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Old 10th Sep 2018, 11:05 am   #10
Ian - G4JQT
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Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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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