will not worry much about it and move on.  If I ever desire to use this circuit or one like it for my own design though I’ll need to understand it better.

In other news, I ordered another enclosure like the one I used for the Bitx-20.  That one came from Hendricks kits, which has since changed to “Pacific Antenna”.  Although not currently listed on their webpage, the enclosure box is still being sold separately from the Bitx kits.  James, KA5DVS, at Pacific Antenna was very courteous and assembled and shipped everything to my QTH.

I really liked this little box and the accompanying KD1JV digital dial frequency counter display, which fits nicely into pre-cut holes meant for that display.  I decided this would be perfect for the SW40 kit.  The pair set me back a bit price-wise, and I’m sure there are cheaper ways to go.  In the future I’ll need to solve my boxing problems a little more cost effectively.  But for now I’m pleased with this.

Blue Pacific Antenna box and KD1JV digital dial in bag

Blue Pacific Antenna box and KD1JV digital dial in bag

For me, 40 m is really the king of the ham bands for CW operating.  So I want this rig to be one that gets used a lot.  In the future I may well have multi-band rigs which will make many of my current ones obsolete.  But given constraints on my shack time of late, I think I may be relying on this one for awhile.  So, I’m happy it will be in a good enclosure with a decent readout.

Probably nobody else is building these older SW kits now.  On the off chance that someone is, these upcoming blog entries might be of interest to compare notes.

I’m thrilled to be at the bench again.  I’ve got my 6 m running on my Kenwood TS-2000 in the background and I’ve snagged a few grids in the past month as well, as people break the squelch on the 50.125 watering hole, alerting me to an opening.  At the same time my homebrew 40 m receiver, the Mrad-40, is busy playing SSB rag chews, often on antenna topics (I recently learned what a “BOG”, or Beverage on the Ground, antenna was).  I find that soldering and measuring and building things goes better with that background accompaniment, along with of course the “beverage” of choice: coffee.

More to come soon, stay tuned.

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