Dave Richards AA7EE

February 9, 2010

The Fort Tuthill 80 – A Direct Conversion Transceiver For 80M

A week ago, the Arizona ScQRPions released their Fort Tuthill 80M TX/RX kit. I’d been keen to build it ever since I became aware of it (thank you AE5X) due to a long standing interest in DC receivers, and the fact that the first transceiver I ever built was also for 80M and also utilized a DC receiver. That was back in the early 1980’s.  I brought that rig with me to the US when I moved here from the UK in 1987 and then fried several of the active devices in it by accidentally connecting the power the wrong way round.  Yes I know – for some reason I didn’t want to use a diode to polarity-protect it (probably didn’t want to lose the 0.6V forward-bias voltage drop).  At the time I was going through a phase of my life in which I was distinctly uninterested in amateur radio, so rather than replace the fried devices, I threw the whole thing out and somewhat regret that decision to this day.  I don’t remember it suffering from any of the problems normally associated with DC receivers – microphony and hum pickup. However, this could very well be a case of my looking through the past at rose-tinted glasses.  As it was the first transceiver I had ever built, it was the apple of my eye, and well, proud parents can be quite good at ignoring flaws in their offspring.

OK, here’s what you get for your $53.  The silvery bag contains the active devices, and you also get 2 sets of decals:

Considering the amount of work that goes into designing something like this, designing the PCB, sourcing and ordering all the parts, as well as bagging them all up ready for delivery to you, the end consumer, $53 is a steal. Dan N7VE, the designer of this transceiver, knows a great deal about active filtering in receivers and has applied his expertise and knowledge to the design. Look here for a presentation he gave on the subject of active filtering in receivers.

A closer look at the board that Dan designed for this radio:

One of the many great things about the internet is that manuals for kits can be more detailed, with more pictures than ever before.  Dan’s manual makes building this little radio a lot of fun, and with the help of the Yahoo Group, expert advice from the designer, or other builders, is not far away.

The build went smoothly.  As tends to be the case with these things, I stayed up all night to finish it off and ended up finally hitting the hay at 9am.  I find that it’s easy to get so engrossed in a project that I’ve barely noticed that it’s something like 3am.  By that time I’m within spitting distance of finishing – or so it seems.  I’m not fast at doing things, tending to get distracted easily by things like the need for coffee breaks, the urge to look at something on the internet etc, so what some might call an 8 hour kit build, is closer to 3 times that for me. Next thing I knew it was 9am, but the board was finished, and all the external connectors temporarily connected:

The board with external connectors temporarily attached. On the far right just above the middle you can see the 2 PA transistors epoxied to the heatsink. Just above that is the trimpot for controlling output power. The VFO toroid is on the far left about 2/3 of the way up the board.

Initial impressions are favorable. Although some of the capacitors are microphonic (to be expected in a DC receiver), I can tell that this is not going to be a problem in use, especially when the board is mounted on standoffs in a case.  The other main problem with DC receivers is the issue of hum pickup.  I’m a little concerned, because I am getting quite a lot of hum pickup through the antenna connection. I’m hoping that enclosing the board in a metal case will help.  An enclosure will be arriving later this week, so we’ll see how that helps.

The receiver is sensitive and VFO stability seems to be good enough for regular usage.  Using my FT-817 as a reference,  I measured about 60Hz of drift in an hour from the VFO after it had already been on for several hours, operating in a room of reasonably constant temperature. I then measured the frequency drift every hour for 6 hours. After 6 hours, the VFO had drifted 90Hz higher than the original frequency; the maximum drift from the original frequency within the 6 hour period was 150Hz. Not bad! I intend to fix the VFO toroid more firmly to the board with a nylon nut, bolt and washer to help improve the resistance of the VFO to physical impact.

At this point, my main concern is that of hum pickup in the receiver.  We’ll see what happens when I’m able to install it in a metal case. To be continued………..

Postscript – nothing better to do with my time this morning than stare lovingly at the PC board and ponder on what a thing of beauty it is:

The board after disconnecting the temporary knobs, switches, power etc and before mounting in a case.

