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Old 4th Jun 2015, 1:10 am   #81
Colin Ames
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Default Re: MiniMod pantry transmitter

Andrew,

You can use an oscillator with a different frequency. Digikey, here in the USA, have an Epson oscillator with a frequency of 1.544MHz. See here:

http://www.digikey.com/product-detai...216-ND/1021909

Reduce the value of L2 to 150uH.

Like you, I have a station that is close to 1MHz, so I am about to purchase one of these oscillators, and a new inductor, and modify my transmitter. The only thing that concerns me is that, unlike the 1MHz oscillator, the replacement is plastic rather than metal. I wondered if the lack of shielding would be a problem. I will find out in due course.

Regards,

Colin
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Old 4th Jun 2015, 1:43 am   #82
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Default Re: MiniMod pantry transmitter

Quote:
Originally Posted by paulsherwin View Post
The problem with all simple tunable microtransmitters is that they tend to drift. A crystal controlled one won't drift, but you obviously need to choose an appropriate frequency.
1MHz would seem to be OK for the list of frequencies you give, though presumably it isn't a standard broadcast frequency (I think NZ uses 9kHz spacing like most of the world outside the Americas).
Yes, we use 9 kHz - but I think that 1 MHz would be too close to Southern Star on 1026 kHz. It's not overly strong into Gore, but it does get there, there are listeners there, and I don't want to risk being close enough for RSM to start looking. Very low power just sounds like a safer option

A little drift won't be a major problem for this application.

Quote:
Originally Posted by Colin Ames View Post
You can use an oscillator with a different frequency. Digikey, here in the USA, have an Epson oscillator with a frequency of 1.544MHz. Reduce the value of L2 to 150uH.
That's always an option, but Digi-Key are showing a $125 minimum order for NZ! element14 have other brands, but all SMD for ones that would work in the AM band.... unless I build something around a 3.7268 MHz crystal and divide it by three, but it's starting to get complex!

It's why I thought trying the Minimod initially may be the easiest option.
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Old 4th Jun 2015, 1:57 am   #83
Colin Ames
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Default Re: MiniMod pantry transmitter

Andrew,

$125 minimum purchase is nuts! The oscillators are only a couple of dollars. I would be happy to order an extra one and mail it to you. Send me a PM with your address if you would like me to do this.

Colin
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Old 4th Jun 2015, 7:46 am   #84
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Default Re: MiniMod pantry transmitter

Quote:
Originally Posted by arjoll View Post
[...] but I think that 1 MHz would be too close to Southern Star on 1026 kHz.
If the nearest used channel is 26 kHz away, that's nearly three channels. Plenty of space.

Simple free-running L/C oscillators do drift a little (depending on temperature and supply voltage), but if properly constructed drift is usually less than the local oscillator of a vintage radio!

As has been said, crystals are a good way to drive a 'pantry transmitter' but to get it on a channel, unless you're using some basic frequency synthesizer circuit or DDS driver (all a bit complicated for generating such a low power device) getting a crystal that operates on a MW channel (multiples of 9 kHz outside the Americas) is quite expensive.

A 1 MHz crystal will be cheap, but at night there's the strong possibility of a 1kHz het from a station on the 999 kHz channel. Although to be honest you run the risk of heterodynes with an L/C oscillator that will rarely be on channel closer that 50Hz, but you can easily tweak it away from any stations.

The whole point of these mini transmitters of course is to provide a very strong local signal, so any weak adjacent or co-channels stations, or electrical interference is completely swamped. The only real advantage of getting it exactly on channel (within a few hertz) is at the edge of your "service area" where any co-channel het will be more noticeable.

Ian

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Old 30th Jun 2015, 1:16 pm   #85
David G4EBT
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Default Re: MiniMod pantry transmitter

I've just built the mini-mod circuit, having made a PCB more or less along the lines of Ian's original layout but with less of a ground plane to give plenty of clearance of the tracks, and have used a BF256C FET for Q1. I carefully checked all the components before soldering them in, including checking the pin-outs of Q1 on my transistor tester to ensure correct orientation. Can't get a peep out of Q1 on the 'scope, so it appears not to be oscillating. I just wanted to be sure that my understanding of which the drain, source and gate are, as annotated on the attached circuit, is correct, if someone can confirm please.

