Common-Mode Hum Issue With VRX-1 Fixed and YouTube Video Posted

In the last post I mentioned a hum issue that I’d been having with the VRX-1 that I bought as a kit from 4SQRP. When I related that it only happens on connecting the antenna, Jason NT7S told me it was probably common-mode hum. For a better description, refer to EMRFD, but my layman’s understanding is that it is caused by signal from the local oscillator being radiated by the antenna, picked up by AC wiring in your house, and re-radiated with the 60Hz component from your AC supply imposed on it (which the receiver then picks up).  The solution is to prevent the receiver from radiating, and an RF pre-amp can do this.

The first place I looked for a suitable schematic was Todd VE7BPO’s site and lo and behold, I found one (not – this link no longer works, as the preamp was on Todd’s old site).  It uses a J310 JFET in common gate configuration.

I’m feeling the need to make some kind of excuse for the messy look of the circuits I have built Manhattan style. Part of the reason is that I’m not as disciplined with my building technique as are masters of Manhattan like Jim Kortge K8IQY and Chuck Adams K7QO. The other part of it is that I think I need to recalibrate the way I look at my projects and reign in my OCD tendencies. With an experimental project like the VRX-1, it was inevitable that bits would get added on, and it would be a work in progress for a while.

Here’s the completed RF pre-amp board:

And here it is installed in the VRX-1. It cured my common-mode hum problems beautifully, by the way:

If you’d like to hear how it sounds, I created a YouTube account for my ham radio shenanigans, and the video of the VRX-1 is here:

Advertisements

First Forays Into Manhattan Construction – Crystal Oscillator and a Peaked Lowpass Filter

With the exception of NT7S’ VRX-1 receiver I’ve had no experience with Manhattan construction. Once the majority of my recent flurry of parts orders came in, I figured it was time to start building a few things Manhattan style, and to start with some simple circuits.

I found a good supplier of PCB material. His eBay username is abcfab. He has a lot of PCB listings – different sizes, thicknesses and materials, and based on the shipment I received from him, it’s good stuff. The cuts were clean and the cut corners were square:

Although I wanted to make my own Manhattan pads, I saw W1REX’s MeSQUARES on the QRPMe site and was curious to try them. They arrive as one sheet of 300 square pads. The material is scored so that you can easily break the pads off with a pair of pliers:

The material these pads are made from is fairly thin (about .03″) and I would normally be concerned about the possibility of shorts to ground with thin pads like this, but Rex has designed them in such a way as to minimize the possiblity of that happening – the square pad of tinned copper on top of the pad doesn’t extend to the very edge – there is a small margin around the tinned area.

The pads are easy to use.  I wanted to put something simple together to try them out, so I built a crystal oscillator in an Altoids tin:

If I were building a more complex circuit in a tin, I wouldn’t build it directly onto the base, as it can get tricky holding the soldering iron at a low enough angle to make the joints. It’s definitely not the way to go if you need a fairly decent component density. I like the MeSQUARES, but wish they were available in smaller sizes too for circuits requiring greater component density. A few days later, I ordered some MePADS which is the same thing, but for mounting IC’s. I’d show you those, but they haven’t arrived yet.

OK, onwards and forwards (or however the saying goes).  I built NT7S’ VRX-1 Direct Conversion Receiver for 40M a while back. It worked well and could hear plenty of stations, but I must admit there was a fair amount of hum.  This is a common problem with simple DC receivers and I’m not sure if it has something to do with my layout, or perhaps whether something I did with the wiring caused a ground loop. (Perhaps feeding the final audio amp with a balanced input wold help eliminate some of it?) After completing it, I put it aside and made a mental note to come back to it one day. Incidentally,  KE7GKM has been making lots of QSO’s with his homebrew station using a VRX-1 on the receive side, so it must be something to do with the way I’ve laid out or wired my circuit. The other thing about simple DC receivers like this that my inexperienced CW ears would benefit from, is some audio filtering.  It seemed like a good excuse to build the active peaked lowpass filter that was designed by W7ZOI and featured on VE7BPO’s site. I built it on a scrap of PCB from Dan’s Small Parts and Kits. While the pieces of board from abcfab on eBay are cut nice and square, the small pieces from Dan’s are not. This is not necessarily a problem if you’re using them to build circuits on, but will require a bit more work if you’re using them to build enclosures. Here’s the completed filter:

