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Old 12th Sep 2018, 11:24 pm   #21
Half a Mullard
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Default Re: Building the 'Minimod' AM modulator-questions!

Please feel free to ask if you have any further questions.

Certainly will Nick, thank you!
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Old 27th Sep 2018, 12:29 am   #22
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Default Re: Building the 'Minimod' AM modulator-questions!

Apart from the polystyrene caps, which were rather expensive as they were only in multiples of 10, I sourced all the caps for the project from RS. I am planning to build three of these units, so caps in multiples of 5 were fine for me.
All of the following were acceptable from a size, functionality and price point of view.
So, as requested, here is a list. I tend to use RS as they have no minimum order and the bits will be there next day. Anyone can set up an online account.

C1, C13 Polystyrene 270pf sourced from Ebay
C2 Polystyrene 100pf sourced from Ebay
C3 Ceramic 10nf /50V RS 5381196 qty 5
C4,C11 Electrolytic 330mfd /35V RS 2286789 qty 5
C5, C7, C10 Ceramic 47nf /100V RS 5381348 qty 5
C6 Ceramic 4.7nf / 100V RS 5381304 qty 5
C8, C9 Electrolytic 10mfd / 35V RS 2286717 qty 5
C12 Ceramic 680nf / 50V RS 5381607 qty 5

LM386N IC RS 5342911 qty 1

Hope that helps.

At the moment I haven't finalised the audio input circuitry. Mono audio, from the headphone jack of the computer, goes straight to the 0.68mfd capacitor, C12.
Having played with it, for my audio sources, some form of auto level control would be really useful to ensure the modulation depth stays at maximum without distortion.
I'm sure there must be a 8 pin DIL IC that is an auto level control? I've not investigated yet.

Cheers
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Old 27th Sep 2018, 4:28 am   #23
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Default Re: Building the 'Minimod' AM modulator-questions!

NE570, SA571, not 8 pin though and possibly only in smd now.

The old 8 pin Plessy SL6270 was the go to chip for this sort of thing if you can find any.

I note that BG Electronics http://bg-electronics.de/shop/produc...23/language/en lists them in smd and there was a whisper a few years ago that a special run of these had been made, so you may be lucky.
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Old 27th Sep 2018, 9:42 pm   #24
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi
I have just found this thread after reading the one posted by Coopzone.
I made a Minimod and found the frequency drifted, after a few hours it was almost off station altogether.
I used mica capacitors for the tuned circuits, could this be the cause, are Polystyrene better.
Or could it be something else causing it to drift.
I even fitted a zener diode in the supply to the VFO.

Mike
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Old 27th Sep 2018, 10:50 pm   #25
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Default Re: Building the 'Minimod' AM modulator-questions!

Many thanks to Nick, I have placed an order with RS for the capacitors. It makes it so much easier with the codes! Fortunately I have been able to locate the polystyrene caps from my own stock.

I am also indebted to a very kind member who has helped me greatly so I hope to make a start very shortly! I'll report back as to how I get on.
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Old 28th Sep 2018, 1:21 am   #26
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Default Re: Building the 'Minimod' AM modulator-questions!

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Hi
I have just found this thread after reading the one posted by Coopzone.
I made a Minimod and found the frequency drifted, after a few hours it was almost off station altogether.
I used mica capacitors for the tuned circuits, could this be the cause, are Polystyrene better.
Or could it be something else causing it to drift.
I even fitted a zener diode in the supply to the VFO.

Mike
Hi Mike,

They're not particularly known for drift and no one else has mentioned it, although it is possible with a simple L/C oscillator. I found on my prototypes that the frequency only drifted by a couple hundred Hz from 12V to about 4.5V so I didn't bother with VFO voltage stabilizing with such a simple circuit, so I guess the zener made little difference on yours? Drift at normal temperatures didn't seem to be a problem, and the circuit itself doesn't generate any significant heat.

The article is the first link in the following thread: https://www.vintage-radio.net/forum/...68&postcount=1

The inductor cans should be grounded as should one pin of the unused coil in the VFO inductor - that will help. (See article.) Maybe also just check that L1 is the right way around, with the tap towards the earthy end - it's not centre tapped.

Assuming it is the MiniMod that's drifting, it is most likely to be C1 or C2, and the mica caps you're using here should be fine. But try polystyrene if you have some of the right value. Is the circuit sitting somewhere that's getting warm?

Drifting right off the tuning suggests a change of a few kHz. That seems unusual at normal temperatures.