I've also attached pics of the PCB layout, component overlay and the completed PCB for interest.

Any advice would be appreciated - I know that scores have been successfully built, and I'd like to add mine to the list in due course!
Attached Thumbnails
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Old 30th Jun 2015, 4:37 pm   #86
Ian - G4JQT
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Default Re: MiniMod pantry transmitter

Mmm... I'm not sure about 'scores' being built...

Anyway, sorry to read you're having problems.

I've never come across a BF256 (or 2N3819) having insufficient gain to prevent oscillation, so all I can suggest is that you check the SGD leads again. But your layout looks to be correct regarding using a BF256 for Q1.

RF output isn't completely flat across the frequency range, so just give L1 a tweak to see if it starts it oscillating, although again, I've never found one yet that won't start at all.

And DC (about 8V) on Q1 drain?

That's a very neat layout you have made by the way.

Ian
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Old 30th Jun 2015, 5:17 pm   #87
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

Check that L1 is correctly orientated - the direction of the winding relative to the tap is critical, and if it's the wrong way round it won't oscillate. I'll leave you to guess how I discovered that!
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Old 30th Jun 2015, 6:14 pm   #88
David G4EBT
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Default Re: MiniMod pantry transmitter

Thanks for reading the post Ian and Phil, and for your comments - my first thoughts were that the FET leads were incorrectly orientated, but from what you say, I think not. I set the cores of L1 &L2 at the top as suggested in the Bulletin article Ian, so I'll tweak L1 and see if it comes to life! I could also try a 2N3819, but the gate and source leads will need to be twizzled over each other to fit the PCB.

Thanks for your kind comments on the layout - you did the original Ian - all I've done is to 'plagiarise' it, retaining similar component placements, and adding an on-board limiting resistor which I've called 'R9' for an ultra-bright LED. With a 1K5 resistor, the LED is still bright enough but only draws 5 mA, so doesn't add too much to the overall consumption.

Re the orientation of the coil, it will only fit onto the PCB one way round Phil as the primary only has two pins whereas the secondary has three.

Easy mistake to make though if say strip-board is used to build it.

Am I correct in assuming that if the oscillator is working correctly, I should see a sine wave on the scope at pin 4 of the LM386?

I'll have another furtle around with it tomorrow and see what gives.
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Old 30th Jun 2015, 6:27 pm   #89
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Default Re: MiniMod pantry transmitter

Ah, yes David. You've used Ian's PCB layout. I used my own layout on perforated SRBP board, and got confused when wiring it up. Happily it worked fine once I'd found my mistake, and my second version on Veroboard worked first time.

Is C1 a good quality low-loss component? Otherwise I've no further suggestions, although I'm sure you'll get it sorted soon.
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Old 30th Jun 2015, 6:40 pm   #90
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Default Re: MiniMod pantry transmitter

Quote:
Originally Posted by Phil G4SPZ View Post
Is C1 a good quality low-loss component? Otherwise I've no further suggestions, although I'm sure you'll get it sorted soon.
Yes Phil, all the components are new - not N.O.S, and C1 is a polystyrene cap.

As to 'getting it sorted soon', I hope your confidence isn't misplaced!
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Old 30th Jun 2015, 7:00 pm   #91
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Default Re: MiniMod pantry transmitter

Pin 4 of the LM386 is grounded. Where's the other part of the probe connected?

This is little more than a guess but I'd have thought the the RF would be on the base of the BC109.

Good luck.

- Joe
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Old 30th Jun 2015, 8:01 pm   #92
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Default Re: MiniMod pantry transmitter

As Joe says, IC1 pin 4 is grounded.

RF will be at BC109 base, but not very strong until it appears at the collector, and then only when L2 is tuned to the same frequency as L1.

Incidentally, I've found that rather expensive mica capacitors for C1, C3 and C13 do give more output than polystyrene ones. Cheap ceramic disc capacitors here also work well, but some types are a bit more prone to temperature drift.