A week or two ago, I made a couple of hundred 3/16″  (about 5mm) pads and tinned them all. Now I’m finding that they are rather large, and I’ll most likely only be using them for connection points that have a lot of connections. The trimmer pot nearest you is mounted on these 3/16″ pads. They looked small when I made them, but once I started to use them, they were huge! As the MeSQUARES hadn’t turned up yet, I mounted the NE5532 chip on pads punched from a 3/32″ die (just under 2.5mm).  By bending out the chip leads and trimming them to different lengths, and staggering the pads, managed to do it without any of the pads touching. Pins 6 and 7 were connected together, so I soldered them to one bigger pad. You can get a slightly better view of the way I mounted the IC from this picture:

The two trimmer pots are for adjusting the resonant frequency and Q of the filter (if you take that adjustment too far it starts to ring).

Here’s the board installed on the back wall of my VRX-1:

It is certainly filtering the audio, but of course I still have the hum. Although the filter peaks gently at the frequency set by the trimmer, it is still basically a low-pass filter. I could use something to notch out the 50c/s hum. However, there is so much of it that I think what I really need is to figure out the way I’ve laid out and wired the receiver.

EDIT: I only get hum pickup when an antenna is connected. I just walked out onto my balcony, connected an antenna and the hum was much lower in level than before. I don’t know why I didn’t think of this before, but I live in an old house with substandard wiring and no electrical ground (just the 2 wires at every socket.) I wouldn’t mind betting that if I were to walk out onto the street, the hum would disappear almost completely.

The box that I put the VRX-1 in is one of a really useful line of enclosures from LMB Heeger. Look at this shot of the receiver all cased up:

See the way that 4 little lugs are formed from the top of the case? They stick down just a little and engage with the front and back panel, giving extra rigidity to the enclosure. The aluminum is 16 gauge (0.0508″). The larger sizes aren’t quite as rigid, but the smaller ones like the one above, are, and would be ideal for a VFO or VFO-controlled rig. The above case is 2″ high, the front panel is 4″ wide and the case is 4″ deep. It is LMB Heeger’s Model 143 in their interlocking series, available in crinkly black, painted grey (like the one you see above) and plain aluminum (for that homebrew look). I think Digi-Key carry them, but you can order them directly from LMB Heeger. I’m thinking that a modular station built in these little cases (plain aluminum finish) would look neat. VFO in one, freq counter in another, mixer in another etc etc. Hmmm……

That’s it for now.

No Commercial Rig In The Shack For Now And All Sorts Of Homebrew Plans

I sold my FT-817 just days after mentioning 2 posts ago that it was up for sale.  After a week on Craigslist, I was planning on putting it up in a few of the online ham forums and then on eBay. It didn’t come to that though. In fact my rig sold not because of Craiglist, but because of the post here on WordPress.  I didn’t know that many folk read this blog!  Shortly after posting here that the 817 was for sale and including a link to the Craiglist ad, I received a call from Frank KA8SYV, saying that he was interested, and courtesy of Paypal and Fedex, the deal was finalized. I’d assumed that he found me on Craigslist but then remembered a remark he’d made during the phone call in which he complimented my large CD collection. The only way he could have known about that was if he’d seen this blog first before following the link to the Craigslist ad to get my phone number.  Hope that’s not against WordPress rules 🙂

The day that Frank received the radio, he called to let me know, and we had a great conversation. Frank’s been getting back into the hobby in the last couple of years, and his enthusiasm is infectious.  He told me about a great little 80M indoor loop that he built (which he forwarded details on) and it turns out that he has the kit to build K8IQY’s SS-40 receiver.  I’m envious, as I have really enjoyed the receiver on Jim’s 2N2 transceiver and the SS-40 is also reported to be low-noise. It was just great to converse with a fellow ham who has similar interests to mine – thanks for the phone QSO Frank.  I knew that Frank would be a trustworthy buyer – not only is he a fellow ham who can build things, but he also has a picture of a kitty in an astronaut suit on his QRZ page.  He just had to be cool!