Ian
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Old 28th Sep 2018, 2:26 am   #27
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Default Re: Building the 'Minimod' AM modulator-questions!

While being completely over the top in relation to the Minimod, this might be useful elsewhere - an SL6270 replacement in discrete component form:

https://www.radio-kits.co.uk/radio-r...fier/index.htm

GQRP members will find another version of this in Sprat 155 - SSB Generator.

While designed for a microphone input, it wouldn't take much to mod it for line input.
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Old 28th Sep 2018, 2:34 am   #28
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Terry_VK5TM View Post
NE570, SA571, not 8 pin though and possibly only in smd now.
NE570 is showing discontinued on element14, SA571 is there but SMD. 1426393, NZD8.35. There are a couple of other compandors there, both SMD.

There was another IC that was used in a Silicon Chip project - I have a kitset version sitting in the workshop that dad found in the estate of a guy who used to do church sound. I haven't had a play yet. Jaycar also have a kitset stereo compressor, KC5507, NZD45.90. Online only and $29 off, so probably discontinued.

I tried building a Minimod a while ago, but just couldn't get it to play ball. It's stilling in my "one day" pile.
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Old 28th Sep 2018, 6:34 am   #29
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
Thanks for your reply, I will double check the unused coil is grounded, I am pretty sure it is, and I will try and source some 270pf polystyrene caps.
Yes the zener did not help, the frequency drifting happened after the unit had been on for a few hours and at the moment I cant remember if it went up or down.
Since reading more on this subject of mini AM tx's I have saw that if there is any FM content in the output it can be evident by there being a bit of a dip in the tuning. My minimod was behaving like this, and the best quality and audio volume reception on the radio was with the minimod slightly off tune.
If this is FM causing this are there any particular pointers to stop it. (e.g. layout issues) I will take some photos of my one which is made on veroboard.

I will get the project out again and try some new caps when they arrive.
Thanks
Mike
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Old 28th Sep 2018, 8:58 am   #30
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Mike,

It sounds like you do have significant FMing. Do you have a radio that can demodulate SSB or CW, if so this will clearly show up any FMing. But again, the Minimod is not renowned for serious FMing, at least as far as the ones I have built/sold or those others have built. Nevertheless, with such a simple circuit it is possible. The design was intended to be simple, quite cheap and easy for anyone to build with good performance. That appears to be most people's experience with it.

Apart from obvious neat, short-leaded layout for any RF circuit construction, I can't think of anything that would cause the amount of drift or FMing you describe, although placing the output tuning components next to the VFO or audio input components is probably a bad idea.

Maybe others could describe the amounts of drift or FMing they have?

Ian
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Old 28th Sep 2018, 10:01 am   #31
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
Yes I do have a radio with a BFO control. I have tried it but the Minimod does not seem to sound any different to any other commercial or BBC station, it is possible to turn the BFO to more or less null out any whistles so the audio is pretty clear.
For some strange reason the effect of a null in the centre of the operational frequency seems less noticeable now, and good quality audio can be obtained with the radio tuned to the centre of the frequency..

I tried a 260pf polystyrene cap in the VFO and the frequency instability was worse. I could watch the frequency counter go up 0.1kHz every 10 seconds.

However one thing I have found out today, is the drift in frequency slowly upwards may not necessarily be a result of the Minimod being powered on. I have noticed that if I disconnect the power for a minute or so and reconnect it, the frequency displayed has still risen slightly.
I have ruled out an issue with the frequency counter as the radio being used also has to be adjusted to keep on tune. Testing the frequency counter on my Leader Signal generator seems to give a solid reading.

My suspicions now are maybe there is an issue in the little inductor used in the VFO. As the frequency adjustment is so fine, I suspect that that after setting the frequency the ferrite core settles in the thread and so the frequency moves slightly.
On a normal radio which can be tuned to any frequency this drift may not be so noticeable, but on one of the radios I was testing with, which can only tune to multiples of 9kHz, it is very noticeable.

I think the next option is to swap the 2 coils over to see if that makes any difference.
I double checked and the tap on L1 is the connection with the lowest resistance to ground.

This is a photo of my Minimod, the yellow round thing near the top left is a small trimmer for matching the level of the input device..
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The display on the frequency counter after about an hour of the Minimod being on. I initially set the Minimod to be at 1071kHz.
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Old 28th Sep 2018, 10:51 am   #32
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Default Re: Building the 'Minimod' AM modulator-questions!

