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Old 9th Sep 2018, 12:32 am   #1
1100 man
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Default Building the 'Minimod' AM modulator-questions!

It seems to be the way that one project leads to the requirement for another!
I am repairing a 'volksempfanger' people's set for a friend and he wants to be able to use it with a CD player or other external input.

I've read most of the past threads on the Minimod and it seems to be very well respected. It's only short range but that's fine: I'm quite happy to have it coupled directly to the aerial input if necessary.
I'm not a particularly keen constructor (I'd much rather repair something) and I don't like things with coils:- I don't trust all that RF stuff!! This design by Ian Liston-Smith appeared in the Winter 2011 BVWS bulletin and looks pleasantly simple and the only coils can be purchased for very little money.
As he has previously posted a pdf of the complete article, I hope it's OK to repeat it here.

https://www.vintage-radio.net/forum/...2&d=1491335051

Apart from some capacitors which should arrive next week, I now have everything else to build it. I had contacted Ian in the hope that a PCB might be available but sadly not, so I will build it on a piece of 'perf' board.
I've built several things, some quite large, on Vero board over the years but never used 'perf' board before.
I find the layout seems to sort itself out as I build, so if I build several of them, they will all be different!

The first question that has arisen is that of the FET connections. These are devices I know next to nothing about but it would seem that the drain & source are named from an electron flow point of view. My conclusion is that the source is the more negative one akin to the emitter on a NPN transistor and the drain is the more positive. Would that be correct? They are not marked on the circuit diagram.

I'm using a BF256 and I can't find a definitive pin out for it. David G4EBT had exactly the same problem in this thread:
https://www.vintage-radio.net/forum/...t=98221&page=6

and I'm not really sure what the conclusion was!

So if anyone can shed some light on what the correct connections actually are, I would be most grateful! Google comes up with all sorts of variations

That will do me for tonight,
All the best
Nick
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Old 9th Sep 2018, 2:53 am   #2
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by 1100 man View Post
The first question that has arisen is that of the FET connections. These are devices I know next to nothing about but it would seem that the drain & source are named from an electron flow point of view. My conclusion is that the source is the more negative one akin to the emitter on a NPN transistor and the drain is the more positive. Would that be correct? They are not marked on the circuit diagram.
Yes, Source is negative, Drain is Positive.

The probable reason they are not marked on the original schematic is that the 2N3819 could be fitted either way round.

Re the BF256, if you know the manufacturer of the device you have, go by their datasheet, if not, try it and see. The worst that I can see (from the circuit) will happen is a dead FET.

If you can get a J310 FET, they can be fitted either way.
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Old 9th Sep 2018, 9:29 am   #3
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Default Re: Building the 'Minimod' AM modulator-questions!

If you had a CCCT (cheap Cinese Component tester) it would identify the lead outs and the device for you. Wonderful device for less than a tenner.
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Old 9th Sep 2018, 11:10 am   #4
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Default Re: Building the 'Minimod' AM modulator-questions!

I've got three transistor testers:

Peak Atlas DCA50, which thinks JFETS are diodes
Peak Atlas DCA55 (bought cheap when on offer from Maplins). Sees it as a JFET, and states 'drain and source not identified' but correctly identifies the gate.
Cheapo Chinese jobby, which obligingly gives three options according to which test lead is connected to which pin!

Basically, none are any use whatsoever in this regard, so I relied on datasheets from which I concluded that the correct orientation is as shown in the diagram below.
I get the impression that provided the gate is correctly identified, either of the two other leads will work as source or drain so to that extent, can be considered identical.

The second pic shows an actual BF256C with the maker's pinout I.D. markings.

The third pic clarifies which lead is which on the cicuit diagram.

The fourth pic shows the PCB artwork which Ian Liston Smith, the designer, kindly provided. I assume that it was created in software intended for commercial PCB production, which doesn't lend itself to homebrew as the tracks are rather too thin and the pads too small, so I used (plagiarised!) Ian's layout to create a design which was more suited to homebrew PCB production. Though the PCB may look quite different from the original, all of the pinouts and positioning of components are identical - the only difference is that I added a current limiting resistor, which I called 'R9' to power an LED as a reminder when the Mini Mod is switched on.

