Anyone remember these crude Smartkit Transmitters?

[MiXiN]

My pal is having a good clear out of some of his gear, and amongst a pile of other things he gave me a broken Smartkit 1021 15 Watt VFO Transmitter - an absolute pile of garbage.

I first started on the air with one of these in the 90s, but it didn't look quite like this one so maybe this is a slightly older revision?

A Google of "Smartkit 1021" will show the one I had.

Anyway, after checking the parts, TR3 (2N3924) with the large gold heatsink on was found to be faulty so I replaced it.

After tuning the various stages it's doing 15W, but inductor L8 at the final (8 turns) and the compression trimmer underneath get real hot, so much so that L8 is too hot to touch and has blackened the associated track and melts flux.

I recall this inductor getting really hot and standard trimmers melting on my one from over 20 years ago - but why is this?

With a fan blowing over these everything is fine.

PS - This will not be being used on air and has only been fed into a dummy load, but it's nice to have it running for nostalgia purposes.]

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[Albert H]

I saw one of these in use in Indonesia. They couldn't understand why they had to keep re-tuning it to stay on the same frequency, and why it interfered with nearby TVs!

I was feeling generous, and had some time on my hands (and it was the rainy season), so I stripped it for parts and built them a nice 10 Watt PLL-stabilised rig. The best part of their original effort was the box it was built in - a metal panelled effort with the back made up of a big flat-sided heatsink.

The PLL was made out of logic ICs and was a fixed-frequency job - they weren't going to move frequency any time soon!

The last part of getting the transmitter right was to make a battery charger / PSU for the rig, so that when mains was available, the battery would charge and the rig would run. If the mains failed (an almost daily occurrence), the rig would continue to work for several hours from the caravan battery that was
strapped across the output of the PSU.

Despite the output power reading lower than the "Smart" board had done, Their signal could be heard over a much bigger area. I concluded that a lot of the power seen on their meter was on frequencies well outside the FM Band!

Finally, we assembled a 6dB colinear antenna, and their coverage improved still further. We replaced their nasty RG58 with UR76, and the field strength improved some more!

They ended up with a nice little set-up which worked properly and didn't cause interference!

[MiXiN]

Interesting read, Mr H.

There's definitely some severe harmonics radiated from this thing of mine; In fact, when I adjust some trimmers the SWR/Power meter gives some strange SWR readings at some points.

Going back to my initial post - have you any idea as to why the 8 turn coil gets really hot? It becomes so hot that it actually burns the PCB on the track side and starts melting flux or the PCB composition if a fan isn't directed towards it.

[shuffy]

Going back to my initial post - have you any idea as to why the 8 turn coil gets really hot? It becomes so hot that it actually burns the PCB on the track side and starts melting flux or the PCB composition if a fan isn't directed towards it.

Yeah, the wire those coils are made of at the business end of the board looks far too thin. It's an RF choke, and thin wire makes the skin effect more significant, further reducing the efficiency at high frequencies. What is that, a BLY88? So that coil is probably also sourcing best part of 3 amps DC. Check out the equivalent coils on the NRG boards, they're made of 18SWG wire or thicker.

Actually, I've just compared your board to the ones on google, it's a different layout, but are you sure those 2 coils near the final are in the correct places? I wouldn't expect the coil in the output match to have the 8 turns on it, I'd expect that to be part of the load from the collector to the supply.

[Analyser]

Actually, I've just compared your board to the ones on google, it's a different layout, but are you sure those 2 coils near the final are in the correct places? I wouldn't expect the coil in the output match to have the 8 turns on it, I'd expect that to be part of the load from the collector to the supply.

He's right you know, it looks like those two coils on the output of the board are the wrong way round!

[Analyser]

Just had a quick look online and the coil feeding the trimmer should be 3 turns, diameter 13mm (!) and the supply coil 7 turns, diameter 9mm. So they are the wrong way round.

[yellowbeard]

Jesus - at least it's rugged. I searched it too and I found a guy in South Africa still selling this for £120 - cheeky.

[shuffy]

Has he got the Pantec 3W for £150

[radionortheast]

i built the 1w kit it just hummed, the 4w had a nice star sharped heatsink i kept for agers seem to remember they liked big black inductors...

[MiXiN]

Nice one for the replies necks.

I'll rewind some bigger gauge coils, say 16 SWG, and put them in the correct locations.

If these coils are put in the correct positions, will the heat issue improve and do you think power output will rise?

[yellowbeard]

Yeah for sure the power output should improve, the larger coil is supposed to stop RF going back to the power supply (which has a crap SWR and is sitting on the ground) while the smaller coil presents a lower impedance path out to the antenna. The way they are at the moment the RF prefers going to the power supply and the antenna output is choked. The heat may not improve but it should be more controllable.

[shuffy]

I'd swap the coils over before you do anything else, that might make the heat problem better - one of those trimmers in your output match is most likely sinking most of the RF. If it doesn't make things better, try 18 swg, 16 is a bit thicker than you need on that design.

[FMEnjoyer]

I did have 2 or 3 Zenith Electronics 3W PCB rigs. Back of Practical Electronics mag. In the main they basically created a square wave output. On Off On off with a massive HUM every time. Walked past it and it turned off or changed frequency/drifted. They looked as if they were built ok on a fibre glass pcb. As I new zero about RF circuits and still don't know that much I could never understand that the rig was an at frequency oscillator at the very simplest form and was a terrible terrible design that was never going to work well.

I managed about 400M which was far enough in retrospect.

Wasted endless time and £16.99's on them. It was my first radiated signal. The + and - output pins also arced with a dipole antenna connected every 2-3 seconds. Scary bit of kit all round.

