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Old 19th Dec 2019, 1:43 pm   #1
Argus25
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Default The TRF-One am radio

I have attached some images of a TRF radio that I have just manufactured.

An early Xmas present for myself perhaps.

I am preparing a very detailed article with engineering diagrams.

I have always been a better mechanical engineer than electronic engineer, so for me it is all about physical construction first and selected engineering materials and how to get the best out of those.

The very first radios I built as a boy were made from wooden onion boxes and I used the red sacking for speaker cloth, of course they didn't work, but I made them to look like they should. So a radio should look like a radio I think.

I realized that it had probably been over 40 years since I made a TRF radio with a single variable capacitor gang. Obviously I moved away to superhets and PLL based radios in the quest for selectivity & performance. So Why not go back there, to that single tuning gang, for nostalgia's sake and have another go, with the experiences gained over the years.

Also, another member Jolly_7 was very excited about a MW radio based on a single IC. Looking into this notion I found the vintage LM372 IC intended to be used as an IF & detector, but with some remarkable properties:

The gain stage typically is 2,360 or 67dB while the precision detector stage has a gain of 3 or 10dB.

So a 50uV signal input, modulated ton 80% will produce an audio output of around 280mV rms, or 800mV PP.

The AGC stage has an enormous control range of 60dB with a threshold of 50uV. Therefore, with this IC as the basis of a TRF AM radio, the output level would be expected to be fairly constant even if the signal level increased from 50uV to 50mV.

There is a lot more to this story, but I added a "era appropriate" 741 IC and a suitable output stage with a 40R vintage speaker.

The AGC is astonishingly effective. Weak and strong stations appear at the same volume, so its an odd effect tuning across the band, quite different to any other radio I own.

If I rotate the set (and the rod), the volume stays constant until the ferrite rod is about 90% off axis and it suddenly drops out. Due to the wider bandwidth the fidelity and treble response is wonderful compared to the usual Superhet (selectivity sacrificed of course) however due to the high permeability 1/2 inch diameter ferrite rod the situation is quite good.

I will post the article about this radio when its finished. The Tropical fish capacitors threw in a splash of color.
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Old 19th Dec 2019, 2:58 pm   #2
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Default Re: The trf-one am radio

Beautiful physical construction as always with your projects.
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Old 19th Dec 2019, 7:14 pm   #3
Philips210
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Default Re: The TRF-One am radio

That's highly professional, it looks great. I really like the AGC system, something that you don't often see in TRF designs.
I hope to build your circuit when I can find the time.

Regards,
Symon

Last edited by Philips210; 19th Dec 2019 at 7:16 pm. Reason: typo
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Old 19th Dec 2019, 7:31 pm   #4
kalee20
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Default Re: The trf-one am radio

Quote:
Originally Posted by paulsherwin View Post
Beautiful physical construction as always with your projects.
Second that! It looks awesome!

How does this IC (which I have not come across before) compare with the ZN414?
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Old 19th Dec 2019, 7:35 pm   #5
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Default Re: The TRF-One am radio

It would also be good to extend the coverage to the LW band and possibly SW bands too. Great project.

Regards,
Symon
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Old 19th Dec 2019, 9:56 pm   #6
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Default Re: The TRF-One am radio

No LW in Oz, so not much point in providing coverage, though it could possibly be added with a switched padder cap for European use.

These simple-ish TRF designs were more likely to be used for 198kHz LW in the UK, initially for the Light Programme then for BBC R4. Sound quality was pretty good before the BBC started using brickwall filters to stay within the 9kHz allocation.
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Old 20th Dec 2019, 3:28 am   #7
Argus25
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Default Re: The trf-one am radio

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by paulsherwin View Post
Beautiful physical construction as always with your projects.
Second that! It looks awesome!

How does this IC (which I have not come across before) compare with the ZN414?

Thank you all for the kind remarks.

Unlike the ZN414, the LM372 wasn't actually intended to be used as a "one IC radio", it was typically for a 455kHz IF on the tail end of a mixer and 455KHz IF filter. The IC is good to 2MHz (so there is no SW possibility and as noted by Paul, no LW here).

However for the application I think it makes a great MW TRF radio.

I have just finished the provisional construction article (ignore any typos), there are more photos in it during construction and some detail on the LM372. A pretty advanced part I think, for 1969:

http://worldphaco.com/uploads/THE_TRF-ONE.pdf
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Old 20th Dec 2019, 9:26 am   #8
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Default Re: The TRF-One am radio

Wow, what a beautiful job. Where did the LM372 come from, it must be long obsolete by now? (sorry if I've missed it).
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Old 20th Dec 2019, 9:31 am   #9
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Default Re: The TRF-One am radio

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Originally Posted by paulsherwin View Post
No LW in Oz, so not much point in providing coverage, though it could possibly be added with a switched padder cap for European use.
Just airport beacons and thunderstorms on LW!
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Old 20th Dec 2019, 10:21 am   #10
'LIVEWIRE?'
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Default Re: The TRF-One am radio

LW, as I guess most members will know is only used for broadcasting in the EBU area, which includes North Africa. When I was in South Africa some years ago, I had with me a Braun multi-bamd portable radio, and, like you, Sue, I could only pick up noise on LW, the nearest Broadcasting station being maybe 1000miles to the north of where I was staying. No doubt the fact that it isn't so used is is due to international agreements on the use of spectrum space, but LW would be a good means of providing coverage to remote rural areas in large countries such as Australia & South Africa.
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Old 20th Dec 2019, 1:26 pm   #11
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Default Re: The TRF-One am radio

Beautifully made, don't see any valves though.

