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Old 1st Oct 2018, 7:57 pm   #61
crackle
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
Thats a very kind offer, I will send you a PM.
cheers
Mike
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Old 22nd Oct 2018, 8:57 pm   #62
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Default Re: Building the 'Minimod' AM modulator-questions!

I managed to complete the Mini Mod project with much practical assistance, advice and encouragement from a very kind Forum member!

The capacitors were sourced from RS – the task was made much easier with the product codes to hand kindly posted by Nick. I had the polystyrene ones in stock although they are readily available from ebay. The other components were sourced from Spectrum, very fast and helpful service from Jean.

I was conscious of the potential problem with the BF256 which David has highlighted and was ready to swap over the connections should the unit fail to oscillate. However I was in luck, after connecting a frequency counter to the output I was greeted with a signal. I then connected an audio signal generator to the input and tuned in the radio. Although I was rewarded with a healthy signal unfortunately it landed on a powerful station so I had to adjust L1 to find a clear spot on the dial. I then adjusted L2 using an oscilloscope to find the peak in the signal. The cores are fairly stiff to adjust and I had some difficulty finding a suitable non metallic tool. Eventually I filed and shaped a dental flossing stick and managed to make the adjustments. It is vital to use something with a very square profile, any slight taper will not engage properly with the slots. However as the cores are fairly tight this should mean that they are unlikely to move.

I did note some discussion about frequency drift. Out of interest I connected the frequency counter and observed the readings over time. The results were:-

1.02164 Mc/s at switch on
1.02125 Mc/s after 10 minutes
1.02149 Mc/s after 30 minutes
1.02165 Mc/s after 60 minutes

So that does seem very stable. The unit does give good results although I found it necessary to place it quite close to the receiver. I’ll have an experiment with the length of aerial and possibly an earth connection.

All in all a very neat little unit which gives a good account of itself giving a good balance of component count and performance.
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Old 23rd Oct 2018, 11:56 am   #63
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Default Re: Building the 'Minimod' AM modulator-questions!

Howard built his Mini-Mod on the PCB that I designed back in 2015 (based on the same layout as that of Ian's original design). I recently modified the PCB - still based on the 2015 layout - but leaving more copper on the board. I only did that to reduce the etching time, and hence, the risk of undercutting narrow tracks in the etching process, not that the 2015 layout suffered from that.

Regarding drift/frequency stability, I've just built a Mini-Mod based on the updated PCB and like Howard's, there is minimal drift on mine. At switch on, the frequency was 998.900 kHz, and after three hours, it was 999.283 kHz, so in that time it had only drifted 383 Hz. I dare say that any valved radio with which the Mini-Mod was used would have drifted more than that! (I used polystyrene caps for C1 & C13, as before).

I'm not sure where I sourced my BF256s from, as I've had them for years, but as with the previous Mini-Mod that I made, the pinouts didn't conform to the data. Initially the transistor wouldn't oscillate but on changing the drain and source connections, it worked fine, with a nice clean sine wave about 10 V p-p. I know that Howard got his BF256 from Spectrum, and it world fine straight off.

A current model Peak Atlas DCA55 transistor tester would only identify the Gate, an earlier DCA50 thought a BF256 was a diode, and a Chinese cheapo multi-tester correctly identified that it was an N Channel J-Fet, but would obligingly call any lead anything, according to which test clip went where.

I made a waterslide transfer to 'tart up' the front panel, and a little comb-jointed box in which to house the Mini Mod. It works fine with 45cms of wire as an antenna.

First pic in the 2015 PCB.
Second pic the 2018 PCB.
Third pic outlines the issues I had with the BF256 connections.
Fourth is the front panel transfer.
Last is the boxed up Mini-Mod, much like my earlier one.

Sincere thanks to Ian, J4JQT, for all the enjoyment and intellectual stimulation that I've had from building this great little gizmo, helping to keep homebrew alive!
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Old 23rd Oct 2018, 8:57 pm   #64
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by David G4EBT View Post
I made a waterslide transfer to 'tart up' the front panel, and a little comb-jointed box in which to house the Mini Mod. It works fine with 45cms of wire as an antenna.
Hi David,
Once again, I'm in awe of the lengths you will go to to finish off a project! I don't realistically expect I shall ever get round to putting mine in any form of case, let alone a handmade comb jointed box complete with printed front panel!
I don't suppose you would consider making a small batch of bare PCB's for Forum members? It would certainly make construction easier!

