AM 'Pantry' Transmitters

[boiss]

My best one used an 813 with an EF80 in triode modes as a screen modulator.
Input to 813 running on Ht of 25V was 1mA, Apart from the filament which takes a whopping 36W (on 9V glows a nice yellow colour). Power output <1mW into 50 Ohms, Harmonics & IPs<-50dBm, (noise limit of Spectrum Analyzer and impossible to measure IPs from 100Hz PSU ripple) Pics in Gallery.
Mike

[Yorkie]

It might be worth mentioning for those who don't already know, that forum members Ian, G4JQT, and Peter, 'Kalee20' have both produced designs which have attracted much interest on the VRF, and in Ian's case, also in the BVWS Bulletin (Vol 36, issue 4, Winter 2011) where his 'MiniMod' was described and illustrated fully in Ian's excellent article.

Peter's was a sophisticated design employing (if I counted correctly!) 15 transistors and three IC voltage regulators. It's as good an example as any of how versatile 'perfboard' is. All housed in a nice wooden cabinet evocative of a crystal set. As an indication of the interest that Peter's design generated, the thread had 34 posts and 4663 views before the thread was closed. (Huh?). See this link:

http://www.vintage-radio.net/forum/showt...hp?t=53295

Ian's design was altogether simpler, using one IC (the ubiquitous LM386), two transistors, two readily and cheaply available inductors, a diode and a few Rs and Cs. As an indication of the popularity, to date, the thread has received 40 posts, 4000 views, and is still open:

http://www.vintage-radio.net/forum/showt...hp?t=98221&highlight=Mini+Mod

Impossible to say how many have been built, (including by members of this forum!), but given its simplicity and the Bulletin article, my view would be 'a lot'. Few problems have been reported in building the 'Mini-Mod' and many favourable comments have been made. The original Tolko coils are no longer available, but Spectrum Communications sell an alternative one very cheaply. (both are identical 90uH - the secondary isn't used on one coil, the tap isn't used on the other).

If Ian reads this thread, a little plea from me (the mini-mod' is on my 'round tuit list!):

The Bulletin article didn't give the artwork for the PCB. Is it available, or did you just make the PCB as a one-off?

[Joe]

Indeed, David.

It was easily the most complicated thing I'd built on veroboard but it's so well designed that it worked first try.

- Joe

[Yorkie]

Did you build Peter's ('Kalee20') 15 transistor jobby on perfboard Joe, as per his original, or are you referring to Ian's 'mini-mod' design?

I know that you built the mini-mod, but I didn't know that you'd had a pop at Peter's design.

[Joe]

It was the Mini-Mod I meant.

           

I did have a pop at Peter's but failed utterly. I didn't have the right bits for one thing so am not blaming the design. Again, a PCB would no doubt help but I can only too well understand people not going to the hum-big of producing them.

   

- Joe

[Ian - g4jqt]

Thanks for the great feedback!

It just so happens that I'm looking to have some PCBs made for my MiniMod design. At the moment they are looking to cost about £6 each (2.5" x 2.5"), unless anyone knows of a cheaper supplier. (I've tried the toner-transfer method - unsuccessfully.)

Peter's design will probably have the advantage over mine of being able to give 100 percent modulation. My basic design will give about 80 percent, though in practice this is not really noticeable.

I've read an article in Feb 2014 Practical Wireless (p53) describing in detail the inductors I specified for use in the MiniMod and there's a point which needs highlighting.

The tapping in the tuned winding (L1) is 25 percent from the earthed end. Although I stated this in my BVWS article (and this is also shown in the circuit diagram) it may be worth repeating for anyone else wishing to build one.

I expect anyone who has already built one either measured the resistance to identify the correct end to earth, or referred to the Spectrum Communications information from the link in the article for pin identification.

I've also learnt recently that the BF256B has better defined characteristics than the 2N3819. I've not tried this substitute yet, but will probably be using it for any MiniMods I make from now on.

Providing a PCB layout has proved problematic. I use the free software ExpressPCB. I find it generally very good, but the catch is they really want you to email them your design for manufacture, and copying the 'track only' is not a feature in the software! It can be emailed to be read by anyone with ExpressPCB or printed out on paper.

Regards,

Ian

PS: I can provide pdf of my original article published by the BVWS in their Winter 2011 Bulletin and sheet with the PCB layout for a SAE.

[Joe]

If you can print the PCB artwork onto paper how about sending to a 'Virtual Printer' such as Cute PDF or PaperlessPrinter. These things catch what would normally go to the printer and save it as a PDF, or in the case of the latter, other formats.

Sorry if I'm telling you what you already know.

I've got Express PCB so could have a pop at separating the bits out for you. I make no claims to cleverness but cunning is a different matter.

- Joe

[Radio Fixer]

Hi Ian

Please put me down for 2 PCBs if you get some made. I e-mailed you some time back and said I had all the components but me and Perf board don't get on. I'm so busy with house improvements, and radios in between, getting PCBs is about the only way in the near future that mine will get built.

I am currently using the Peter Lankshear designed valved "Mate". I'm interested in yours as using battery may cut the intermod hum. With careful construction of the Mate it is low but I would like to try one of yours (I was going to post Peter one as a present if I get them built).

My Mate has been running 24/7 for maybe 5 years but Peter must hold the record as his has been running continuously from the 70's.
With 2 feet of wire as an aerial it covers the house for sensitive receivers (tuned RF stage).

I did once take it over to a friends house who was using an SSTRAN and there was very little between them for quality. The SS had possibly a little more treble.

thanks Gary

[Yorkie]

Thanks for the update Ian.

I've got Express PCB and would appreciate the artwork if you could e-mail it to me.

I've sent you a PM.

[Ian - g4jqt]

Hi Ian

Please put me down for 2 PCBs if you get some made.

I'll try and remember. I've made a note, but if you don't hear from me in a month or so send me an email or a PM.

I am currently using the Peter Lankshear designed valved "Mate". I'm interested in yours as using battery may cut the intermod hum.

Just had a look at the "Mate" cct. Doesn't applying modulation in series with the oscillator also produce a lot of FMing?

It doesn't usually matter (my MiniMod 'FMs' very slightly) but if there's another signal on the frequency you can get a 'grumbling' sound if it's FMing more than a few tens of Hz.

It's the price you pay for simple designs without a crystal or DDS generated carrier.

Regards,

Ian