Dave Richards AA7EE

July 7, 2014

The ZL2BMI DSB Transceiver – An Update

Filed under: Amateur Radio,QRP — AA7EE @ 6:51 am
Tags: , ,

A couple of years ago (gosh – has it been that long?) I attempted to build the ZL2BMI DSB transceiver for 80M.  It was an appealing design, being simple, and capable of being built into a compact space. Eric, the designer, had originally conceived it in the mid 1980′s as a very small rig to be used while bushwalking or hiking in his home country of New Zealand. It first appeared in issue 83 of SPRAT, with an updated version being featured some 16 years later in SPRAT 146. This little rig has spanned many years!

My build looked good but proved that although I may be capable of building things that look quite nice, I’m not always able to make them work.  I’m thinking right now of the wise words of a particular ugly construction guru who would most likely look disapprovingly at my pretty layout that, from an RF point of view, didn’t look so pretty. I am not a great experimenter as, if my builds don’t work after a modest amount of troubleshooting, I have a tendency to retire them to a box on a shelf to keep company with the other projects that “almost made it”.

A few days ago, I received a message from Eric ZL2BMI, who noticed that my version of his rig hadn’t lived up to the aspirations of it’s designer. A number of people made some very helpful comments underneath the post, and Eric also had some ideas. Here’s what he said,

“Originally I developed a prototype and then Bob (ZL2ASO) and I developed it further. Bob is very good at making cases and also milling the boards required for the RF amp. We have made quite a number of changes since the first article and my latest one (which measures just 75mm x 50mm x 25mm) is dual 80/40 m and about 5 watts out – and weighs about 110 gm) However, to come to the problem you had with output carrier on transmit - we did not have this problem with our first two or three rigs, or not to any great degree, probably because the power output was not much above 1.5watts. Then it started to show up – particularly with a 10 watt version I built for an amateur who goes hunting and wanted something with a bit more power to use in the backblocks. Looking at the circuit I realized that with the front-end coil tuned to the frequency in use, and still connected to the NE602, it would pick up some signal on transmit, and this would unbalance the 602. I confirmed this by watching the output (no audio in) and shorting the top of the aerial coil to ground – which killed the spurious output completely. I tried a diode switch - but while it helped, it wasn’t perfect (still 0.6 volt across it). Then I played with transistor switches and discovered something I had never realized – the collector of a transistor does not need volts on it to work. The simple fix is this – an npn transistor (small signal type eg BC547 etc) - the collector goes to the top of the aerial coil – the point where the cap goes to pin 2 of the 602. The emitter goes to ground and the base goes via a 10k resistor to the +ve T line. Despite the fact that the collector is at ground potential (via the coil), it has no effect on the tuned circuit with no volts on the base, but switches the signal hard to ground when +ve is applied. We have since modified all of the approx 12 sets we have built (most for others who use them in the field), with the addition of this transistor - usually mounted right on the top of the coil – to great effect.

Eric also writes about my build,

“It’s possible that leaving the input of the NE602 “open” (rather than grounded) may have left it susceptible to RF pickup. Or it may be that there is some other RF problem. We tended to use the same layout for all our rigs, and I know that some who varied the layout too much had problems. I have built about 7 or 8 of these rigs now, and since the addition of the “front end shorting transistor” there have been no problems with the RF “leakage”. I have retrofitted it to all the earlier ones I made for others. I will try to get some photos of my smallest rig in the next day or two and email to you. There are a few other small mods – to stop a “skwark” when going from transmit back to receive – but this is really just a resistor; and one or two others, mostly to do with getting more power out by better matching of the output transistors.”

Looking back at my notes, I did try disconnecting the antenna coil from the input of the NE602 on transmit, but they don’t show whether I actually shorted that input to ground on transmit. It’s very possible that I didn’t try that.  I have a feeling there may also be some problems with my layout.

My head is full of regens now, but I wanted to get this information up on my blog and into the hands of anyone who is thinking of having a go at this neat little rig. Eric, as promised, also sent some photos of his smallest rig. It’s a 2 band 80/40 version. -

ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)


ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)

The antenna coil is the one close to the front panel with a ferrite slug inside, and you can see the transistor he added to short it to ground on transmit -

ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)


ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)


ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)


ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)

Eric also sent along a schematic which looks like the way he gets the higher output power in his newer version.  I do believe he has written something for a future issue of SPRAT on this, so we may get a little more information in the next SPRAT.

ZL2BMI DSB Transceiver – 80/40M Version (Photo by ZL2BMI)

Thanks for the info Eric – and thank you for sending along the photos!




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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