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