Howard, Good to hear you have successfully completed yours- well done!

I am planning to add an auto level control circuit to mine to keep the modulation depth at optimum. As suggested by other forum members, this will be based on the BA3308 chip. I now have one of these to play with so hopefully I can come up with a design whereby the Minimod can be fed from a wide range of input levels.

Cheers
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Old 24th Oct 2018, 7:22 pm   #65
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Default Re: Building the 'Minimod' AM modulator-questions!

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Hi David,
Once again, I'm in awe of the lengths you will go to to finish off a project! I don't realistically expect I shall ever get round to putting mine in any form of case, let alone a handmade comb jointed box complete with printed front panel!
I don't suppose you would consider making a small batch of bare PCB's for Forum members? It would certainly make construction easier!
Nick
Thanks for the compliment Nick, which is appreciated, but as I've said before, I make the boxes with a router jig that I made and a cheap DIY store router I was given, so there's no great skill involved, but clearly, they're time-consuming. I like the 'retro' look, evocative of the crystal set era when everything seem to be in a comb-jointed box! Despite the time taken, if we enjoy doing what we do, (and I do) it isn't a chore. The front panel was just knocked up in MS 'Paint' and printed onto a waterslide transfer ('decal') paper. Again, it's no big deal. There's room for us all in the hobby whatever our approach. Some have very limited workshop space and equipment and lots of demands on their time. Some like to quickly tack things together and put them into any old box they have to hand, 'to see if it works' which is fine, but that's never been my way.

As to 'making a small batch of Mini-Mod PCBs', it does pose a problem (for me or anyone else), in that it's time-consuming and not economically viable, but if there are still forum members who would like to build the Mini-Mod, (which has now been around for four years), to try to help them along the way, I could probably manage to make half a dozen boards for say £6.75 each inc post (to UK only), to cover my costs for the FR4 glass fibre laminate and consumables. (Micro-porous film for the UV mask, UV photo-resist dry film, sodium carbonate for the developer, sodium persulphate for the etchant [not messy ferric chloride], acetone to strip the resist from the PCB after etching, HSS dill bits to drill the board).

Some will say "it makes no sense to make your own boards - you'd get them cheaper than that from China just by sending them a Gerber file".

I'm sure that they're right - professionally made screen-printed PCBs with plated-through-holes are a delight to behold, but ordinarily, I only make individual boards for my own homebrew projects, and I don't do it to save money - perverse as it may seem to some - I do it because I enjoy it. I don't know what a 'Gerber file' is and I dislike with a passion the free software that PCB houses supply as they're not intended for homebrew, but for commercial production techniques. That's a topic for another time.

For anyone who might wish to make their own PCB, the artwork for the UV mask is below.

The first pic shows the track side as it will look when etched, then positive mask in reverse, which is how it should be printed so that in use, the ink side will be next to the laminate when being exposed to UV, and likewise, the negative mask in reverse as is needed for the UV dry-film technique. The reversed positive artwork can also be used for the iron-on laser print technique, of which I've no personal experience, but which some claim to have success with. If it is ironed on in reverse, ('mirrored') the design will be the correct way round.

The second pic is the component layout and an 'X-Ray' of the PCB.

I added R9 to the original design - a limiting resistor for an LED, but if an LED isn't needed, that can be left off.

When I've made half a dozen Mini-Mod PCBs, hopefully over the next few days, I'll put them in the appropriate section - 'items for sale'.

Hope that's of help and interest.

'The woods would be very silent if the only birds that sang were those that sang the best'.
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Old 25th Oct 2018, 10:03 am   #66
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi David,
Sorry, it wasn't my intention to force you into making PCB's for everyone!! Although re-reading my post above it does rather look like it was!
I have been involved in small scale kitchen PCB production in the past (Ferric Chloride- very messy!) so I know how much effort has to go into it. Your suggested price is very reasonable considering the materials as well.

If you do make some, I would love to have two if I may? With stations disappearing from MW, I suspect devices like the Minimod will be even more in demand than previously.

