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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Tue 27, 2017 9:29 pm 
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Felix, can you post a scan of the schematic you're using? I've built two so far, using different circuits, and both worked right from the start. BTW, these chips can't tolerate voltage above about 1.5; have you kept your power at or below that level? They actually work best at around 1 to 1.2 volts.

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 7:29 am 
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fifties wrote:
BTW, these chips can't tolerate voltage above about 1.5; have you kept your power at or below that level? They actually work best at around 1 to 1.2 volts.

On my second MK484 build I thought I would be cute and start out with a 1.2V NiCad battery. It still needed a dropping resistor. :roll:


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 12:54 pm 
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G'day all, IT WORKS AT LAST! It took me ages to realise that I had nothing connected to the ground pin of the MK484! Oh well! I had to remove quite a few turns of the coil to get the LC ratio right and it tunes the full AM broadcast band quite well now. It tends to overload a bit on stronger AM stations but I'll put in the improved AGC modification later....it is getting a little late now. All up, it works quite well and sounds ok patched into my audio system. Regards, Felix.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 1:23 pm 
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Felix wrote:
G'day all, IT WORKS AT LAST! It took me ages to realise that I had nothing connected to the ground pin of the MK484! Oh well! I had to remove quite a few turns of the coil to get the LC ratio right and it tunes the full AM broadcast band quite well now. It tends to overload a bit on stronger AM stations but I'll put in the improved AGC modification later....it is getting a little late now. All up, it works quite well and sounds ok patched into my audio system. Regards, Felix.


Woot! Congratz! For the time being, you can put a 10K pot in series with the antenna on the frontend. So, it will be: ant, small coupling cap (10pF), 10K pot, and wiper on to the MK484. It will act as an RF attenuator. You could even keep it in circuit when you add AGC. More knobs on radios almost always equals more fun :D.

73,

Randy Ab5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 2:39 pm 
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Felix wrote:
I am thinking about building another AM tuner using an MK484 aimed at a really high quality AM tuner with very good audio quality.

Before I bother any further, is the MK484 worthy of this application. Any thoughts or comments? Regards, Felix.


Yes it is very worthy.

The best way to go about doing it is to find a defunct TRF radio that is beyond reasonable restoration efforts and take one tuning cap and RF transformer out of it. That will be your tuned circuit and the RF transformer can even serve as the antenna.

The problem is having to adjust the AVC voltage for strong and weak stations. The cure is a LM-10 set up to provide a larger voltage change for the smaller voltage change at the output of the MK-484. That will drive the input over the larger voltage range it requires and will make the AVC much better.

The output can be fed to an OP-AMP to step it up to line level so it can be connected to any amplifier.

I have a design, but have not implemented the LM-10 yet, although it is on the schematic.

If you would like I can email you the schematic.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 5:18 pm 
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Hey TR,

Tube Radio wrote:

Yes it is very worthy.

The best way to go about doing it is to find a defunct TRF radio that is beyond reasonable restoration efforts and take one tuning cap and RF transformer out of it. That will be your tuned circuit and the RF transformer can even serve as the antenna.


Sorry about the stupid question, but I know absolutely nothing about TRF radios. A good friend is sending me a couple MK484's to play around with, though. Got a link to some good info on them? (Anyone?) Don't want to a knothead about any of this stuff when the chip arrives :D

I'm wondering if I can hang a bandpass filter and a decent low-noise, RF amplifier in front of the MK484? Looking at the internals of the chip, I'm guessing that an RF amp wouldn't be necessary with the multiple stages of amplification before the detector?

What I'd like to have in a rig based on this chip is an external antenna, RF-gain control, bandpass filter, low-noise RF amp, LO, the MK484, and then a 2N3904 driving an LM386, with audio-derived AGC and an s-meter. Is this doable? It won't be the most elaborate radio in the world, but it should be fun to tune around band and see if I can pick up a bit of DX :D.

Quote:
If you would like I can email you the schematic.


I would love to see the schematic, TR.

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 6:29 pm 
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This circuit layout works very well;
Image

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 6:38 pm 
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PM me your Email, Randy and I'll send the schematic either today or tomorrow.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 7:20 pm 
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Tube Radio wrote:
PM me your Email, Randy and I'll send the schematic either today or tomorrow.


My call sign at yahoo.com, just in case anyone wants it.

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 7:28 pm 
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Howdy fifties,

fifties wrote:
This circuit layout works very well;
Image


Wow. Man, that is a really, really simple radio. Even has a darlington pair :D. I'm guessing the little rig would rock the world with a good antenna. What kind of DX do you pick up with the rig, fifties?

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 8:44 pm 
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G'day all, a couple of things please. Firstly as I've used a very old metal case variable tuning capacitor I've realised the need for an insulated knob on the metal shaft and the tuning capacitor is electrically 'floating' in the MK484 circuit and will 'buzz' loudly when touched. Secondly are you all aware of the (Australian) 'performance breakthrough circuit'? http://members.iinet.net.au/~cool386/zn414/zn414.html Regards, Felix.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Wed 28, 2017 10:35 pm 
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Howdy(Southern USA's version of G'Day :D) Felix,

Felix wrote:
G'day all, a couple of things please. Firstly as I've used a very old metal case variable tuning capacitor I've realized the need for an insulated knob on the metal shaft and the tuning capacitor is electrically 'floating' in the MK484 circuit and will 'buzz' loudly when touched. Secondly are you all aware of the (Australian) 'performance breakthrough circuit'? http://members.iinet.net.au/~cool386/zn414/zn414.html Regards, Felix.


