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Old 28th Sep 2018, 6:22 pm   #41
Ian - G4JQT
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Default Re: Building the 'Minimod' AM modulator-questions!

A drift of 2 kHz is still a lot and if common would probably be noted by others. Maybe there's some variation in the coils from Spectrum Communication since I got all mine quite a few years ago. Mine - and those of most others as far as I can see from the photos in various threads - look like these from David's post:

https://www.vintage-radio.net/forum/...2&d=1435664319
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Old 28th Sep 2018, 6:41 pm   #42
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Default Re: Building the 'Minimod' AM modulator-questions!

Its possibly a bit late after 2 years to go back to Spectrum to enquire if they sent the wrong ones.
I wonder if it would be possible for you to measure the resistance of the windings, so I can check against my ones.
Thanks

Mike
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Old 28th Sep 2018, 7:16 pm   #43
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Default Re: Building the 'Minimod' AM modulator-questions!

There are two possible cures to reduce drift.
1- replace the 10K inductors with 10EZ types, RWO6A6408 (red core) tunes MW.
2- I am not sure what type of parallel capacitor has been used, polystyrene or silver mica is preferred, but you could put an NPO ceramic in parallel with a silver mica (or use a standard ceramic, not multilayer type) the two drifts can cancel out.
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Old 28th Sep 2018, 7:49 pm   #44
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Restoration73
Thanks for your advice, what value NPO and Mica capacitors would you suggest. and where can I get 10EZ or RWO6A6408 inductors from, I cant find anywhere on the internet that sells them.
Mike
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Old 28th Sep 2018, 7:57 pm   #45
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by crackle View Post
Its possibly a bit late after 2 years to go back to Spectrum to enquire if they sent the wrong ones.
I wonder if it would be possible for you to measure the resistance of the windings, so I can check against my ones.
Thanks

Mike
If they sent the wrong ones, they probably wouldn't tune up on MW.

Unfortunately the coil data has no DC resistance info.

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

Look for: 10mm 10K RF/IF COILS 90u0L.
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Old 28th Sep 2018, 9:20 pm   #46
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Default Re: Building the 'Minimod' AM modulator-questions!

Dividing R3 and adding a zener may stabilise the oscillator. The highest value Toko 10K was 45uH, so the Spectrum ones are specials. All inductors recover from adjustment
but if this is inconvenient put a small foil trimmer say 30p across the 270p.
6408 coil is 360uH with a tapped primary and low-Z secondary and may tune with a
lower C. I have a few for the receivers I designed using TDA1072A so let me know if
you would like any.
I prefer a Colpitts oscillator with a quartz crystal in my own modulators.
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Old 28th Sep 2018, 9:28 pm   #47
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Default Re: Building the 'Minimod' AM modulator-questions!

I hooked up mine to the frequency counter this evening to see what was going on.

Initial frequency 1.08534 Mhz
After 5 mins 1.08316
Then pretty stable for 1/2 hr. Last two digits drifted slowly up, then slowly down.
Then after an hour from switch on, frequency decreased to 1.07985
It then started to climb again.

I considered the most likely candidate would be the FET. Sure enough, a squirt of freezer spray sent the frequency rocketing back up to over 1.085. Granted, I can't find a straw for the freezer, so several components got hit, but when I placed a finger on the FET, the frequency started to fall pretty quickly.
I wonder if it would be worth sticking the FET to a small heatsink just to try and stabilise the temperature it runs at?
Maybe there is a more 'electronic' way of achieving temperature stability?

All the best
Nick
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Old 28th Sep 2018, 9:30 pm   #48
David G4EBT
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Default Re: Building the 'Minimod' AM modulator-questions!

Quote:
Originally Posted by Ian - G4JQT View Post
A drift of 2 kHz is still a lot and if common would probably be noted by others. Maybe there's some variation in the coils from Spectrum Communication since I got all mine quite a few years ago. Mine - and those of most others as far as I can see from the photos in various threads - look like these from David's post:

https://www.vintage-radio.net/forum/...2&d=1435664319
I can confirm that those were obtained from Tony Nailor, at Spectrum.
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Old 28th Sep 2018, 10:10 pm   #49
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Default Re: Building the 'Minimod' AM modulator-questions!

