Dave Richards AA7EE

July 1, 2011

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.

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8 Comments »

  1. Dave, your hum problem really bothers me. Perhaps you could loan me your VRX for a bit so I could poke around in it and see what’s going on?

    Comment by Jason Milldrum — July 1, 2011 @ 11:54 pm | Reply

    • Jason – I’m reticent to take up any of your time, when you don’t have that much time to spare. I know the problem is something that I’m doing and not something inherent in your design. We should talk on the phone sometime (or I guess we can keep doing the Twitter/e-mail thing!)

      Comment by AA7EE — July 2, 2011 @ 12:20 am | Reply

  2. Jason – I edited the post. The hum is only apparent when an antenna is connected. I also just noticed that when I walk out onto my balcony and connect about 30 feet of wire as an antenna, the hum is much, much lower in level. I live in an old house with substandard wiring and no electrical ground – this may have a lot to do with it.

    Comment by AA7EE — July 2, 2011 @ 1:24 am | Reply

    • OK Dave, that makes some sense. I had thought of asking you to terminate the antenna port in 50 ohms resistive and tell me if it was still there, but I didn’t want to insult your intelligence! But years of troubleshooting for a living has taught me to check the obvious stuff, even if you think you are sure that’s not the problem.

      I would recommend that you read EMRFD pages 8.8 – 8.9 on Tunable Hum. I think this may be what you are experiencing (I’ve known that the very simple design of the VRX-1 is susceptible to these types of issues). If you really want to be sure, you could breadboard that hum probe in Fig 8.16.
      (BTW, EMRFD Chapter 8 is probably the best and most comprehensive reading on DC receivers that you will ever see).

      If the problem is tunable hum, then I think that a preamp would go a long way towards solving your problem, by isolating the LO from the outside world). I would try a simple low gain JFET preamp on the front end to try to kill the hum.

      Good luck!
      Jason

      Comment by Jason Milldrum — July 2, 2011 @ 6:15 am | Reply

      • I should have been more specific about the preamp. It should be a common-gate JFET amp. Tunable hum is caused by LO leakage out of the antenna port. The VRX-1 is susceptible to LO leakage due to the simple single-ended mixer. Because the common-gate amp has excellent reverse isolation, this goes a long way towards taking care of the LO leakage problem.

        Jason

        Comment by Jason Milldrum — July 2, 2011 @ 6:51 am

      • Thanks Jason. I found a great common-gate JFET pre-amp circuit on VE7BPO’s site, and I think I have all the bits for it. Taking the weekend off, but this should be an easy thing to try out in the next few days.

        Comment by AA7EE — July 2, 2011 @ 5:12 pm

  3. I admire your great invention looks real neat…..Great Job!!!

    Comment by peter — August 15, 2011 @ 8:23 pm | Reply

  4. Nice job!

    73s, David F4EPU

    Comment by F4EEPU — April 5, 2014 @ 2:41 pm | Reply


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