After practicing my chassis bashing skills for some time I decided on another QRP Valve transmitter project for 40mtrs, some time back I had built a QRP valve transmitter which has served me well and still does on 40mtr CW but with the skills learnt on that project and an increasing skills on the chassis bashing side of things I thought perhaps a nice simple xtal controlled in a more elaborate case would be an interesting project for the coming winter months.

Many years ago I had been invited round to a local Radio Ham who was a retired Embassy Radio Operator, and in his shack was a LabGear LG 300, a real meaty vintage CW Transmitter, it had a real Marine look about it, taller than it was wide with rack handles and a real business like look about it, I longed to own one, but as the years passed and things moved on other priorities took centre stage, recently I had a chance to own one but the realities of it were sadly that is was simply to big and heavy for my shack,

So after coming to terms with never going to own an original I thought I might build a QRP valve transmitter along the same lines so to speak, I decided on the same shape case which would push my shaky metal working skills to the limit, with the transmitter in the top half of the case and a solid PSU in the bottom half, the front would hinge out for access and maintenance

The actual transmitter would be simplicity itself Xtal controlled with an EF80 pentode in a trusty Modified Colpits Oscillator, followed by a 5763 PA, I had been collecting bits for some time and this seemed an ideal time to make a start, the summer of 2004 was rapidly coming to a close so I made a start in the garage hoping to get the case and chassis finished and large components mounted before the car took priority in the garage for the winter, (heavy chassis bashing being frowned on indoors) My wife on seeing the case for the first time called it a Bird Box !  So much for the vision of the fairer sex! Case more or less completed time to add the major components the chassis handles and meter, then tuning caps and valve holders, the heavy mains PSU components being fitted later as its easier that way owing to the weight, and at last the unit started to look like a transmitter instead of a bird box!

I decided to use an under swing meter mainly as it’s a bit unusual and I happened to have one, and I thought it would add a bit of character to the finish item, I decided on a Pi Tank PA Antenna matching unit as these will match almost any bit of wire into the Transmitter PA and after all the transmitter will be very low power and an efficient match will be really important, I also decided to bring the HC6U crystal holder out on to the front panel with a small variable capacitor across it as a means of shifting the frequency a few Htz and easy to change frequency with a different crystal

Now the time has come to fit the PSU components, the mains transformer and smoothing choke are from an old Pye link receiver and really a bit big for this project but it never hurts to under run a Power Supply, the smoothing caps are computer switch mode types with a rating of over 450v these new type electrolytic caps are far superior to the old type, I’m hoping to get away without a stabilised supply for the xtal oscillator but I’ll have to see how the rig performs.

The case is looking quite full now and quite heavy, you can also see the antenna sockets now mounted, I’ve decided on a relay operated net/rx/tx control all that remains before the wiring up starts is the fitting of through grommets, coils and tag strips, I will probably make the osc and PA coils when I start to wire it all up, first job will be the wiring of the PSU mains circuitry and DC supplies.

And now the wiring of the PSU and other power related wiring HT LT and Mains, I always find this part a real chore, also whilst having a break I decided that this transmitter will be on 20mtrs, mainly as I already have a 40mtr home brew tx and I thought although 40mtrs CW is my favorite band I should try something new, and as hopefully this little transmitter will take me round the world I had better fit a few simple refinements,

as you will notice two new controls have appeared on the front panel, these are xtal switching and Break in delay, the reasoning being plug in xtals would be ok but it would be handy to have a couple of frequencies built in so to speak and one of the real limitations of my 40mtr HB rig was the hand tx/rx changeover so on this rig I have built a simple break in time delayed tx/rx switch, on pressing the key two relays come in instantly, one fires the transmitter PA and the other changes the antenna to transmit and keys the xtal oscillator the second relay is delayed drop out, so whilst the first is keying the PA the second keeps the antenna in tx mode and the xtal osc on, when keying stops the second relay drops out after 100ms or 500ms according to which time constant is selected, very simple no fancy electronics just a couple of relays and a few capacitors, although it did mean a LT DC supply which I made up from the heater windings on the transformer, that part is now wired up and tested, and the valves are now glowing at last and the 275v HT tested and ready to connect up to the valves

The drop down chassis idea has proven to be a bit of a problem, mainly as when constructing it drops the transmitter chassis below bench working level so I find myself almost bent double trying to wire it up, also it makes it very difficult if any more holes need to be drilled and nuts and bolts fitted, I’m sure it will prove itself for servicing after completion but at the moment it’s a bit of a curse, but that’s in the nature of homebrew from scratch, you can dream and plan for ages but its not until you start that the unseen difficulties start to arise!! next job AT LAST is the wiring up of the xtal oscillator and PA

good fun Ron Ayling 18.9.04 ronayling@hotmail.com