I think once the correct adjustment point of L1 is found the frequency would be pretty stable, or at least stable within 1kHz of the desired frequency which would probably not be noticed too much.
But the problem is adjusting L1 to try and get the desired frequency is like trying to adjust an old clock with a pendulum on a piece of elastic. The slightest touch of the coil slug sends the frequency rocketing up or down, then you have to wait an hour for it to settle back into its groves.
I know it sounds daft, but I am convinced this is what is causing my problem.
Thanks for the advice everyone, and thanks Nick for doing those tests.
Mike
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Old 29th Sep 2018, 8:08 am   #50
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Default Re: Building the 'Minimod' AM modulator-questions!

It is early days yet but I think I am almost there.
I have added a small trimmer cap (T1) in parallel with C1. It gives a finer adjustment than L1.
I have also replaced the original L2 coil with a toroidal one. It has 2 windings one is thicker wire than the other and I used that as the primary.
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Not sure what the full spec of it is, but it works well with any length of aerial, giving a signal better than the original L2 even when it was tuned.
But the toroidal one does not need to be tweaked, it works well with 18 inches of wire, 5 feet or wire or even better, connected to my RF earth. (which does have about a 12 foot drop down the side of the house to the earth rod)
Last night I found a new clear channel on MW 1188kHz and set L1 approximately. I adjusted T1 the new trimmer turned it off and left it for the night. This morning the frequency had drifted up about 8khz whilst the TX was turned off, possibly change in temperature in the room had caused the larger than expected change, as well as the ferrite slug settling in the core of L1. So I reset T1 without touching L1, left the TX on and when I returned just over an hour later the frequency was still reading 1188.04kHz.
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Old 29th Sep 2018, 10:38 am   #51
Ian - G4JQT
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Default Re: Building the 'Minimod' AM modulator-questions!

This does highlight a worst-case scenario when using a simple LC oscillator to generate RF. My bigger modulators use a crystal, but as outlined in the article, obtaining one for an MW channel is very expensive. Cheap 1 MHz crystals and other non-channel MW crystals are about, but if crystal stability is chosen it might as well be on a channel. As Mike has fond it's not really possible to set a precise frequency by adjusting L1 - it's too coarse.

But the whole point of the MiniMod was do design a very simple, comparatively cheap low power modulator, hence the L/C VFO. And after all, the vintage radios it will likely be used with have an L/C VFO/LO and most don't have as much drift as Mike is experiencing with his MiniMod circuit.

It's odd - and probably significant - that it drifts in one direction even when unpowered.

BTW, if you're using a bigger inductor for L2 (output), then if you remove the 390R resistor you might get more output power. I added it for two reasons; it keeps the power down to a 'safe', very local range, and it stops the DC through the 90uH coil from saturating the small core on modulation peaks.

Ian
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Old 29th Sep 2018, 11:04 am   #52
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi

The Minimod is performing well, I have no criticisms of the design it is doing what it was designed to do. Connected up to my central heating pipe there is a very strong signal all over the house and it fades out about 30 yards up the road. I measured the power dissipated in R8 and it was about 2.5mW (1v 390r) when using a 5 foot aerial wire, it is possibly more when connected to the central heating.

I would imagine that if I had not studied the frequency output and tried to align it with a MW channel then I would never have suspected anything wrong. The listening radio would just need to be slightly retuned periodically.
The coil used as L1 seems to be reasonably stable after about an hour following adjustment, it makes no difference if the TX is powered on or off.
The current RF stability is about +1kHz when first turned on & -1.5kHz after a couple of hours so it is still more or less on channel.

Here is a trace on the scope of the output from the Minimod. RF frequency was about 1186.5kHz and the modulation frequency about 400Hz. There was no aerial connected, only the scope wires.
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Things I notice are the lower lobes seem larger then the upper lobes, and what are those 2 additional wavy lines in the lower half.

Mike
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Old 29th Sep 2018, 11:54 am   #53
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Default Re: Building the 'Minimod' AM modulator-questions!

Looks like there are a lot of harmonics in the signal there! Is your output now un-tuned? Depending on how tightly scope probe is coupled to the antenna, you may get an uneven scope trace, especially if the output circuit is un-tuned. I think that's why it looks like that. Switch the probe to X10 and connect it directly to the collector of Q2. L2 will probably need retuning. and see what it looks like then.

===

I've just done another frequency-stability test on my own MiniMod:

Switch on from 'cold'; 1025.4 kHz; five minutes 1024.9 kHz; ten minutes 1024.8 kHz; fifteen minutes; 1024.9 kHz; thirty minutes 1024.7. That drift is not noticeable on my 9 kHz synthesized Sony ICF-SW 7600GR when set to 1025.00 kHz.