As a result of that sale, the band coverage here in the shack is a little slim, but that’s fine by me.  I have the Tut80 on 80M, the Norcal 2N2 on 40M, and NT7s’ VRX-1 direct conversion receiver – also on 40M. My 80M activity is sparse, due partially to the fact that my only antenna is a co-ax fed inverted vee for 40, so 99.5% of my activity is on 40M CW.  Most amateurs would consider this a very meager ham-shack situation, but it’s working out well.  I never have to try and figure out which band to operate on, and I spend zero time adjusting the ATU. The 2N2 is always on so when I want to operate, I just turn the volume up on the rig.  Also, the lack of a commercial transceiver is a very good incentive for me to build equipment for the other bands.  I like what not having ready-built gear does for my creative juices.  If I want to go on 20, I have to sit down and figure out what to build for that band, as I’m not about to buy another commercial rig just yet.

The plan here at AA7EE is to spend more time building and experimenting.  I don’t design circuits, but I have enough gumption to build from schematics, and have been wanting to get lots of practice in Manhattan construction for a long time now, so the plan going forward is to build stuff. With that in mind, I spent a day or two putting in a big order with Dan’s Small Parts And Kits, as well as a small order for toroids and some transistors from W8DIZ –  “The Toroid King” (how can you say no to 50 x 2N3904’s or 2N3906’s for $3?)  The goal is to have a big stock of the most commonly used components, as well as enough PCB material to make Manhattan pads and build cases. Once I have a well-stocked junk-box, I can see what I’m using from it and order ahead to keep it stocked up. Hopefully, most projects will only require me to order just a few parts, with the rest coming from the personal stash of parts.  As a kid I had a big junk-box of parts, but it consisted mainly of donations from local and benevolent hams with the contents of a few self-purchased “grab bags” thrown in. I’ve never purposely ordered bulk amounts of all the common resistor and capacitor values and types, along with quantities of commonly-used semiconductor devices. What pleasantly surprised me was that (as long as Dan comes through with this order, and it’s good stuff) if you look for deals and buy from the right places, it needn’t break the bank.

I can’t wait to have my own personal arsenal of resistors, fixed capacitors and trim caps, diodes, transistors, pots, toggle switches, DBM’s (Dan has ADE-1 DBM’s for $4.50 each), ICs and who knows what else. The teenage me would have been so excited to know that I would one day achieve heaven on earth 🙂

Once everything arrives, here’s the plan of action:

 

* Build an active bandpass filter for NT7S’ VRX-1 DC receiver. I’m excited to see what what Jason’s receiver sounds like with audio filtering

* Build a regen receiver for 80M so that I can listen to the weekly West Coast AM net – and maybe mod it for 40M

* Consider building a QRP AM TX for 40, which can later act as an exciter for a linear amp (you think I’m crazy enough to try AM QRP on 80 and 40?)

* Build K8IQY’s 2N2-20 Manhattan-style

* Remove the Tut80’s polyvaricon and modifiy it for tuning with a varicap, so I can use a 10-turn pot, then put the Tut80 in a more stout case made of PCB material

*Lots more things.

 

The above are just ideas and we’ll see which ones come to fruition and which ones remain in my head, but I think I have plenty to keep me occupied, especially considering the fact that I work at a snail’s pace (solder one resistor, take a swig of coffee and look out the window, solder another resistor, take another swig of coffee and play with the kitty etc etc.)

It’s going to be so much fun!

Software Defined or Hardware Defined – Which Way to Go?

