UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 2nd Jul 2019, 8:41 pm   #1
mictester
Triode
 
mictester's Avatar
 
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 29
Default Another "Pantry" Rig for the IC-allergic

I cheated slightly - there's a three-terminal regulator chip in there, but that could be replaced by a series-pass transistor and a zener diode - at the cost of some stability - but the aim was largely achieved. The 100n ceramic caps either side of the regulator need to be as close to its pins as possible to prevent RF getting in there.

The circuit's nothing particularly special and is made up of a Colpitts oscillator (for stability), a buffer stage (with a bit of gain) to prevent the variable loading of the final "pulling" the oscillator frequency about, and a Class C final, which is operated at half the supply rail voltage under conditions of no modulation. The only slightly critical component is the NPO 220p capacitor in the oscillator. I had an ordinary orange-topped ceramic there at first, and the drift was horrible.

The modulator provides the nominal half rail voltage, and waggles the supply to the final up and down with the applied audio. It also provides a small measure of audio filtering.

It's worth making sure that the emitter of the modulator transistor never quite reaches zero, so that the final is never quite completely cut off - this reduces the (already small) harmonic output considerably.

I used 3N3904 transistors throughout only because I've got a few thousand of them (thanks Ebay!). My brother made one using BC546 transistors throughout, and it worked just as well as mine. Use whatever devices you have - within reason! Most of the component values aren't too critical - if you haven't got the exact values, "close enough" will probably work.

It's worth pre-processing the audio that's modulating the rig - compression and limiting will help this little device be heard above the noise floor! My prototype - thrown together in the traditional 2oz tobacco tin - worked well over nearly a kilometre when given a good earth and a 5m "bit of wire" dangling from a tree, powered from a "12V" gel-cell (actually about 14.4V fully charged) on a well chosen "quiet" frequency.

Have fun!
Attached Thumbnails
Click image for larger version

Name:	Mini-MW.png
Views:	281
Size:	4.0 KB
ID:	186102  
mictester is offline   Reply With Quote
Old 2nd Jul 2019, 8:45 pm   #2
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,236
Default Re: Another "Pantry" Rig for the IC-allergic

Very nice. I've added it to my list of projects to build. Thanks for taking the time to write it up!
MrBungle is online now   Reply With Quote
Old 4th Jul 2019, 12:00 am   #3
JohnBG8JMB
Tetrode
 
Join Date: May 2006
Location: Poole, Dorset, UK.
Posts: 69
Default Re: Another "Pantry" Rig for the IC-allergic

Thanks...nice and straightforward. A question:
How much input for full modulation?
73
John
JohnBG8JMB is offline   Reply With Quote
Old 4th Jul 2019, 2:54 pm   #4
PsychMan
Octode
 
PsychMan's Avatar
 
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,209
Default Re: Another "Pantry" Rig for the IC-allergic

Are small axial leaded inductors ok for the coils?
PsychMan is offline   Reply With Quote
Old 4th Jul 2019, 4:11 pm   #5
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 18,062
Default Re: Another "Pantry" Rig for the IC-allergic

Do you actually need the 7805 in there? Presumably it's there to improve frequency stability and minimise FM.
paulsherwin is offline   Reply With Quote
Old 4th Jul 2019, 4:47 pm   #6
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,236
Default Re: Another "Pantry" Rig for the IC-allergic

I'm going to build this over the weekend.

Quote:
Originally Posted by PsychMan View Post
Are small axial leaded inductors ok for the coils?
I wouldn't use one for the 47u inductor. They are terrible things for long term stability. I'd probably use a spectrum communications 45u0L IFT.

Quote:
Originally Posted by paulsherwin View Post
Do you actually need the 7805 in there? Presumably it's there to improve frequency stability and minimise FM.
You would need it for long term frequency stability on a colpitts.