When my Mini Mod didn't initially work, I eventually resolved the problem by changing over the leads of the BF256. Three years down the line, it still works a treat.

I've since made a few others as presents for friends, which have worked straight off.

I know that few are equipped or inclined to make homebrew PCBs, but many people have had success in using the laser printer 'iron on' technique, which obviates the need for a UV light box or developer. Thus, if the artwork below is 'flipped' to reverse it, and if it's printed on a laser printer onto glossy paper so it comes out 'the right way around' when ironed on. If the paper is then soaked off, it should leave the toner in position ready for etching the PCB. The worst that can happen if the process fails is that you wash off the print from the PCB, clean the board and have another go. There are countless youtube videos showing how to do it, though I'd stress that I have no personal experience of using the technique.

I hope that helps a bit.

Good luck in your endeavours Nick.
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Old 9th Sep 2018, 10:32 pm   #5
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by David G4EBT View Post
I've got three transistor testers:

Peak Atlas DCA50, which thinks JFETS are diodes
Peak Atlas DCA55 (bought cheap when on offer from Maplins). Sees it as a JFET, and states 'drain and source not identified' but correctly identifies the gate.
Cheapo Chinese jobby, which obligingly gives three options according to which test lead is connected to which pin!

Basically, none are any use whatsoever in this regard!
Hi David,
I had rather hoped you would respond as it was your posts in the original thread I had been reading.
I must confess I have never felt the need to buy a transistor tester. I have always tested bipolar types in circuit with an analogue meter. Black to the base of an NPN and away you go! I never got used to doing it with a DMM as the leads are the wrong way round!!
I don't know how a FET would test out though on the AVO:- although it's a bit academic as it's not here anyway!
I have a BF256b so I'll go with your drawing and see what happens!

Second anomaly concerns the coils. One winding has a tapping which I assume is the primary? This is fairly obvious for L1, the oscillator coil but not so for L2, the aerial coil.
I would have assumed that the primary (the side with three pins) would be connected to the BC109 with the tapping unused. The secondary with two pins would go to the 270pf cap and the aerial.
Looking at your PCB layout, it looks like your L2 coil is connected the other way round- ie the three pin side goes to the aerial?

I have been involved in several projects some years ago where we made our own PCB's. I seem to remember that the artwork had to be printed on paper from an inkjet printer and then photocopied onto transparent film and then UV exposed onto photoresist board. Getting enough density from the photocopies was always a problem. We did have access to an early laser printer but had issues with accuracy. We were making eurocard sized boards with a dozen or so TTL chips and a CPU plus a full width edge connector. Double sided print as well!!
My mate did the artwork with only a manual PCB routing program- no autoroute! His head very nearly exploded! I drilled the million holes with a pillar drill and loaded the boards.
All done in his flat with minimal equipment- madness really but we did manage to produce pretty good boards! Ferric chloride and something nasty to develop the boards which got upset on the kitchen floor and dissolved all the cardboard boxes....

I guess there are some much better ways to do it now. Even getting small runs of boards made nowadays is quite cheap.
But for this project it's not taken long to build it on the perf board so a PCB is not really necessary but would make a much more professional looking job.

Many thanks for your help and to the other respondents to the thread.
All the best
Nick
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Old 10th Sep 2018, 2:49 am   #6
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Default Re: Building the 'Minimod' AM modulator-questions!

The article you posted does (in the paragraph RF Output, second page), mention that the low impedance (secondary) side of L2 is connected to R8 and the BC109.

But it is not that intuitive when looking at the schematic.
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Old 10th Sep 2018, 8:44 am   #7
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Terry,
Thanks for the reply. I had read the article a couple of times but failed to note the significance of that sentence! I just assumed that the driven winding would be the primary. Good job I noticed the orientation of the coil when studying the PCB layout and asked the question!
I wonder how many others have fitted it the wrong way round? I imagine it would probably work but with lower output?
I'd best go and remove mine and fit it the correct way round....