[radionortheast]

I did have 2 or 3 Zenith Electronics 3W PCB rigs. Back of Practical Electronics mag. In the main they basically created a square wave output. On Off On off with a massive HUM every time. Walked past it and it turned off or changed frequency/drifted. They looked as if they were built ok on a fibre glass pcb. As I new zero about RF circuits and still don't know that much I could never understand that the rig was an at frequency oscillator at the very simplest form and was a terrible terrible design that was never going to work well.

I managed about 400M which was far enough in retrospect.

Wasted endless time and £16.99's on them. It was my first radiated signal. The + and - output pins also arced with a dipole antenna connected every 2-3 seconds. Scary bit of kit all round.

dipole was 2 big fat blue wires about 50cms long each, mine didn’t do anything but hum and produced all different carriers, i didn’t have much at the time except a really cheap steeletone scanner. I think they called them a pantek..http://www.gareth.net.nz/nrgworkshop/3_ ... ematic.htm

i remember the circuit diagram the transistors where facing each other both seemed to be attached to the same coil, they where oscilator, the other 2 transistors where just for audio. if you where designing one of these transmitters problaly wouldn't be the way to go about it need audio amplifier - the oscilator - then an amp..But one of my first transmitters that got out beyond a km was just a power oscillator, using a 2n2219 transistor…it problaly was more stable than this but suffered the same problems, nothing beat the excitement of hearing it in the park

[Albert H]

My first rigs were built for the higher frequency end of the medium waveband. The first few used VFOs and would drift quite a bit. I then got some crystals cut, and I stayed on frequency. I used an old Philips tape recorder (valved) as the modulator, which could either amplify my (very crude) studio or could play a pre-recorded programme!

The first aerial was a "sloper" which went up a nearby tree. The earth was a buried galvanised dustbin! The first serious rig used an 807 valve as the (modulated) final, and did about 12 Watts unmodulated and peaked at about 50 Watts. This effort went about 25 miles in every direction - in the daytime! It was useless at night. I can certainly relate to the thrill of hearing my first "station" on the air!

My first VHF transmitter was also valved, and used a QQE06-10 as the final, getting around 8 Watts. It was also a VFO-based effort, but didn't drift significantly, as I'd learnt how to build stable oscillators by that time, and VHF receivers in those days had "AFC", which was designed to tie the receiver to the transmitted frequency, to compensate for the inherent drift in the receiver's local oscillator. The other bonus was that domestic receivers would "follow" a slightly drifting VHF transmitter!

My earliest VHF stuff used transistors in the early stages - usually a ¼f oscillator, buffer, then two doublers to get to the final frequency. The last stage of the exciter would include a fairly tightly tuned filter to make sure no ½f or ¼f got through, and was usually a pair of BSX20s or similar. This would drive into the valve stage(s). The biggest one I built used a 4CX1000A as the final, and did around 800 Watts. This was the first of them to have a PLL!

After that, everything went solid state. High voltage components were getting difficult to find (and expensive) and the valves got ridiculously expensive. My first all solid-state rigs used BLY90s for the finals - 50W out of a single one and 90W output a pair.....

[FMEnjoyer]

Love these stories notheast. Albert H thanks for the time you use to make your contributions.

Keeps the feeling going.

[MiXiN]

I too remember those "push pull" 3W crude Transmitters that were available in the back of various electronics magazines; wasn't it Bull Electrical & some other supplier that sold them in addition to a wide variety of other random things?

I think I managed around 1/2 mile out of mine and was chuffed to bits even though it had a severe hum.

I'd love to get hold of another, not to use, but just for the nostalgic blast of the past feeling of being a teenager again.

[radionortheast]

yes the advert was usually on the back cover of electronics world, with a blue border and white with black text (i had to beg my dad to send off cheques to them course there was no internet then) they mostly sold disco equipment, looked like junk! you where problaly better off with one of the survilence bugs on the other page, forget who sold them, the most powerful one was 0.2 of a watt, but it had different stages aswell as ferrite core for tuning, i got mine about 1km weakly. (some people drive round in the cars in things that can go that distance!) it was kind of exciting at the time, that was the furthest i’d gone as i’d just used local oscilators out of fm radios managed to get them weakly about 500 meters away.

[MiXiN]

I remember those 0.2W bug Transmitters you recite as well.

Smartkit are still selling these after all these years just like that 15W thing I have - www.communica.co.za/catalog/Details/P0731195960

[Albert H]

Back when I was experimenting with crude VHF transmitters, I came across the Dutch "Stentor" board. This actually made some efforts to work properly, and had some neat ideas - voltage regulation for the oscillator, using the grounded heatsink of the final transistor to screen the output end from the oscillator, and it even had a (basic) output lowpass filter. At 15 Volts, it ran really hot (which led to drift) but it would deliver 5 Watts or so if all the trimmers were carefully peaked!

The transistor line-up was a BF245 FET for the oscillator, with a 78L09 as its supply regulation, a 2N2219 and an MRF237 (or SD1127) for the final. Just three transistors, and all that power!

I remember putting one on a spectrum analyser, expecting the worst. However, if everything was peaked for maximum power on the frequency you'd chosen, it was remarkably clean! Adding an extra stage of filter to the end made it pretty acceptable! If you kept the supply down to about 11 Volts, it gave about 3 Watts and didn't drift so badly.

Everyone knew that they drifted, so I built one in an aluminium box, with a FLL circuit concealed underneath the board. As you turned the tuning pot (which normally had a span of about 4MHz), it would go up the band in 200kHz jumps. When you stopped turning the pot, it stayed exactly where you'd left it! You could run it for hours without it drifting. I told people that I'd spent hours selecting just the right capacitors and so on to stabilise the oscillator against temperature drift..... Nobody could understand how a "Stentor" could be so stable!

I might still have one here somewhere.....