Dave Richards AA7EE

July 30, 2012

The Other Half Of The ZL2BMI DSB Transceiver – The Transmitter!

In the last blog post, I described the receiver part of my build of the ZL2BMI DSB transceiver and noted that I’d been having problems with the TX transmitting a constant carrier, even with no modulation.  Everything seemed to be OK up to and including when I built the driver stage. On adding the BD139-16 final though, pesky RF feedback started having it’s way.

During my early years as a radio DJ, one of the pieces of advice I received from my program director was that if I didn’t know something, it was better to not talk about it at all than admit it on the air. As a general rule, it’s probably better to spend time imparting what you do know to your listeners, than to waste time telling them all the things you don’t know.  However, occasionally admitting to holes in your knowledge, I think, increases your relatability and well, is just more honest. In the same way, after my partial success with the G3WPO DSB80 rig, I decided that I wasn’t going to keep blogging about the circuits I built that didn’t fully work. However, a very nice comment from Rogier on my last blog post about the receiver in the ZL2BMI DSB transceiver made me change my mind.  Rogier said that even if my circuits don’t work, they still look good (or words to that effect). Well, I’m not posting the pictures here because I think they look good (after all, what good is a nice-looking circuit that doesn’t work?) but for two reasons. Firstly, to share what I’ve been up to, and secondly, someone may look at my layout and know why I’m having the problem with RF from the final getting back into the earlier stages of the rig and causing it to emit a constant residual carrier (of about 3/4 watt if I remember correctly – not an insignificant amount in such a rig).

So without further ado, here’s what my ZL2BMI transceiver looks like from above.  I hadn’t yet added a TX/RX switch, so the power supply line is connected in the TX position, and the 12V supply to the receiver is disconnected:

My version includes a mic pre-amp as used by G3XIZ and detailed on the GQRP site:

I added an extra 01.uF cap from pin 8 of the NE602 to ground in order to try and stop RF getting where it wasn’t supposed to go.  I also temporarily disconnected the output of the mic amp from pin 1, which didn’t help. I’d read that some people have experienced problems with RF getting into the antenna input coil on TX, so temporarily removed that coil, but that didn’t make a difference either.

Another view of my build:

It is now sitting on a shelf until inspiration (or help from someone else) moves me to take another look. I’m taking a brief break from building but am feeling the desire to build a kit :-)

Rogier – was it you that I heard Bill Meara mention in the Soldersmoke mailbag in the just-released episode 145?

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14 Comments »

  1. Great looking manhattan style breadboarding. I’ve put a link on my web (www.w5dor.com/W5DOR-Oner.html) to guide others to see ur work. I purchased some MePads & MeSquares for my own breadboarding projects. Thanks for such fine construction examples.
    Gene, W5DOR

    Comment by Gene — July 31, 2012 @ 3:50 pm | Reply

  2. Insanely Jealous of your constructing capabilities, I thought I had done a great job on my first kit build for some years ( a little Rock-Mite 40) but it’s rather like putting up the ugly duckling when I see the kits you are building here! Wonderful blog and great photos, I’ve added you to my excuse for a blog, (http://morsesagas.blogspot.co.uk/) I hope you don’t mind? – 73s Richard M0AUW

    Comment by Richard Hull — July 31, 2012 @ 4:53 pm | Reply

    • Richard – you live in a lovely part of the country. I grew up about halfway between Worcester and Stratford, near Alcester. Glad that you found the problem with your Rock-Mite. Have you worked anyone with it yet?

      Comment by AA7EE — July 31, 2012 @ 5:14 pm | Reply

  3. Hi Dave, That is a great project you are working on. Now you have me motivated to give this circuit a try. I heard Bill N2CQR talk about the circuit on SolderSmoke. I went looking for the circuit on the internet, but just now learned that you are building one also. I believe that some of my best learning experiences come from making mistakes in my circuits, being honest, and then l persevere until I get the circuit going. Sometimes I feel like Thomas Edison in trying this and then trying that, but I stick with it. I look forward to watching your progress with this circuit.

