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Old 17th Oct 2006, 9:05 pm   #1
adibrook
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Default The Junk SW TRF!!!

yay!!! I got a working SW radio.

Its basically a regen with an untuned RF amp and a couple of stages of audio.

It works amazingly well, with stations being recieved from as far as Russia. One station which i remember is 1945KHz. I even herd the odd bit of morse!

The RF side of it works well...and ironically, the biggest problem is AF instability at high volumes.

I used a 6BW6 and 6AT6 for audio just because i happened to have them handy. They make a good audio amp, and on most staions the sound quality is actually amazing. I never expected such good preformance.

Selectivity is abit bad, and very very gently nudges are needed to the tuning cap to tune it, but its not anywhere as bad as i thought.

The crude RF amp is a later addition, and makes selectivity worst, but alot more sensetive.

The tuning coil turns are thesame as on the diagram.

http://img.photobucket.com/albums/v4...shortwave1.gif

I will upload the pic of the actual device soon.

What do you think?
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Old 17th Oct 2006, 9:35 pm   #2
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Default Re: The Junk SW TRF!!!

Good stuff Adi!

The selectivity in a simple design like this will largely depend on the coil design and construction, though I suspect the EF80 is damping it somewhat which is why you are finding selectivity better without it. It may be better to do away with the EF80 and rely on regeneration for gain, which will peak the selectivity nicely. You want to get the set to the stage where it almost oscillates.

Paul
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Old 17th Oct 2006, 9:47 pm   #3
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Default Re: The Junk SW TRF!!!

Well done! It certainly looks a novel design for a TRF to me - where did you get the design?

What do I think (you ask) ?
Well . . . . . .

The reduction in selectivity, when the untuned RF amp is added, could be due to the primary of the coupling transformer (between the RF amp and the detector) damping the tuned cct. at the grid of the detector. The anode dc load of the RF amp is 5k6. (Seems far too low to me. I would have expected something like 100k - 220k). This, in parallel with the O/P impedance of the RF Amp. could be heavily damping the tuned cct. - depending on the degree of coupling between pri. and sec. of the transformer.
Try increasing the physical spacing between pri. and sec. You'll lose some overall gain, but the selectivity should improve.

The AF instab. is probably due to the combination of the 0.47uF and the "stability trimmer" (presumably "A.O.T."). Try 10k fixed from the HT end of the 150k to HT and replace the 0.47uF with something like 8uF, or more.

I'm also a bit puzzled as to why there is a LF choke in the HT feed to the RF Amp. It may be better to use this component as a decoupler for the HT to the 6AT6, if the above doesn't help.
However, if this choke isn't an LF choke but an RF choke (cct. shows it as an LF choke) then that comment doesn't apply - and the arrangement, as shown, makes a bit more sense.

I'm sure you get lots of comments from others.
Over to you, YC . . . . .

Al.
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Old 17th Oct 2006, 10:27 pm   #4
adibrook
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Default Re: The Junk SW TRF!!!

Hey.

Sorry i was tired, and missed some stuff out.

The choke is an RF choke from an SMPSU. I put it in because i run out of suitable resistors. I have no idea what value it is, but i guess its better than nothing. Have i drawn it wrong?

The design isnt really one in particular. The RF preamp is from some TRF project i saw some time (complete with the low value resistor) and seemed to have saved a copy of. The Regen is from a diagram someone sent me using a EF91 because i didnt want to have a physically movable reaction winding.

The AF amp is just somethgin i threw together out of parts i had lying round the work bench.

The instability doesnt bother me too much yet because i never turn it up to high volume, and at lower volumes its stable.

I'm not sure, but i think the selectivity got worst because thers more stations avalible now. Also, i tested the non-preamp design at about 18.00. Now its 22.20...and sw activity increases at night i think.

But dont get me wrong...the selectivity is not bad at all for ahome made piece of junk. I dont have any reduction gearing on the tuning cap, so tuning to a particular station is fiddly.

The coil is wound on a drain pipe using random wire i think for a TV deflection coil.

The annoying thing now seems that thers a huge area of nothign, then about 20 stations crammed into about 3 degrees of tuning cap movement, and then nothgin again.

So far i'v had very good reception of:
Radio Belarus
Unknown german station
Unknown polish shation
Family radio (crazy religious station from the states by the looks of it)
Somethign on 1945KHz (russian i think)

This is cool. It even works
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Old 17th Oct 2006, 11:54 pm   #5
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Default Re: The Junk SW TRF!!!

Quote:
Originally Posted by adibrook View Post
The coil is wound on a drain pipe using random wire i think for a TV deflection coil.

The annoying thing now seems that thers a huge area of nothign, then about 20 stations crammed into about 3 degrees of tuning cap movement, and then nothgin again.
SW is organized into standard 'bands', both broadcast and amateur. This has been the case since the 30s though the boundaries have changed now and then. There are indeed big regions without any broadcast transmissions in them. I don't know why this should be (apart from the legalistic stuff) - maybe some of the forum radio amateurs know?

I believe plastic isn't a great former for use in coil winding. The traditional former is an old bog roll centre, which works very well You can experiment with all sorts of different coil designs. One possibility is to wind one on an old IFT scavenged from a scrapped valve radio.

Paul
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Old 18th Oct 2006, 2:16 am   #6
adibrook
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Default Re: The Junk SW TRF!!!

OK...Here it is.

Pins in wood contruction as usual.

http://img.photobucket.com/albums/v4...k/P1010073.jpg

However, maybe i will build it into a proper shassis (baking tray!) soon. This is just a prototype.
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Old 18th Oct 2006, 1:48 pm   #7
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Default Re: The Junk SW TRF!!!

Quote:
Originally Posted by G8DLH View Post
The anode dc load of the RF amp is 5k6. (Seems far too low to me. I would have expected something like 100k - 220k). This, in parallel with the O/P impedance of the RF Amp. could be heavily damping the tuned cct. - depending on the degree of coupling between pri. and sec. of the transformer.
Try increasing the physical spacing between pri. and sec. You'll lose some overall gain, but the selectivity should improve.
Al - the anode load of an untuned RF amp has to be low, since the stray capacitances will be the 'real' load on the output. A few tens of pFs will only have a reactance of a few k-ohms at the top end of MW. Reducing the coupling or tapping the primary down will improve the selectivity though. I'd use an EF85 rather than an EF80 for smoother gain control.

Neil
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Last edited by Kat Manton; 18th Oct 2006 at 2:31 pm. Reason: Fixed quote
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Old 18th Oct 2006, 5:46 pm   #8
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Thumbs up Re: The Junk SW TRF!!!

Quote:
Originally Posted by Neil Breward View Post
Al - the anode load of an untuned RF amp has to be low, since the stray capacitances will be the 'real' load on the output.
Neil
Agreed: my error.

Al.
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