The main project going on in the AA7EE  “shack” (for which read “main room of my studio apartment”) has been, and will continue to be, the monumental task of committing my sizable collection of cassettes, CDs, DAT tapes and broadcast carts to backed up hard drives.  This is an ongoing project which I expect to take several years. Over the course of a 22 year career as a DJ, I amassed a lot of stuff that is becoming wieldy and expensive to lug around, so it’s time to start consolidating.

So whenever I’m listening to the radio or soldering something, I’m often also ripping a CD, scanning the artwork, or calling up my friend Antoinette and trying to give her as many of the CD’s that I just ripped as possible. I spent 22 years accumulating stuff (opens in a new browser window) and I now want a good portion of it gone (thanks Antoinette!)

All that aside, the amateur radio goings-on here have included building a neat little direct conversion receiver for 40M – the VRX-1 designed by NT7S and sold by the 4SQRP club. It’s a cool little DC receiver.  I’ve been having a bit of a problem with the input bandpass filter, so it’s on the back burner for a while, but it’s been a fun Manhattan building experience:

NT7S’ fun DC receiver got me all charged up.  The first successful TX/RX I ever built was an 80M DSB TX/RX designed by G4JST and G3WPO and published in the UK magazine “Ham Radio Today” in March 1983. It utilized a direct conversion receiver, to which I added an audio filter built from a 741 op-amp.  It was my first experience with DC receivers, and I remember being surprised that such a simple receiver could sound so good. Jason’s VRX-1 re-introduced me to the pleasures of DC receivers, as well as the technique of Manhattan construction (my first time), so by the time I’d finished construction, I was all primed up and ready to swoon at any direct conversion receiver that might flit it’s tail feathers at me.

John AE5X’s post couldn’t have come at a better time. Allow me to repost this picture of the 80M direct conversion TX/RX that the Arizona ScQRPions will be providing a kit for very soon:

The details are here. I was beyond excited when I found about this (thanks John).

I have a mental list of several different QRP rigs that I want to build, among them the Weber Dual Bander, and either the Elecraft K1 or K2. However……SDR has been on my mind too recently.  I built the SoftRock Lite II for 40M a few months ago and was impressed with the performance for such a simple piece of hardware, and low price too.  Of course, the simplicity of the hardware and the low price is a bit misleading because the signal processing is all done in software.  This is the beauty of SDR though – if a better demodulator or filter is available, you just download it.

I’ve been using the SoftRock to monitor the CW portion of 40M, and then once I see a signal, I can work him on the main rig, which is currently a Norcal 2N2/40.  Yesterday, I built a combined switch and dummy load into an Altoids-type tin so I can easily accomplish switching the antenna between the SoftRock and the 2N2/40.  The 2N2/40 is a cracking little rig – a sensitive low-noise receiver with a stable VFO (after the intial warm-up period).  It has a nice narrow crystal filter too which is great for working people, but not as convenient when you’re trying to find stations to work.

So….what I do is use the SoftRock to look at a wide portion of the band.  With my soundcard, I can look at a 96KHz-wide slice of the band on my screen, and the minute I see a station, flip over to the 2N2/40 and work him. It works well but it got me to thinking – why switch over to a traditional radio to work a station that I find with SDR? Why not just get an SDR transceiver and avoid having to switch over to a hardware defined radio?

Hardly original thinking.  I’m sure it’s the same thought process that has led many an amateur to adopt an SDR rig as their main station radio. FlexRadio are about to introduce their Flex-1500, which is a 5 watt all HF band SDR transceiver.

Mighty tempting and with my limited amateur radio budget, I’m now wondering whether to continue building all the QRP transceiver kits I’ve had my eye on, whether to build the SoftRock v6.3 HF TX/RX, or whether to go for broke and get the Flex-1500 when it comes out.

In the meantime, I have a KD1JV Digital dial on order, which will turn the Norcal 2N2/40 into an even more usable little radio.

Oh, and I have 1,000’s of CD’s to rip and perform hi-res artwork scans.  It’s not as if I’ll be sitting here twiddling my thumbs.