    Best 73,
    Al, N8WQ

    Comment by Orville A. Jones — August 1, 2012 @ 2:09 am | Reply

    • Al – Eldon WA0UWH was kind enough to e-mail me and suggest a couple of possible reasons for the trouble I’ve been having with the transmitter in this little rig. One of his suggestions sounds like it may be the issue, and that is my grounding technique. I think I need to create a separate ground for the high-current circuit (i.e. the PA transistor, and possibly the driver) and ground that to a location away from the low-current circuits. I need to do a bit more reading on this, but I think he may be onto something.

      I have so much to learn, but learning new things is fun!

      Comment by AA7EE — August 1, 2012 @ 2:26 am | Reply

  4. Dave,
    Very nice looking rigs! I noticed that the BD139 is directly bolted to the PCB that acts like a heatsink. You might have shorted the collector of BD139 to the ground! Just check that it isn’t so. Just check that before proceeding with the rest of my advice.

    Usually, the power chains that are too simple have problems of not being controllable. They are a bit like a homebrew car without brakes. Usually, our circuits like this one have been built by so many people that most of the bugs are ironed out. Too frequently, they don’t work as advertised and can either give up or step up our homebrewing capability by being able to measure and diagnose the problem. Let’s do that now.

    I suppose you can parallel up two 100 ohm resistors to create a dummy load. The transmitter, however low powered should always be tested with only a proper load. So, first, take out the BD139 and see that the 2N2222 is providing a few milliwatts of power (my guess is that it should be around 1 to 2 V peak). Then put the BD139 back and measure the power again, this time at the output of the BD139.

    If you say that there is ‘residual’ power, can you see which frequency that is at? is it in 7 MHz band or outside? I wouldn’t suspect any carrier leakage at this point in our investigations. It looks more like a case of self-oscillations of the BD139. Attaching a 50 ohms load should calm things down a bit,

    however there are few deficiencies in this minimum-part-count circuit. The bypass on 2N2222 as well as BD139 is ineffective. I would add another FT37-43 with 10 turns as an RF choke between power supply and the 0.1uF on the power line of 2N2222., I’d add another 0.1uf in parallel to the 100uF on the power line of BD139 and isolate it from the power supply through another FT37-43 with ten turns.

    The 2N2222’s output impedance will be in the region of a few hundred ohms. The BD139’s input impedance will be in the region of few tens of ohms. There is clearly a mismatch there. Hence, I’d suggest that you add a tap at about 2 turns from the power side on the inductor at the collector of 2N2222 and move the capacitor feeding the base of BD139 to that point, providing it with a lower impedance drive.

    I hope this gets us somewhere. Best of luck!

    Comment by Ashhar Farhan Vu2ESE — August 2, 2012 @ 5:29 am | Reply

    • Ashhar –

      Great ideas – thank you very much. I did have the output of the BD-139 connected to a 50 ohm load when testing. Also, the entire case of that transistor is plastic, so the collector is not shorted to ground.

      I’m taking a short break from this rig, but your ideas are excellent ones and I will try them out when I start experimenting with it again.

      Comment by AA7EE — August 2, 2012 @ 5:45 am | Reply

  5. IT Will probably start FMing when you use the internal 612 osc – i’ve been there, i built a similar design some years back – i had to add a seperate VXO. iF you drive the mic audio up youll notice what i mean.. g0ebp

    Comment by tony — October 1, 2013 @ 8:01 pm | Reply

    • I did have a bit of FM’ing, and also had other problems with it unfortunately Tony. I was getting RF feedback from the driver and final which in retrospect, was most likely due to me not doing enough to isolate those 2 stages from the earlier stages of the TX. I didn’t follow up on it but am now building a similar design which has a separate 2-transistor (oscillator and buffer) VXO with a ceramic resonator. I have learned from this experience and will build the driver and final on the other side of a partition made from double-sided PCB material.