Hum Problem Solved – I haven’t begun to put the FT80 in a case yet, but the hum issue has already been resolved.  The radio was connected to an antenna via an LDG Z11 tuner, which was powered by an unsmoothed wall wart transformer.  On unplugging the wall wart from the wall, the hum all but disappeared.  There is still a very low level of background mains hum, but only at the level you’d hear in a mains powered receiver with a well smoothed power supply. At this point, the radio is very usable in just the current bare board situation; things can only get better when it is installed in a metal case. Dan N7VE made the point that if you mount the board close to the bottom of the case, as the traces are on the bottom of the board, and the top of the board is mainly ground plane, then the traces will be sandwiched between two ground planes. The FT80 looks like it is going to be a very usable little transceiver. I’m really looking forward to when QRP Kits start stocking versions for other bands.

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5 Comments »

  1. Dave, you didn’t waste any time with that build. I haven’t even started mine yet so I’ll be interested to hear how you like yours once you get it on the air. And how the hum issue goes once you get it installed into a case. Which case will you be putting it into?

    I plan to start (and hopefully finish) mine this weekend.

    73,
    John AE5X

    Comment by haskelltx — February 10, 2010 @ 1:23 pm | Reply

  2. I have a case by LMB Heeger arriving today. It’s their interlocking chassis Model 141 in black. It’s a larger black version of the case I used for the receiver in this blog post: http://aa7ee.wordpress.com/2010/01/11/software-defined-or-hardware-defined-which-way-to-go/ It’s not quite the dimensions I wanted, and if I hadn’t already ordered it, I’d be thinking about the Ten Tec TPB-41. This case will give me 6″ of width – room for a vernier dial and a digital readout a la KD1JV if I decide to go that route. There will also be plenty of room for a keyer and an audio amp to drive a speaker – all things I may want to add. I like the fact that the board is small enough that you can mount it in a small case with no reduction drive or digital readout etc, but can choose to go the whole hog if you wish. In it’s minimal version in a cheap case, it’s quite an affordable little radio.

    I talked to Dan about the hum issue. He’s also had the experience of hum with a bare board that goes away when you touch a ground point. When I touch ground, the hum doesn’t completely go away, but is greatly minimized. I’m very hopeful that a metal case will solve this problem. He also made the point that because the ground plane is on the top of the board and the traces are on the bottom, when you mount the board in a metal case, the traces are effectively sandwiched between two ground planes, which should do a lot to help.

    Wow, I intended this to be a brief comment. Enjoy the build, and welcome back to radio from the world of photography, though ham radio doesn’t afford as many opportunities to meet beautiful women. I can’t imagine that the line “Would you like to come to my place and build a direct conversion transceiver” works quite as well as “I’d love to help you build your portfolio” :-)

    Comment by aa7ee — February 10, 2010 @ 3:38 pm | Reply

  3. I think I’ll take the circuit board to Fry’s tomorrow and see if they have something in stock that will be a good fit and not too expensive. Normally they have a pretty good selection.

    But I am a bit hesitant right now to buy a case with the idea of later expansion without first knowing how the rig will perform, ie it may not be worthy of the add-ons, etc…but hopefully that won’t be the ‘case’.

    Regarding the photography, I do music-based stuff here in Houston and need an ongoing port to show what I do. Most of the images are eventually manipulated, composited, etc to present an “idea”. It is fun to have an image in mind and then (hopefully) actually making it with camera, lights, etc.

    I’ll keep that ham radio line in mind though – now if I could only do it with an English accent… ;-)

    Comment by haskelltx — February 10, 2010 @ 7:28 pm | Reply

  4. Hum problem solved John. See postscript at the end of this blog. You probably won’t experience this but FYI just in case.

    PS – the first set of lights I ever bought were designed for portable use and had very weak modeling lamps. That was a mistake, as good strong proportional modeling lamps would have been a great help in learning how to set up lighting. You live and learn.

    Comment by aa7ee — February 11, 2010 @ 6:20 pm | Reply

  5. [...] first part of the build was covered in this post.  I wasn’t too sure about what I thought would be the tricky tuning that may result from [...]

    Pingback by The Tut 80 Complete « Dave Richards AA7EE — February 28, 2010 @ 9:33 pm | Reply


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