Many thanks
Nick
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Old 10th Sep 2018, 9:14 am   #8
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by 1100 man View Post

Second anomaly concerns the coils. One winding has a tapping which I assume is the primary? This is fairly obvious for L1, the oscillator coil but not so for L2, the aerial coil.
I would have assumed that the primary (the side with three pins) would be connected to the BC109 with the tapping unused. The secondary with two pins would go to the 270pf cap and the aerial.
Looking at your PCB layout, it looks like your L2 coil is connected the other way round- ie the three pin side goes to the aerial?

Nick
Hi Nick,

Yes, the tapped winding of L2 goes to the aerial, but the tap isn't used. I can best quote from the article, which makes this clear, and I've annotated the component placement sketch of the PCB I designed, which uses exactly the same layout and connections as does Ian's original PCB.

Here's a quote from the original article:

'L1 & L2 are both 90H variable inductors. These are tapped at 25% from one end and have a low impedance secondary winding. The secondary is not used in L1 and the tapping is not used in L2. One end of L1's winding should be grounded to aid frequency stability'.

End quote.

The sketch below will hopefully make this clearer.

I've also attached the original circuit so it can be compared with the layout.

Good luck with the project Nick!
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Old 10th Sep 2018, 9:46 am   #9
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Default Re: Building the 'Minimod' AM modulator-questions!

Just one observation on this circuit...

The J-FETs used have very large - absolutely huge - variation from part to part allowed in their data sheets. The 2N3819's max current (at Vgs=0) can be anywhere between 2 and 20mA and if you bias it for 0.2mA current the gate-source voltage may have to be anywhere between 0.5 and 7.5 volts.

What this means is that not all examples of those FETs will work in the circuit, and you may be unlucky and find one which isn't a go-er. You may have to try ones from a different batch before you get lucky.

So if you're making an early foray into the world of field effect transistors, don't be discouraged if it takes a bit of work to make it go. It isn't something you've done wrong. It is VERY difficult to design sure-fire circuits with JFETs. And simple looking circuits usually aren't the sure-fire ones.

2N3819 is THE original popular JFET. Later part types came in graded bands so the spreads were reduced, a bit like the coloured dot of paint on early bipolar transistors, though some parts didn't get suffixes, the different grades sometimes got different type numbers like 2N4391, 2N4392, 2N4393.

David
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Old 10th Sep 2018, 11:05 am   #10
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Default Re: Building the 'Minimod' AM modulator-questions!

Well, after all these years the little thing is still generating interest. That's very rewarding!

I think everyone has said more or less what I would have covered.

Just a few points.

I did use the 2N3819 to start with, but as has been said, their spec has a large spread and I had one or two which didn't work very well (as radio Wrangler predicted!), so decided to use the BF256. But as has been noted, the pinouts are different and the clear layout posted by Dave G4EBT show this. As far as I know, few if any component testers can be relied upon to identify the drain and source of all fets. Some are truly symmetrical, some probably are not - I don't really know. Different manufactures of the same device are apt to label them differently - so please refer to the manufacturers data sheet.

Most of the components are not critical in value or type - except those mentioned in the article. However, I did find that a cheap ceramic capacitor used for C13 dropped output by a bout 3dB!

Be sure that L1 is the correct way around. I think the coil's data sheet should give a clue where the tap is. But again, Dave's layout makes this clear.

I didn't include a layout in the original article, partly because it forces people to source all the components of exactly the right size when this isn't necessary. And as mentioned, the layout I did eventually post in one of the threads was really only for my guidance as I used it to wire the components up underneath, not to print boards from. Although if I'd known how many I was going to make I would have made a proper PCB layout with the correct pad and track sizes!

Resistor R8 is used to limit the current through L2 and stop it saturating and distorting the waveform on modulation peaks. I haven't tried it, but I expect primary and secondary coils wound on a small ferrite toroid with the same inductances wouldn't need R8 and would probably give a bit more output...

