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Minimalist Superhet Radio.

Domestic Valve & Transistor receivers, Radiograms, Gramophones, Amplifiers, Hi-fi, Speakers, Record players, Music centres, Tape machines, Cassette players & Jukeboxes
 
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Re: Minimalist Superhet Radio.

Post by Terrykc » Fri Dec 20, 2013 2:14 am

Crackle wrote:... I turned the amplitude from the signal generator right down to minimum and went through calibrating the dial again. I found that the higher frequency "image" was the strongest all the way down through the SW frequency band to the last 3 MHz, where it became impossible to tell any difference between the 2 images and on the lowest frequency, 6.5MHz, the lower image was slightly stronger at this point ...

It sounds as if the tracking between aerial and oscillator crosses over between the top and bottom of the band!

Because the aerial input lacks sufficient selectivity because both signal and image frequencies are so close together, you can resolve them both but, if you turn the generator output down, as you have done, you can differentiate between the wanted (strongest) and unwanted (weaker) signals.

This indicates that the aerial tuned circuit is tuned to the unwanted image rather than the wanted signal. As you tune down the band, the input gradually becomes tuned to a point half way between the wanted and unwanted signals, so they are both equal (but lower than they should be!)

Go back to one end of the band. Inject the frequency marked on the dial and align the aerial circuit for maximum output. Do the same at the other end of the band and repeat both adjustments until no improvement is possible.

You should then find that the strongest reception will be at the correct signal frequency all along the band and that this will match the scale markings ...

 
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Re: Minimalist Superhet Radio.

Post by crackle » Fri Dec 20, 2013 8:28 am

Hi
I presume for SW this is done in the same way as for MW, e.g. adjust coil for low frequency end and capacitor for high frequency end.

Thanks
Mike

 
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Re: Minimalist Superhet Radio.

Post by Michael Watterson » Fri Dec 20, 2013 10:54 am

yes.

 
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Re: Minimalist Superhet Radio.

Post by Terrykc » Fri Dec 20, 2013 12:24 pm

Mike,

Perhaps a few drawings will give a better understanding of the problem you are having.

I have followed the convention of the wanted signal being below the local oscillator so, in the left hand figure below, the image frequency of a MW or LW signal that you are more familiar with would be so far to the right that it would be off the screen. However, using the same (relatively) low IF for SW can result in the unwanted image also appearing in the passband as shown.

Tracking.png
Tracking.png (8.64 KiB) Viewed 2055 times

In your case, the tracking of the oscillator and aerial circuits appears to be wrong so that, at some stage across the band, both signals are received at equal strength as shown in the centre. Carrying on to the ultimate tuning error would completely reverse the received signal levels as shown at the right.

In the second figure, the bold line shows the wanted peak response across the band should ideally correspond to the wanted signal rather than wander from one extreme to the other as shown at the bottom.

Tracking2.png
Aerial tuning wanders from one side of the local oscillator to the other.


The SW coverage on most 3-band receivers usually extends from the 49m band to the 19m band at least and usually also takes in the 16m band as well - see http://en.wikipedia.org/wiki/Shortwave_ ... cast_bands - but in your case the range stops short of these bands at both ends.

If you are having trouble getting the tracking correct, this may be the cause of the problem and I would suggest that you start again with the oscillator alignment so that the full range from 5.8MHz to 18MHz is covered. You will, of course, need a new SW dial ...!

 
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Re: Minimalist Superhet Radio.

Post by crackle » Fri Dec 20, 2013 5:05 pm

Hi Terry
Thanks for taking the time to explain that, I have a good understanding now. I think the clue I should have used was in the Ferranti 345 spec, which states it was 16m to 50m. So I will start again with the SW alignment and try and set the bottom frequency to about 5.8MHz and see where that ends up at the top of the scale. In theory as I am using the same coils and tuning cap from the Ferranti it should be about 19MHz.
At the same time I might have another go at LW and try to get that band covered a bit better.
Mike

 
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Re: Minimalist Superhet Radio.

Post by Terrykc » Fri Dec 20, 2013 6:47 pm

Trevor, 99% of my diagrams are drawn using the free Paint program that comes with Windows.

For schematics, I've built up a master file of symbols: every time I create a new one, I stick it in my master file ...

 
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Re: Minimalist Superhet Radio.

Post by crackle » Sat Dec 21, 2013 4:23 pm

I have now got Long and Medium waves almost perfect and even got rid of the nasty instability I was suffering.
dial cal.png

You can see that the band coverage now is almost exactly the correct frequencies.