You can side step some of these concerns by using a 1MHz CMOS oscillator module instead of a VFO but you can't tune them away from 1MHz.
MrBungle is online now   Reply With Quote
Old 4th Jul 2019, 11:37 pm   #7
Biggles
Nonode
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,153
Default Re: Another "Pantry" Rig for the IC-allergic

Would those oscillator modules produce a nice sine wave or are they designed to interface with logic? I have a couple of crystals in the spares box around about the right frequency. I may try these as and when I get round to building the circuit. It's definitely on the list of projects to try. (along with all the others). I would imagine the inductors could be home wound if you have an inductance meter.
Alan.
Biggles is offline   Reply With Quote
Old 5th Jul 2019, 1:09 am   #8
mictester
Triode
 
mictester's Avatar
 
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 29
Default Re: Another "Pantry" Rig for the IC-allergic

A little further information:

The original prototype was built "Manhattan"-style on a piece of copper-clad PCB material. I used about 4" X 2", and put the completed board into an aluminium box 6" X 4" X 1" that I had lying around for years. The audio socket(s) were phono type, and the RF out was through a BNC. I used a zener diode on the oscillator supply at first - 560Ω from the 12V supply to the top of a 5V6 zener worked quite well, but the temperature coefficient of the zener diode combined with the thermal drift of the oscillator to make things worse! The 78L05 was a quick fix.

My oscillator coil was initially an axial choke - it oscillated OK, but drifted badly, so I dug out an Amidon FT37-61 ring core, and calculated that I needed 29 turns for 47H. I put mine a bit higher up the band, by winding 26 turns, and the 40p trimmer would allow me to tune from about 1500 - 1650 kHz. 32 turns put me down to roughly 650 - 800 kHz - as the American cousins would say "YMMV"!. I wound the coil with (roughly) 30swg enamelled copper wire that was actually from the secondary of a dud transformer. Once the turns were on, I held them - and the whole coil - in place with candle wax. The oscillator is surprisingly stable considering its simplicity.

It's fully modulated with a few hundred millivolts of audio - however, you can scale the gain of the modulator by changing the feedback resistor. A fairly crude (transistor and FET-based) audio limiter was scaled for the 0.775V nominal output, and this worked well with the little transmitter. I also did a little audio filtering before the modulator, rolling off the top end at 6.5 kHz, but introducing a (sort of) pre-emphasis HF lift from about 1 kHz upwards, which made the overall sound "brighter".

I was considering designing a PCB for this thing, but haven't found the time. Between my brother and myself, this circuit has been replicated about half-a-dozen times now. None of them are exactly this circuit - the component values aren't "set in stone" - I'd just use the first "approximately right" part that I found on my bench!

Improvements:

Is it worth spending much time on something as crude as this? The oscillator stability could be improved by careful selection of components or (better) by using a crystal. The output matching and filtering could be optimised, and a crude ATU could be constructed to load it into the "dangly bit of wire" more efficiently. You could polish this thing endlessly if you wanted to - the circuit shown is just the starting point!

Last edited by mictester; 5th Jul 2019 at 1:13 am. Reason: Typo
mictester is offline   Reply With Quote
Old 5th Jul 2019, 1:10 am   #9
mictester
Triode
 
mictester's Avatar
 
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 29
Default Re: Another "Pantry" Rig for the IC-allergic

Quote:
Originally Posted by Biggles View Post
Would those oscillator modules produce a nice sine wave or are they designed to interface with logic? I have a couple of crystals in the spares box around about the right frequency. I may try these as and when I get round to building the circuit. It's definitely on the list of projects to try. (along with all the others). I would imagine the inductors could be home wound if you have an inductance meter.
Alan.
Those little oscillator blocks are (generally) designed to clock logic circuits, so mostly give a good squarewave.

Last edited by mictester; 5th Jul 2019 at 1:11 am. Reason: Bad English!
mictester is offline   Reply With Quote
Old 5th Jul 2019, 9:25 am   #10
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,236
Default Re: Another "Pantry" Rig for the IC-allergic

Thanks for the write up mictester. Much appreciated. It's an excellent little circuit.

On the oscillator blocks, the square wave doesn't really matter that much if it's a very low power output. The fundamental signal is what you are mostly interested in and there's plenty of that. I have actually built a small CW beacon that uses a 74AC14 as an osc and PA and a crystal modulated by a PIC12C509 driving a 2n7000 to disconnect the oscillator. The matching circuit between the 74AC14 output and the antenna is done with a naturally low-pass LC network and then followed by a 5 pole LC filter so what hits the antenna is pretty harmonic free. I was surprised at "how digital" a transmitter can be! It's also a lot more efficient than running class A amplifiers through each stage.
MrBungle is online now   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 8:25 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.