And one last thing, C12 680nf is a slightly awkward value to find. A small electrolytic might have been preferred here, but there's no suitable polarising voltage. Nevertheless, an easier to source 470nF capacitor here gives a very slight reduction in bass response, but I doubt it's noticeable in practice.

Anyway, thanks for the continuing interest. Maybe one day I'll see the MiniMod for sale on ebay for 1.99 including postage like some other kits.

Ian
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Old 11th Sep 2018, 9:00 am   #11
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi David,
Thanks for the clearly annotated layout drawing- it makes it much clearer now. I'll go with your FET pin out and see what happens! I've now removed the L2 coil and turned it round.
Most of the caps came from RS (including the 0.68) but they wanted fortunes for the polystyrene ones; over a pound each in packs of ten which gets expensive when you need several different values! So they are coming from various Ebay sources and should turn up today. Hopefully I can finish it off tonight and see if it works.

David (RW), thanks for the extra interesting information about FET's- they are not things I've ever played with before.

Ian,
I'm sure the need for and interest in the Minimod is probably greater now than ever. With MW stations disappearing, an easy to build modulator will be essential.
I'll report progress tonight if the rest of the caps turn up.

All the best
Nick
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Old 11th Sep 2018, 9:42 pm   #12
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Default Re: Building the 'Minimod' AM modulator-questions!

I have a circuit for a valve AM modulator which I will get around to building sometime. I think it uses an ECC82 or if you want a more vintage feel, an ECC35 (6SN7).
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Old 11th Sep 2018, 11:15 pm   #13
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Default Re: Building the 'Minimod' AM modulator-questions!

Well, would you believe, the thing actually works!! In view of the FET and the questionable pin out, I had very low expectations that it would oscillate. However, I connected the battery and scoped the base of the BC109 driver transistor and low and behold there was a thumping great sine wave of about 1Mhz!!
Injecting a 1Khz tone (found on the internet: where else!) to the 0.68mfd input capacitor, produced a nice AM envelope as shown.
I adjusted L2 for maximum amplitude and then dug out a valve radio to try it out on.

I had not listened to a medium wave radio for at least 10 years and had forgotten just how bad it sounded (no, don't throw things at me!). Hearing music limited to 4.5 Khz came as a bit of a shock! I remember, years ago, when radio 1 was on MW, Steve Wright described it as "muddy radio"! I always looked forward to the times when it was allowed to use radio 2's FM transmitter.

Once my ears had got used to that MW sound again, and with the right type of music, it actually sounds pretty good. I'm sure a better radio would help as well: this Defiant is pretty crappy!
So, all in all, my conclusion is that this is a great little design which is easy to build. I probably spent about 2 hours in total building it and asking the questions about the FET's pin out and the coil connections. My construction was probably more crammed than was necessary, measuring only 2.25" by 1.5". I'm very glad it worked first time as removing components would have been a real pain. I think the 'perf' board was actually more convenient than Vero board as you don't have to worry about breaking tracks.
So thanks Ian for designing such a handy modulator: I will be building several more I think!

All the best
Nick
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Old 12th Sep 2018, 8:43 am   #14
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Default Re: Building the 'Minimod' AM modulator-questions!

Nick - is there any FM on board? You can check for this by carefully tuning to the centre of the transmission and seeing if there is a quiet bit in the middle. I built a number of similar mini-transmitters back in the 60's & 70's and they all FM'd to a certain extent.
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Old 12th Sep 2018, 9:08 am   #15
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Default Re: Building the 'Minimod' AM modulator-questions!

I would be very interested in trying to construct one of these. Could I ask a couple of questions though, are Spectrum Communications still trading and are the coils still avaliable? Also I am a little confused regarding the FET's - which are the correct ones to use please? I think the perf board as used by Nick would be easier for me as I am not too confident in trying to construct a PCB. Any advice would be much appreciated!
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Old 12th Sep 2018, 10:34 am   #16
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Default Re: Building the 'Minimod' AM modulator-questions!