I have not been able to get SW setup to my satisfaction, it is now worse than it was the first time I aligned it.
The problem of aligning the radio is not helped when starting from scratch like this, as there are no reference marks to align to.
Reading the Ferranti alignment procedure again indicates that the same marks are used for aligning all bands.
I have now spent over a day aligning this radio using different reference points to see which worked best.. Now that LW & SW are near perfect I thought I would use these to create my reference points based on the MW alignment points of 600 & 1500 kHz and use these as the reference marks for the SW. alignment points of 6.67 & 15 MHz. It all seemed to work but the sensitivity of the SW band was very poor, with hardly anything being picked up, so it seems SW is hardly worth bothering with. (it was much better before the first time I calibrated it with many stations tool loud and severely distorting the OP).
I will leave it till tonight and test it again just in case there are generally poor reception conditions on SW at the moment.
Mike

 
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Re: Minimalist Superhet Radio.

Post by Terrykc » Sat Dec 21, 2013 5:33 pm

Crackle wrote:... I thought I would use these to create my reference points based on the MW alignment points of 600 & 1500 kHz and use these as the reference marks for the SW ...

Yes, Mike, those are quite common for MW. The idea is not to use the extreme ends of the bands as the tracking can be a compromise. By using points away from the ends, any tracking errors are averaged out in an S Bend fashion.

Provided that the SW Aerial coil peaks nicely - it shouldn't really be any different to previously - the alignment should be OK but SW conditions can, and does, vary considerably, dependent on a number of factors.

I don't know if anybody publishes them these days - might be worth a web search - but Wireless World used to publish graphs every month showing forecasts of the daily and monthly Maximum Usable Frequency ...

... they varied considerably at times!

 
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Re: Minimalist Superhet Radio.

Post by Terrykc » Sat Dec 21, 2013 6:51 pm

Mike, I've just found this:

http://rsgb.org/main/operating/propagation-predictions/

From the amount of black it doesn't look as if conditions are particularly good this month anywhere!

Although intended for amateur use you can still find a lot of useful information such as the poor conditions mainly in the middle of the day - which could well have a bearing on the poor reception you reported earlier!

Clicking on a place name brings up a graph for the path to it which shows both maximum and lowest usable frequencies throughout the day.

 
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Re: Minimalist Superhet Radio.

Post by Michael Watterson » Sat Dec 21, 2013 11:44 pm

SW is amazingly variable. One day you might think the 20m long aerial is disconnected from the professional receiver and the next the internal loop aerial on Battery Vidor is lively across the bands.

If day time is OK, it's usually 14MHz to 21MHz. If night time is good, it's usually 3MHz to 9MHz approx.

Different bands for dawn /dusk and according to if station is in southern Hemisphere within +/- 3 hrs or if it's far east or west.

At night just after dusk on 80m Canada will appear then later the US Eastern Seaboard then later SW such as Texas.

This is why the same stations are duplicated on different short wave bands.

 
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Re: Minimalist Superhet Radio.

Post by crackle » Wed Jan 22, 2014 4:07 pm

I have finally finished my little radio project. After 4 attempts at creating the dial I have settled for this one.
It is printed on laser slide transfer paper and then masked the 3 scales and sprayed the back a cream colour so that the station names and numbers showed up.

It is called a Tarv "Miniranti"

Thanks to everyone for your help throughout the project and inspiring me to make it.

Mike
Attachments
Tarv Miniranti.jpg

 
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Re: Minimalist Superhet Radio.

Post by Michael Watterson » Wed Jan 22, 2014 4:25 pm

Loks gud!

 
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Re: Minimalist Superhet Radio.

Post by sideband » Wed Jan 22, 2014 4:48 pm

Excellent job. Looks very professional. What is reception like now seeing as you added SW as well?

A TV next?


Rich

PS anyone else built the superhet yet?

 
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Re: Minimalist Superhet Radio.

Post by Brianc » Wed Jan 22, 2014 6:12 pm

That's a super little set, Mike. As Rich said, it looks like a real 'un! :qq1 . Well worth the effort.

 
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Re: Minimalist Superhet Radio.

Post by crackle » Thu Jan 23, 2014 2:51 pm

Hi Rich
My normal AM aerial is 30 feet of wire from the eaves of the house to a tall tree, loads of stations on MW and about 4 stations on LW including a good strong reception on R4. On Medium wave using 6 feet of wire as an aerial it picks up about 6 of the stronger stations with plenty of volume, nothing on LW though. I have not tried SW on the short aerial.
Mike

 
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Re: Minimalist Superhet Radio.

Post by crackle » Wed Feb 12, 2014 9:13 pm

Do you think this circuit for the KB BM20 qualifies as a Minimalist Radio.

Mike
Attachments
KB BM20 circuit.png

 
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Re: Minimalist Superhet Radio.

