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Old 13th Jan 2017, 11:32 am   #1
crackle
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Default Another Minimod TX

I have been thinking of building a pantry transmitter for some time now. Originally I was thinking of a crystal controlled 2 valve design based along the lines of something Murphyv310 posted on here back in 2011.

But after reading Phil G4SPZ's thread I have now opted to build the Minimod designed by Ian Liston-Smith. Mainly because I hoped its relatively simple and easy to understand design would contribute to keeping the size nice and small.

Like Phil I am using Veroboard as I have a large sheet of it I bought at a boot sale some years ago. (seller had no idea what it was)
I have done my own layout aided with being able to refer to one of the layouts that Phil used just to double check.
Here is the layout.
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Most of the components came from my spares, but I had to order the coils, which arrived very fast, the next day, from Spectrum Comms. I also had to order some 270pf and 100pf caps and some LM386 op amps.

Next was to cut out a piece of veroboard from the long piece I had. I used a Dremel type tool fitted with a thin 1 inch cutting disc. I also used the tool with a small milling bit to make the cuts in the copper tracks.
To make things easy I used a sideways reversed image of the layout drawing and reduced the scale to 1:1.
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Adding the components started with fitting the coils. These required the matrix board to be adapted slightly to enable the pins to fit through without stressing them and additional holes to be drilled for winding tap pin on each coil and their mounting lugs.
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Fitting the rest of the components was very straightforward, following the life sized printed copy of my layout.
Here is the finished board, the LM386 is missing as they have not arrived yet.
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Tests done so far:
The oscillator is working, I have a range of 1554kHz with the slug wound all the way in to 1014 kHz with the slug wound out.
I have found a spot which seems fairly clear at 1071kHz on the MW.
The oscillator on its own could be picked up with a radio just a few inches from the oscillator with no aerial and no output stage as the OP amp was missing.
For the next test I shorted pins 5 & 6 on the IC socket to connect power to the BC109. I adjusted the output coil to peak the signal and found that the signal would cover the room but not very strong outside the room using about 1 m of wire as an aerial and no earth. I have planned to not use an earth as I was hoping to make the finished Minimod portable and you dont always have a convenient earth to hand.

Am I right in thinking the output power can be adjusted by reducing the value of R8?. Would an RF choke work well here.

I will update later when I have tried it fully working with some audio modulation.

Mike
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Old 14th Jan 2017, 1:23 pm   #2
crackle
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Default Re: Another Minimod TX

The LM386 ic's arrived today and when I tried it out I discovered I had left out 2 cuts in the veroboard on the audio input circuit.

Here are some revised drawings.
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The voltage measured across R8 is 1.1volts which appears to give a power dissipation in the OP transistor of a fraction over 3mW.
It could do with being a bit more I think, reception is limited to about 6 feet, unless I connect up a 10m long wire.

I wonder can anybody advise if I have done something wrong.
The transistor used in place of the BC109 is a CV10806.

Thanks
Mike



Mike
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Old 14th Jan 2017, 2:04 pm   #3
Nuvistor
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Default Re: Another Minimod TX

I thought the MiniMod was designed for low output and required being close to the radio. That's what mine is like on the 50's valve radios I have if I want it to fully " quiet" the background.
Frank
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Old 14th Jan 2017, 5:40 pm   #4
crackle
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Default Re: Another Minimod TX

My statement in the post above is incorrect. The figure of 3mW is the power dissipation in R8. The overall power is 4.57v x .0028A = 12.7mW so am I correct in thinking the power dissipated in Q2 is about 9mW.
I think Ian says in his article on the Minimod that the power was about 20mW.
By the way I substituted a BC108 in place of the CV10806 transistor just in case the military one was iffy. I did see a tiny fraction more volt drop across R8, indicating more current flow, so maybe the CV10806 is not so good.

Frank, I think you are correct, maybe I am expecting a little too much from it.
But I also find the reception on the radio, when about 4 feet from the TX, has a quiet background but the modulation is also quiet, noticeably quieter than when tuned to the adjacent stations.

Oh well, it was fun and interesting putting it together, and it must be said that one needs to be wary of putting out too much signal so as not to interfere and attract unwanted attention. But I was hoping for it to work around the house.

I have ordered some mica caps for C1, 2 & 13, as recommended, to replace the ceramic ones I used as it does tend to wander off channel over 10 min or so, to see if the frequency stability can be improved from what it is at the moment.

Mike

Last edited by crackle; 14th Jan 2017 at 5:51 pm.
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Old 14th Jan 2017, 7:21 pm   #5
Nuvistor
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Default Re: Another Minimod TX

It doesn't need a lot of mod to be quite loud, the frequency stability is excellent, I only had it drift slightly when the battery was low, I now have an external battery box with AA cells, last a lot longer than a PP3.
The one I have Ian built.
I can hear a weak signal on a transistor portable around the house but it's very weak.
Frank
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Old 14th Jan 2017, 9:37 pm   #6
mark_in_manc
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Default Re: Another Minimod TX

My minimod works OK in the same room, but I needed to ideally use it in a neighbouring room. I connect it to the headphone out on my stereo and play records through it - this is handy as I can mess around a lot with drive level and LF / HF pre-emphasis. It struck me that the aerial would be radiating a good bit in a room with a large metal receiver in it - the radiator! True enough, connecting the aerial on the receiving set next door to a heating pipe brought in a lot of signal. Not sure what this implies about the earthing of my heating pipes.

Actually, a VHF61 in the attic with a LONG wire aerial wound round the roof space picks it up well too, with no help from the heating system.
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