Looks good Nick. Layout is not particularly critical. The coils are screened (or should be if you earth the covers!), but the LM386 can get a bit 'feisty' if the decoupling caps are not located close to the pins.

As the article describes (found at first link in post one above) there is a small amount of FMing, mostly due to there being no VFO buffer or VFO voltage regulation, just to keep the circuit simple. In any case it is completely undetectable in normal AM use. However, it is detectable on strong modulation peaks if you listen on an SSB receiver.

To keep things simple, I suggest the fet to use is the BF256. Spectrum Communications is still operating (I think some one in this thread recently bought the coils from them), so they can supply the 90uH TOKO 10mm coils and also the BF256B fets. They sell the majority of the other parts, including the LM386 and BC109 (although any equivalent silicon NPN transistor will do).

I have no commercial connection with Spectrum Communications, but the proprietor sells lots of commonly used components at very reasonable cost. His website components list is dated 5 August 2018. Here: http://www.spectrumcomms.co.uk/Components.htm

If you look on this forum there is at least one other MiniMod thread. The longest one is here: https://www.vintage-radio.net/forum/...hlight=minimod

Regards,

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Old 12th Sep 2018, 11:26 am   #17
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Default Re: Building the 'Minimod' AM modulator-questions!

Many thanks for the links and advice Ian, much appreciated. It would be useful to be able to purchase most of the components from one source, I'll drop them an email.
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Old 12th Sep 2018, 9:29 pm   #18
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Howard,
I would definitely recommend building this. I'm not an RF sort of person and didn't want a design where I had to get involved winding coils. Being able to buy them easily and cheaply was the deciding factor for me.

I kitted up to build three units so I ordered 6 coils (90UOL is the stock code), 3 BF256B FET's and 3 BC109 plus 10 390R resistors from Spectrum. Total cost including postage was 8.45 and their service excellent.
All the caps, apart from the polystyrene ones, came from RS Components. They are great as there is no minimum order & free postage. Anyone can set up an online account. I can give you all the stock codes if that would be useful?
Ebay was the most economical source for the polystyrene caps.

If you get the FET from Spectrum, the pin out given in David's picture above is correct. Be sure to take on board the correct connections for L1 & L2: I got confused about L2!

By design, this is a low powered device so you won't be able to receive it all round the house. It's aerial needs quite close coupling with the radio's aerial. I simply twisted a few turns of both aerial wires together and it works well.
I only need this to work with single sets so that's fine for my needs.
The perf board works well as you just use the component leads as connecting wires. No risk of forgetting to cut tracks like with Vero board!

Hope that helps,
Cheers
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Old 12th Sep 2018, 10:10 pm   #19
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Nick

That's very helpful information, thank you very much. I feel reassured as not only am I not very RF, my experience with semi conductors is very limited too! I would also feel more confident using the perf board. I've been doing some research into making your own PCB's but feel its a bit outside my comfort zone at the moment although its something I really should try to get involved with.

Thanks also for the references for Spectrum. Their prices seem very reasonable. Regarding the caps, I have an account with Cricklewood who I see do stock the polystyrene caps although not the 270pf although I'm pretty sure that I have one or two in stock.

I'll definitely have a go at it and report back The advice regarding the coils and FET's is very useful, I'll check that very carefully.

Coincidentally David has very kindly reserved one of his FM/AM converters for me. This will give ultimate flexibility as I can listen to FM radio plus the facility to play MP3's and even Radio Caroline from an old smartphone. I certainly won't be short of things to listen to!

Thanks again
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Old 12th Sep 2018, 10:31 pm   #20
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Default Re: Building the 'Minimod' AM modulator-questions!

Ah, a fellow Caroline Listener As Ian said, layout is not critical and a PCB is not necessary. Making one PCB is a lot of effort to go to and there is no shame in using perf board or Vero! Ferric chloride is very messy especially in one's kitchen!
Please feel free to ask if you have any further questions.
Happy construction,
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