Thank you for the detailed post and for sharing your artwork.

Many thanks
Nick
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Old 26th Oct 2018, 9:14 am   #67
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Default Re: Building the 'Minimod' AM modulator-questions!

Thanks for your comments Nick.

No - no pressure!

I hadn't wanted to appear to be whinging, just to say that while I can cope with making a small batch of say six boards, due to time constraints I can't offer an open-ended service to make PCBs. I appreciate that few have the facilities to make their own boards, so as far as I can, I try to be supportive, especially given that this rather nifty and deservedly popular little project was originally conceived and designed by a fellow forum member back in 2014.

In some respects, it's a little easier to make half a dozen boards than one-offs as I use sodium persulphate for the etchant - a clear liquid that turns pale blue in use, but which has the slight drawback that it needs to be heated in an etchant bath (AKA a cereal container!) to 45 C. I do that by using two aquarium heaters with their thermostats overridden. It therefore makes sense to etch a bath of boards at the same time. (No fish are harmed!). No short cuts with the drilling though.

As I said in my earlier post, I'll get on with making six boards, and will reserve two for you Nick. I'll PM you to sort out the details.

Hope that helps.
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Old 26th Oct 2018, 4:57 pm   #68
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Default Re: Building the 'Minimod' AM modulator-questions!

David

I'd like one if it is possible. My forays using Ferric Chloride are long in the past and were never very successful. Your work is impressively neat!

JB Nick
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Old 26th Oct 2018, 5:28 pm   #69
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Default Re: Building the 'Minimod' AM modulator-questions!

Just a reminder to anyone thinking of building a Minimod, there are some minor circuit changes since the article was published in the BVWS Bulletin, Winter 2011.

C12 can be reduced to the slightly easier value to source of 470nF rather than the original 680nF, with no signigicant change in bass response.

Q1 FET is now BF256 as it has more predictable performance than the original 2N3819. Note the pinouts are different, although David's layout takes this into account.

R2 can be reduced from 10k to 3.3k or 2.2k for very slighty better signal output with the changed FET.

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Old 27th Oct 2018, 3:20 pm   #70
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Junk Box Nick View Post
I'd like one if it is possible. My forays using Ferric Chloride are long in the past and were never very successful. Your work is impressively neat!
I'll reserve one for you Nick and will drop you a PM for details.

I've just finished a batch of six PCBs, so with two going to Nick '1100 man' and one to you, that leaves three available, which I'll list in the 'items for sale' section of the forum. The boards are drilled and tinned, and I've done a 'test fit' with a coil in the L1 & L2 positions to make sure the coils fit in OK. I've soldered eight terminal pins for the off-board connections, and have also fitted a turned pin DIL socket fitted for the IC.

Thanks for your interest, and kind words Nick.
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Old 28th Oct 2018, 11:24 am   #71
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Default Re: Building the 'Minimod' AM modulator-questions!

Just a reminder that the two coils for the Mini-Mod can still be obtained from Spectrum Communications, which is a small outfit - Tony Nailor and his wife Jean. You can also get the BF256 FET, the BC109, the LM386N-1 IC and no doubt most, if not all, of the other bits from them. From their website, as of 5th August, 2018 the prices were:

10mm 10K RF/IF COILS: Two are required – 90uH, type ‘90u0L’, listed at 80p each.
BF256 FET 50p each, BC109C 15p,
LM386N-1 £1.00.

P&P is £1.50.

http://www.spectrumcomms.co.uk/Components.htm

When I buy stuff from Spectrum, I usually ring up and place the order with Jean, on 01305 262250 and pay by debit card so they don't have to stand PayPal or credit card charges.

Incidentally, some forum members may be aware that from 2004 Tony Nailor wrote an excellent long-running series in Practical Wireless entitled 'Doing it by Design' and 'Technical for the Terrified' which suddenly ceased about a year or so ago. I'd wondered why, and the reason is stated by Tony on his website. I mention that not for debate or comment, simply for information.

Hope that's helpful.
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Old 29th Oct 2018, 10:03 am   #72
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Default Re: Building the 'Minimod' AM modulator-questions!

Just a note to say that I've received ten further requests in all for PCBs and have contacted those members by PM to say that I'll fulfil those requests.