Unfortunately Mike, it looks like something is very wrong for yours to drift by some kHz. Your layout looked fine, so as you've found, that only leaves L1, C1, C2 or possibly D1, Q1 as suspects...

(Actually, just disconnect one end of D1 and see how that affects things, especially drift.)

Ian

Picture of one of mine below as sold.
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Old 30th Sep 2018, 11:56 am   #54
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Default Re: Building the 'Minimod' AM modulator-questions!

My Minimod - built using a J301 FET and a 2N3904 for the output transistor, using the coils from Spectrum Comms - drifted about 400Hz in 10 minutes from switch-on. I thought that I could try some means of stabilisation to eliminate the little bit of FM, too.

I built a "huff & puff" CMOS stabiliser that locks in 1kHz steps. It worked OK, but used three CMOS ICs and a crystal oscillator module, which seems like overkill for a little, simple AM modulator! It was weird tuning the VFO, as it went in jumps (usually bigger than 1 kHz, since it's difficult to tune that slowly).

My next effort - and the one I'm using now - used an MC145106 PLL IC (scavenged from an old, broken CB radio) and a 4608 kHz crystal for the reference. It can now be tuned in 9 kHz steps over a small range - from about 970 kHz to 1170 kHz - using a green LED as the varicap. Frequency is set with an 8-way DIP switch.

The other addition I've made is a quad op-amp (TL074) and a FET to limit the audio bandwidth and provide compression and clipping (to get rid of over-modulation). The little box now accepts pretty much any audio source without twiddling the levels!

The whole thing is powered from a "10V" DC adaptor, which actually gives well over 12V off load, through a 7809 regulator. The useful range is about 15m through two walls, using about 2m of dangly wire as the antenna!

I suppose that I've gone away from the original - rather nifty - concept of a cheap and simple AM modulator to feed old receivers, but it's been fun adding bits to it to improve its performance!
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Old 30th Sep 2018, 7:16 pm   #55
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Default Re: Building the 'Minimod' AM modulator-questions!

You must let us have the complete circuit - I'm sure a lot of us will be very interested in it!
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Old 30th Sep 2018, 7:45 pm   #56
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
I tried disconnecting one end of D1 and it made no difference that I could detect to the drift or the operation of the Minimod.

Mike
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Old 1st Oct 2018, 12:07 pm   #57
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Default Re: Building the 'Minimod' AM modulator-questions!

I have made some more mods to my circuit.
I have replaced the Spectrum coil for L1 with a Toko yellow coil. I broke away the cap that was in the base. I removed the small trimmer cap and by experimentation found that a 56pf polystyrene cap across the coil gave me the 1188kHz frequency I needed + or - a little for adjustment.
I have set the frequency, the nearest I can get is 1188.13kHz, it has been spot on that for 30 min now.
I am now going to turn off and see what the frequency is when I turn it on again in an hour.
This really does now look like the coil supplied by Spectrum Coms was sub-standard.

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Old 1st Oct 2018, 2:11 pm   #58
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Default Re: Building the 'Minimod' AM modulator-questions!

That's looking promising Mike. Did you ever swap the two coils over - o/p to VFO to test?

From what you describe it seems it was a mechanical fault, rather than the wrong type of core, although who knows, a high resistance internal soldered joint to one of the pins or some loose turns might have been the cause.

I admire your perseverance. If it had been me I may well have just chucked it on the "this design is a load of c***" pile!

I have heard of the gate diode in FET oscillators causing drift problems (and in practice they don't seem to do much anyway in a very low power oscillator), but seemingly not this time.

Anyway, let us know the final outcome.

Ian
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Old 1st Oct 2018, 3:51 pm   #59
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Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
I did swap the L1 L2 coils around and the results were the same bad frequency drift.

However although the frequency stability tests have now proved very good with less than 100hz drift over an hour, that is not the end to the problems.
After the last stability tests I connected the audio to find it is very distorted. I believe there is significant FMing and cant seem to stop it.

I think I may take the view you expressed and donate this to the failed experiments pile.

Unless anyone can come up with a cure to the FMing, I wonder if putting the tuned coil back on the OP would help.

Mike
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Old 1st Oct 2018, 4:08 pm   #60
Ian - G4JQT
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Default Re: Building the 'Minimod' AM modulator-questions!

Any chance you could send it to me? I'd quite like to investigate this, and I'll send it back to you whether or not I can fix it...

(Unfortunately I won't be back at my workshop for a couple of weeks.)

Thanks.

Ian
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