      Dave
      AA7EE

      Comment by AA7EE — October 1, 2013 @ 8:33 pm | Reply

  6. Hey Dave!

    Beautiful workmanship as usual. Tell me what method do you use to line up your squares??? Do you eye-ball them? Do you mark straight lines on the copper clad matl??? . . . and then, erase it??? Do you put tape on the copper clad and then pull it off when finished??? I do my construction with MeSquares similar to yours but mine always seem to be not so straight. Maybe I’m being too critical of my assy techniques. Well, anyway, nice work !!! I always enjoy looking at your workmanship and wishing mine looked at least half as good as yours!

    My next project is to assemble a receiver to go with my N3ZI DDS VFO. I’ll start with a simple NE602 mixer in a dc rcvr using a Hi-Per-Mite. It should be rather simple with a single tuned circuit input from antenna to NE602, a direct connection from the DDS2 to pin 6 of the NE602 and then a direct connection from pin5 of the NE602 to the Hi-Per-Mite. “Direct connections are possible because these modules already have coupling caps.

    My next step is to place another section between the 1st mixer and the Hi-Per-Mite that consists of a 4 xtal IF filter, a MC1350 IF amp (so I can add AGC later), another NE602 detector and then the Hi-Per-Mite. Of course I’ll add the BFO using another xtal just like those in the IF filter.

    If all that works I might next add a DSP DNR (digital noise reduction) circuit and a switch to switch between the 500Hz IF bandwidth of the xtal filter or the 200Hz bandwidth of the Hi-Per-Mite.

    Might next add a few 2N7000 switches to mute the receiver section when keying the xmtr I’ll add next.

    To add a transmitter (xmtr) I’ll add another NE602 to generate a signal at the received freq for the xmtr. I’ll follow that with a DTC to get a signal at the same freq as the received signal and then a transmitter consisting of a buffer, driver and final amp sections. (2N3904, 2n2222 and IRF510).

    Well, I’m just a rambling on here and need to get to bed. Thanks again for such fine examples of workmanship with your manhattan style construction.

    Thanks and 73,
    Gene, W5DOR
    gene@w5dor.com
    http://www.w5dor.com

    Comment by Gene Dorcas — October 2, 2013 @ 1:23 am | Reply

    • Gene –

      I figure out where I’m going to place them, and mark the rough position by scratching the copper-clad with a needle or jeweler’s screwdriver. Then, as I have already lightly sprayed the laminate with lacquer, I scratch the lacquer away from the area I want to glue the pad to. I also roughen up the underside of the pad with a craft blade to give the glue something to key to. I use superglue in the gel form, put a small blob on the board, and place the pad onto it with pliers. I then have about 10-15 seconds to press the pad down and re-adjust it (both of which I do with a jeweler’s screwdriver). The business of the exact alignment is done by simply eyeballing it. Every now and again, I get it wrong and have to decide whether to leave the pad as it is, or whether to pry it up with a blade and try again.

      Good luck with your receiver project. From time to time, I check the QRP-Tech group on Yahoo and see you in there. Great work – Chuck is a tireless and enthusiastic leader!

      73 for now,

      Dave
      AA7EE

      Comment by AA7EE — October 3, 2013 @ 4:08 am | Reply

  7. Hi Dave – interesting what youve done.
    Mine was the same block idea – but i ended up doubling from a 3580 ceramic resonator to get 7MHz.
    I also had to clamp the rx tuned circuit/612 input on TX, to stop another form of feedback .
    I worked some stations though it was hardwork!

    Tony g0ebp

    Comment by tony — October 3, 2013 @ 1:20 pm | Reply

  8. Just to restate what I said in half a dozen tweets, move the collector toroidal inductor on the bd139 away from the collector choke on the 2n2222, toroids are self shielding but not at point blank range. If that doesn’t fix things you can try running a separate +dc power bus for the bd-139 away from the tx chain with lots of capacitance to ground on the dc end of the choke and maybe a couple ferrite beads. Because it is a phone transmitter it may also be worthwhile to add a couple small electrolytic bypass capacitors in addition to the ceramic to keep audio off of the dc power bus. Bob Pease was a proponent of ceramic+tantalum+electrolytic.

    Matt Burns
    kc8com

    Comment by Matt Burns kc8com — October 15, 2013 @ 6:17 am | Reply


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