As explained in the thread below, I'm not in a position to accept any more requests due to time constraints and other commitments:

https://www.vintage-radio.net/forum/...d.php?t=150861

Sorry if that sounds a bit unhelpful, but as I said in post #67 above, I can't offer an open ended commitment to producing PCBs.

It's gratifying to note the renewed interest in the Mini Mod.
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Old 29th Oct 2018, 1:08 pm   #73
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Default Re: Building the 'Minimod' AM modulator-questions!

Figuring that David would get asked for more pcb's than he has time to commit to, I have drawn up his pcb layout in a CAD package.

After private correspondence with David, the (Gerber) files to enable having the boards made in a commercial factory will be posted in this forum sometime tomorrow.

I just need to do one last double check before they are posted, but don't have the time before tomorrow.
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Old 30th Oct 2018, 5:44 am   #74
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Default Re: Building the 'Minimod' AM modulator-questions!

Gerber files in the attached zip as promised and a jpg of the pcb.

If anybody finds a mistake, please post here so others are in the know and I will fix up the files as required as I haven't made any pcb's from these files (and won't be doing so).
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Old 30th Oct 2018, 8:02 am   #75
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Ian - G4JQT View Post
Q1 FET is now BF256 as it has more predictable performance than the original 2N3819. Note the pinouts are different, although David's layout takes this into account.
R2 can be reduced from 10k to 3.3k or 2.2k for very slightly better signal output with the changed FET.
Ian
I was wondering if someone could explain more about changing the value of R2. Is this recommended change to reduce the extra output from Q1 or is it to help boost the level from Q1, I didnt quite understand it.

thanks
Mike
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Old 30th Oct 2018, 5:57 pm   #76
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Terry_VK5TM View Post
Gerber files in the attached zip as promised and a jpg of the pcb.

If anybody finds a mistake, please post here so others are in the know and I will fix up the files as required as I haven't made any pcb's from these files (and won't be doing so).
Thanks for going to all that trouble Terry. The jpg looks excellent - it puts my modest efforts to shame!.

Can you or anyone else recommend a free (and 'hassle free') viewer to enable the Gerber files to be viewed please?

Now that Terry has done the 'spade work' in creating the Gerber files, if anyone (higher up the food chain than me!) wishes to get any Mini-Mod PCBs made by a PCB outfit, all they have to do is to send the files and ask for a quote.
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Old 30th Oct 2018, 9:35 pm   #77
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Terry_VK5TM View Post
Gerber files in the attached zip as promised and a jpg of the pcb.

If anybody finds a mistake, please post here so others are in the know and I will fix up the files as required as I haven't made any pcb's from these files (and won't be doing so).

Hi Terry
Well done it looks excellent.

What size is the PCB? Most manufactures will want to know.


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Old 30th Oct 2018, 10:45 pm   #78
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Default Re: Building the 'Minimod' AM modulator-questions!

It's 75mm square Frank, as shown in the pic in post 65.
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Old 31st Oct 2018, 12:32 am   #79
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Default Re: Building the 'Minimod' AM modulator-questions!

Thanks David.

That jpg from the files is a straight transfer of your drawing into the package, with only minor position adjustments, so all the credit goes to you for the original.

And for the curious, it was done in Sprint Layout 6, a very homebrewer friendly CAD program.

This is one of those perennial projects that crops up at regular intervals, so worthy of having that little extra done to keep it up to date.

Can you or anyone else recommend a free (and 'hassle free') viewer to enable the Gerber files to be viewed please?

Free I can do, hassle free, not so sure. They do need a little understanding of the Gerber format to get sensible output from. This is the one I use (free version):

https://www.pentalogix.com/t/software-products/viewmate

Frank.C - the gerber files contain the board outline layer that defines the size, which as David said, is 75 x 75mm (well, 74.9 x 74.9 if you want to be really really accurate).
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Old 31st Oct 2018, 12:46 am   #80
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Default Re: Building the 'Minimod' AM modulator-questions!

Just because I like to fiddle occasionally (don't know why, but drawing up pcb's relaxes me), here is a version I did at the same time which is 50.8 x 50.8mm (or 2" x 2" in old money).
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