Haven't read the article yet on the ZN414, but I'm really liking the 1st schematic, especially the biasing scheme. At least it has bias :D. The schematic's I'm seeing so far remind me of the Pixie II QRP transceiver -- minimalist design at its finest -- but maybe a bit too simplistic to be considered a high-quality receiver?

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Thu 29, 2017 5:57 am 
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ab5ni wrote:
Howdy fifties,

fifties wrote:
This circuit layout works very well;
Image


Wow. Man, that is a really, really simple radio. Even has a darlington pair :D. I'm guessing the little rig would rock the world with a good antenna. What kind of DX do you pick up with the rig, fifties?

73,

Randy AB5NI

Take a look on my thread about it;
viewtopic.php?f=12&t=213093&p=1785094#p1785094

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Thu 29, 2017 7:45 am 
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fifties wrote:
ab5ni wrote:
Howdy fifties,

fifties wrote:
This circuit layout works very well;
Image


Wow. Man, that is a really, really simple radio. Even has a darlington pair :D. I'm guessing the little rig would rock the world with a good antenna. What kind of DX do you pick up with the rig, fifties?

73,

Randy AB5NI

Take a look on my thread about it;
viewtopic.php?f=12&t=213093&p=1785094#p1785094


Nice thread. I'm really liking on Tube Radio's design.

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Thu 29, 2017 6:39 pm 
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Felix, the way to solve that issue is to ground the capacitor frame then where the capacitor frame connected to the coil install a .01uF capacitor to ground.

That eliminates the buzzing issue while not affecting the circuit at all.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Thu 29, 2017 7:06 pm 
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Tube Radio wrote:
Felix, the way to solve that issue is to ground the capacitor frame then where the capacitor frame connected to the coil install a .01uF capacitor to ground.

That eliminates the buzzing issue while not affecting the circuit at all.


I'm going to be using your circuit when I get the MK484's in, TR. I'll let you know how it pans out.

73,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Thu 29, 2017 7:31 pm 
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Ok.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Fri 30, 2017 10:43 am 
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Further experiments.

G'day all, with a simple circuit like this there is plenty of potential for 'tweaking'. I was originally using a simple transistor stage to follow the MK484 but I was bothered by a high level of audio distortion which I blamed on the MK484 itself, however to do a check on this, I ran the MK484 audio output directly into my amplifier and yes without the additional gain stage the volume was lower, but the audio was perfectly clean.

So it seems that the transistor stage was contributing lots of audio distortion. This seems quite likely as I was using a BC549 transistor with a high Hfe and it is quite possible that either the bias was incorrect and 1.5 v is a pretty low voltage for a transistor amplifier in any case! In any case the MK484 seems to deliver low distortion audio output. I might try the effect of connecting a separate audio stage with much lower overall distortion. All quite interesting stuff though. Regards, Felix.


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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Fri 30, 2017 12:48 pm 
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Hello Felix,

Felix wrote:
Further experiments.
G'day all, with a simple circuit like this there is plenty of potential for 'tweaking'.


And then some :D.

Quote:
I was originally using a simple transistor stage to follow the MK484 but I was bothered by a high level of audio distortion which I blamed on the MK484 itself, however to do a check on this, I ran the MK484 audio output directly into my amplifier and yes without the additional gain stage the volume was lower, but the audio was perfectly clean.


I'd imagine that transistor selection for the audio chain is critical, especially if you want to void distortion. A run-of-the-mill 2N3904 or 2N2222 probably isn't going to cut it (as far as noise goes); however, I'm thinking that if you used a transistor tester that provides hFE, you might be able to select one of the "tamer" transistors in your junkbox, thus avoiding noise. The problem with that, though, is that you might have to add an additional stage of amplification, and adding that additional stage is going to add more noise :D. Man, you can't win for losing :D. Seriously, though, the additional stage of amplification shouldn't be too bad when calculating noise figures, as long as the transistors used are closely matched in hFE and the gain is distributed evenly among them.

Quote:
So it seems that the transistor stage was contributing lots of audio distortion. This seems quite likely as I was using a BC549 transistor with a high Hfe and it is quite possible that either the bias was incorrect and 1.5 v is a pretty low voltage for a transistor amplifier in any case!


The simple circuit that was posted is meant to be used with germanium transistors and high-impedance headphones, and the transistor you are using is BJT silicon.
Basically, what you need to do is re-bias the circuit to work with a silicon component or find some kind of germanium transistor to shove in there and test. This is probably why you are hearing all that distortion. The BC549 isn't being turned on too often.

Quote:
In any case the MK484 seems to deliver low distortion audio output. I might try the effect of connecting a separate audio stage with much lower overall distortion. All quite interesting stuff though. Regards, Felix.


Like I mentioned, you should be able to get away with using a BJT like the 2N3904 or the 2N2222. There really isn't any need to use anything more sophisticated than that, mainly because the BCB is usually very noisy to begin with, Felix. With that in mind, you don't need an RF amplifier below 14 MHz as well. You can add one, however, and some folks do, but it really isn't necessary.

73, and hope this helped,

Randy AB5NI

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 Post subject: Re: The MK484 as a high quality AM tuner.
PostPosted: Jun Fri 30, 2017 3:22 pm 
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If an RF amp was added then some way to control its gain using the AVC would be needed for optimum operation unless you like manually controlling the RF gain control. I suppose a LM-10 could be set up to do that.

At one time I tried to add a second tuned stage which added some selectivity, but I couldn't get it to track exactly with the other section and couldn't figure how to add the AVC to that stage so I went back to a single tuned stage.


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