I built the minimod onto perfboard (with a top ground plane) that worked perfectly first time - just lucky I guess..... this isn't the usual outcome!

Made to fit a standard plastic box as per the picture. I 'nearly' fell for the reversed-coil problem but realised how it should be wired before assembly. The scoped output signal is 'huge'!
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Old 28th Sep 2018, 11:02 am   #33
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Default Re: Building the 'Minimod' AM modulator-questions!

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As the frequency adjustment is so fine, I suspect that that after setting the frequency the ferrite core settles in the thread and so the frequency moves slightly.
Sorry I didnt phrase that very well I should have put; As the frequency adjustment is so sensitive, I suspect that that after setting the frequency the ferrite core settles in the thread and so the frequency moves

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Old 28th Sep 2018, 12:33 pm   #34
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Default Re: Building the 'Minimod' AM modulator-questions!

@Mike: Layout looks fine. Is that a mod-level pot you've added? The fact that the FMing didn't sound any worse than an real station is promising. Swapping inductors over may help pin it down. The fact the radio only tunes in 9 kHz steps shouldn't really be a problem, unless the station drifts a lot which means you can't re-tune sightly, but that shouldn't be necessary. Most modern radios have really tight IF selectivity, so that has an effect if the station is not within about 1 kHz of a channel. BTW, I also have that counter.

@Kellys: What inductors did you use? Just curious as they don't look like the 90uF ones from Spectrum Comms.

Ian
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Old 28th Sep 2018, 1:17 pm   #35
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Default Re: Building the 'Minimod' AM modulator-questions!

Mike, it seems weird that nobody else has noticed any frequency drift although it is possible that nobody else has used one with a 'modern' set with frequency synthesis tuning!

Are you sure that there isn't a change in room temperature during the same period that you've noticed the drifting - in which case the temperature coefficient of C1 might be responsible.

Apart from that, the FET could be responsible - I certainly don't think you moving core theory is responsible - but, if the core is noticeably loose in the former, why not add a piece of elastic, a very narrow strip of PTFE tape or just some core locking compound, if you have any?

Oops! Just spotted in post #31 that you've already eliminated C1 as the cause
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Old 28th Sep 2018, 2:39 pm   #36
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Default Re: Building the 'Minimod' AM modulator-questions!

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@Kellys: What inductors did you use? Just curious as they don't look like the 90uF ones from Spectrum Comms.

Ian
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Old 28th Sep 2018, 3:44 pm   #37
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Default Re: Building the 'Minimod' AM modulator-questions!

I tried one on a synthesised tuner (Sony 2010D) to check the frequency setting and did not notice any drift.

Ken
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Old 28th Sep 2018, 3:53 pm   #38
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Default Re: Building the 'Minimod' AM modulator-questions!

I have come to the conclusion that the frequency drift happens regardless of if the Minimod is switched on or not.
I set the frequency dead on 1071kHz and then turned it off, went back to it a couple of hours later and turned it on, the frequency was 1076 kHz
I have experimented with screwing the ferrite core right down to the bottom against the paxolin. The frequency was in the region of 1640 and when I came back later it was on 1641kHz. I am going to look for some strands of knicker elastic and try to jamb that in the threads of the ferrite, I am hoping it will force the ferrite over so that the threads mesh together tightly so there is no play for it to move.

Mike
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Old 28th Sep 2018, 4:30 pm   #39
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
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Quote:
Originally Posted by Ian - G4JQT View Post

@Kellys: What inductors did you use? Just curious as they don't look like the 90uF ones from Spectrum Comms.

Ian
Don't you recognise them? They're the ones you sold me!
Oh. They must have been old equivalents. But I can't really remember them. The ones from Spectrum have red rings round the core which is smaller, and yours have the yellow-painted core.

(I must be having a senior moment!)
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Old 28th Sep 2018, 5:01 pm   #40
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Default Re: Building the 'Minimod' AM modulator-questions!

I have come to the conclusion that it is definitely an issue with L1.
I have fitted a very thin strand of knicker elastic and the core is very tight now, so tight that I had to re-profile the crappy plastic trim tool after screwing the core back in.
On first tests I have found that the frequency drift now only seems to be about 2kHz, (reduced from 4 to 5kHz) and that is normally upwards. So what I have done is set the frequency to 1069 and waiting to see if it creeps up to 1071.

Maybe I am being too fussy, I would be interested to hear others comments who have used a frequency counter to test the output stability when using the Spectrum coils.

I have just realised that my coils don't have a red ring around the core, as mentioned by Ian above.

Mike
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