Ian
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Old 30th Jun 2015, 9:14 pm   #93
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Default Re: MiniMod pantry transmitter

Oh, polystyrene, eh? I'm deeply suspicious of those, as the internal connections to the foils are susceptible to losing contact with the wire lead-outs due to heat conduction from soldering, leading to the classic and increasingly common intermittent open-circuit fault. Above 70 deg C the capacitor's value can change irrecoverably. I'd suggest trying either a disc ceramic or silvered mica in both tuning positions, as Ian recommends; both these types worked well for me, the SM types producing slightly greater output.
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Last edited by Phil G4SPZ; 30th Jun 2015 at 9:16 pm. Reason: Afterthought
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Old 30th Jun 2015, 9:34 pm   #94
David G4EBT
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Default Re: MiniMod pantry transmitter

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Originally Posted by ThePillenwerfer View Post
Pin 4 of the LM386 is grounded. Where's the other part of the probe connected
So it is Joe! I ought not to need a degree in the blinding obvious to have not made that blunder!

That said, there's nothing at the source or drain of Q1 or the base of Q2.

I used polystyrene caps for C1, C2 & C13 s those were what was illustrated in the photo in the Bulletin article.

It's always been my understanding that they have good stability, but if need be, I can try SM or disc ceramic ones.

I'll take a closer look tomorrow, and some voltage checks etc.

Thanks for you input guys.
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Old 1st Jul 2015, 5:09 pm   #95
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Default Re: MiniMod pantry transmitter

Sorted!

I removed the BF256C FET to test it, (on my cheapo Chinese Multi-tester), and whereas the stated pin-outs on the data sheet are 1,2,3 = D/S/G it came up with 1,2,3 = S/D/G. I thought that odd, so I switched test leads to pins 1 & 2 over, and they then came up with D/S/G. Maybe this says more about the tester than the FET, but I decided to change the drain and source leads of the BF256C over, and bingo - a nice clean sine wave on the 'scope, about 3V p-p.

(I do, incidentally, have a Peak DCA50 Transistor Tester some years old, which allegedly tests FETs, but never has done - it thinks they're diodes).

Clipped about 1 M of flex draped on the floor to the antenna pin, and with the cores at the top end of the coils, found a clear spot on the dial at 325M (922.4 kHz) and peaked L2 for maximum signal. (Ian pointed out in the Bulletin article that it's best to steer clear of 930 kHz - the second harmonic of 465 kHz which could cause a nasty heterodyne. No discernible problems, but when I finally set it up I can always tune it lower down the band to get well away from 930 kHz).

As I write, it's belting out Talking Heads 'Psycho Killer' from my I-pod on my Ekco A22. Nice undistorted output with the volume on the I-pod turned down and turned up on the radio.

Just need to box it up now!

A couple of links that might be helpful:

A list of MW stations - just click on the frequency of interest, and it will list the stations. Useful in finding an empty slot on the dial:

http://www.mwlist.org/mwlist_quick_a...area=1&kHz=927

Convert Wavelength to Frequency and vice versa:

http://www.unitconversion.org/unit_c...avelength.html

So, many thanks for the original design Ian, and for the PCB layout which I shamelessly plagiarised, and thanks also to everyone who responded to my post.

I've got my happy face on today!
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Old 1st Jul 2015, 6:48 pm   #96
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Default Re: MiniMod pantry transmitter

Glad you got it sorted.

- Joe
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Old 1st Jul 2015, 7:19 pm   #97
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Default Re: MiniMod pantry transmitter

Well done, David. Happy listening!
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Old 1st Jul 2015, 8:59 pm   #98
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Default Re: MiniMod pantry transmitter

Glad you got it sorted.

You're welcome to 'plagiarise' the layout!

Ian
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Old 1st Jul 2015, 9:53 pm   #99
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Default Re: MiniMod pantry transmitter

It's always a good idea to check the leadout of an unfamiliar transistor on a transistor tester. There are so many variations that even normally rock solid references like Towers sometimes get the leadout wrong.
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Old 2nd Jul 2015, 12:38 pm   #100
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Default Re: MiniMod pantry transmitter

I thought the same - until last weekend, when I tested a new (on tape) PN100 to confirm the pinouts on the Fairchild datasheet, didn't give sensible readings either way. Didn't work in-circuit, so I tried a brand new BC549 from element14 (they were $0.025 so I bought 20 for stock ) - same thing, gave gain of zero. This one also didn't work. Gave up, tried a scraggy looking PN100 I had lying around without testing first, and it worked perfectly.

I think the tester in my Dick Smith DMM might like eating transistors, so might stick some gaff over the holes so I don't forget and try using it again
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