Post by Michael Watterson » Wed Feb 12, 2014 10:28 pm

Quite a few sets use only one LO coil for LW & MW and combine R4 & R7 and C10 & C14. So not absolutely minimalist. :)

 
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Re: Minimalist Superhet Radio.

Post by sideband » Thu Feb 13, 2014 12:10 am

Something not right with that circuit.Surely C15 audio coupling should be connected to the junction of C13/R5/R6 rather than the screen grid of the IF amp......?



Rich


PS. Not really minimalist as it's a full superhet even though it doesn't have an AF amplifier. Some people might refer to it as a 'short' superhet but I always think of a short superhet as lacking an IF amplifier (like the minimalist).

 
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Re: Minimalist Superhet Radio.

Post by Cathovisor » Thu Feb 13, 2014 12:52 am

sideband wrote:Something not right with that circuit.Surely C15 audio coupling should be connected to the junction of C13/R5/R6 rather than the screen grid of the IF amp......?



Rich

Nothing wrong with it at all. The BM20 is a reflex set, using the screen grid as the anode of a virtual triode.

 
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Re: Minimalist Superhet Radio.

Post by sideband » Thu Feb 13, 2014 9:57 am

I did wonder but it still didn't look right.....

Not seen that arrangement before. Interesting...might look out for one of these myself. I was thinking that there should be a choke in the anode and the audio is taken from there and therefore didn't immediately think of it as reflex.

Clever! Saves the cost of a choke and further filtering.


Rich

 
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Re: Minimalist Superhet Radio.

Post by Cathovisor » Thu Feb 13, 2014 12:42 pm

Quite a few sets used this arrangement, and by far screen extraction of the audio was the most common. The clue is in the component values - too small for "normal" decoupling. Where anode extraction is used you're still unlikely to find a choke; too expensive.

Pre-war examples include some of the 1934/35 Ferranti Lancastrias, and the Ekco AD65; post-war you have the BM20 here and IIRC, the Pye 47X.

As discussed on the BM20 thread elsewhere, the BM20 is of course, the somewhat collectible "Dalek" set. The wooden version - the BM30 - is significantly cheaper.

sideband wrote:Some people might refer to it as a 'short' superhet but I always think of a short superhet as lacking an IF amplifier (like the minimalist).

Traditionally the "short" superhet is one lacking an AF stage and by far the one you're most likely to encounter, with Pye and Ultra being big fans of it - ones lacking an IF amplifier are much more rare, perpetrated solely by Cossor IIRC (I'm sure Mike T will be along to correct me later). Hence the development of high-slope output pentodes such as the AC2/PenDD and the EBL31.

 
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Re: Minimalist Superhet Radio.

Post by sideband » Thu Feb 13, 2014 2:13 pm

Just shows how terminology changes. I distinctly remember reading a book many years ago describing the 'short' superhet as lacking in IF amplifier. I suppose that any version of superhet lacking either an IF amp or an AF amp could be referred to as 'short' seeing as it lacks a stage.

'Thinks'....I wonder if a superhet with an RF stage or an extra IF amp could be classed as a 'long' superhet. :aaj



Rich

 
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Re: Minimalist Superhet Radio.

Post by Till Eulenspiegel » Wed Jan 21, 2015 11:23 am

From the Radiomuseum. The EL8 valve audio output valve was developed for use in small radio receivers. Suggested valve line up for a low cost superhet: ECH4 frequency changer. EF6 straight pentode detector. EL8 output. Different valves, but a similar concept as our minimalist radio.

http://www.radiomuseum.org/tubes/tube_el8.html
From Rtype: http://www.r-type.org/addtext/add031.htm

Till Eulenspiegel.

 
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Re: Minimalist Superhet Radio.

Post by Davek0974 » Wed Jan 21, 2015 4:44 pm

I'm very busy at work today :cch :cch and have just read this entire thread from the beginning.

I have one thing to say

WOW! what a thread, excellent reading and some fantastic work going on there.

I'm working on my first DAC90A and the information discussed here is very informative, the circuits are pretty similar too.

Great stuff

 
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Re: Minimalist Superhet Radio.

Post by Davek0974 » Wed Jan 21, 2015 7:39 pm

Murphyv310 wrote:
Davek0974 wrote:I'm very busy at work today :cch :cch and have just read this entire thread from the beginning.

I have one thing to say

WOW! what a thread, excellent reading and some fantastic work going on there.

I'm working on my first DAC90A and the information discussed here is very informative, the circuits are pretty similar too.

Great stuff

It is a great thread and good to see its been revived again after a year. Will you have a go? I still play about with mine, its great fun, especially on the frame aerial I have.


I'm sort of going the other way :qq1

I have just about fixed my DAC90A now and am in the middle of building my own pantry transmitter with a single valve.

After that is done, then maybe I'll have a bash at a receiver too.

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