Peter
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Old 20th Dec 2019, 3:21 pm   #12
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Default Re: The TRF-One am radio

Quote:
Originally Posted by 'LIVEWIRE?' View Post
LW, as I guess most members will know is only used for broadcasting in the EBU area, which includes North Africa. When I was in South Africa some years ago, I had with me a Braun multi-bamd portable radio, and, like you, Sue, I could only pick up noise on LW, the nearest Broadcasting station being maybe 1000miles to the north of where I was staying. No doubt the fact that it isn't so used is is due to international agreements on the use of spectrum space, but LW would be a good means of providing coverage to remote rural areas in large countries such as Australia & South Africa.
LW's susceptibility to thunderstorm-inteference would surely have been an issue - after all, in many areas they didn't even bother much with MW broadcasting - instead they used the so-called 'tropical bands' at the lower-end of shortwave (60, 90, 120 Metres).
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Old 20th Dec 2019, 3:52 pm   #13
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Default Re: The TRF-One am radio

The vast distances in Australia would have made LW fairly pointless. LW coverage is just right for Europe, with a single transmitter covering a single typical sized country, but there wouldn't be much benefit in covering a quarter of NSW or QLD. The Aussies just used SW to cover the rural areas, and followed a North American city based model with relatively low powered MW transmitters for their urban coverage
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Old 20th Dec 2019, 8:49 pm   #14
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Default Re: The TRF-One am radio

Hi.

I just came across another interesting reference to the LM372 being used in a TRF setup. It's in Radio Amateur, Nov 1972. https://www.radioexperimenter.us/rm-...-receiver.html

Regards,
Symon
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Old 20th Dec 2019, 8:58 pm   #15
Argus25
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Default Re: The TRF-One am radio

Quote:
Originally Posted by Andrew2 View Post
Wow, what a beautiful job. Where did the LM372 come from, it must be long obsolete by now? (sorry if I've missed it).
It is not a very common IC.

National Semiconductors appear to have produced it in the late 1960's along with other similar IC's for signal processing and radio work. It may well not have been taken up by manufacturers very often, I have yet to see an appliance or radio with an LM372 in it, but there is probably one out there.

The idea was that you replaced the entire radio's IF amplifier with the LM372 and one 455kHz ceramic filter.

Later it appears other manufacturers took up the idea of a single AGC controlled IC to replace the usual IF amplifier in transistor superhet radios, the other one I know of is the Motorola MC1350.

Part of it may have been the cost at the time. These IC's were built on a ceramic substrate and encapsulated in a TO-5 diameter metal can and have gold plated lead wires, made like a mil spec part. They probably were an expensive part back in 1969.

When Electronics Australia built a TRF radio with one in it, it was a way to get the readers interested in using IC's which were revolutionizing electronics back then and also to show how, with the one part, you could get a very large amount of signal gain.

There is more info on the LM372's specs, including its internal schematic, in the article:

http://worldphaco.com/uploads/THE_TRF-ONE.pdf
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Old 20th Dec 2019, 9:34 pm   #16
Argus25
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Default Re: The TRF-One am radio

Quote:
Originally Posted by Peter.N. View Post
Beautifully made, don't see any valves though.

Peter
Thanks !

As I mentioned in the article though I had built other kinds of radios, superhets etc (with valves too) and wanted to do something different.

One of the things about plain TRF radios, they always seem to be struggling for gain (at least the ones I have owned & built before).

The LM372 has a total max gain of around 77dB or close to 7000. But of course that would be no use and quickly it would be overloaded by the large range of signal levels at its input, but it has built into it the amazing AGC that simply levels off its output over a 50uV input threshold. Totally perfect for a TRF radio it would seem. It would be bulky and power hungry to acquire that gain and the control of it with valves and it would no longer be a low power consumption baby radio which is what I was aiming for.
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Old 20th Dec 2019, 11:42 pm   #17
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Default Re: The TRF-One am radio

Incredibly beautiful build and I also love reading your rationale and creative thinking around the use of the IC based on its idiosyncratic characteristics!

Lovely story.
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Old 20th Dec 2019, 11:54 pm   #18
Argus25
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Default Re: The TRF-One am radio

Quote:
Originally Posted by astral highway View Post
Incredibly beautiful build and I also love reading your rationale and creative thinking around the use of the IC based on its idiosyncratic characteristics!

Lovely story.
Thanks.

Though Electronics Australia should get the credit for using the IC this way, I just changed the physical build and put a better audio system on the LM372's output.
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Old 21st Dec 2019, 1:21 am   #19
Argus25
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Default Re: The TRF-One am radio

I managed to get a better photo of the pcb, my camera is a little difficult to get everything in focus for an angled shot.

The IC sockets are made by Augat. The transistor sockets are Teflon (I think in past from RS or Farnell). The green 104 ceramic caps are vintage parts made by the Corning Glass Works. The date code on the LM372 is 6921 and on the 741 is 7518.

I'm getting hooked on listening to it now, as the audio is very clear and pleasant, possibly aided by the wider bandwidth than the usual superhet and the fact there are no transformers to upset the response and cause distortion in the audio.
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Old 21st Dec 2019, 2:08 pm   #20
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Default Re: The TRF-One am radio

Very intriguing and neat! I'd not come across the LM372, and honestly expect never to see one in the flesh.

Around the same time as it appeared. Mullard came out with the TAD100, which was a MW/LW-superhet frequency-changer/IF-strip/detector/audio-driver in a DIL package. Add a complementary-pair of transistors (AC127/AC128), a few tuned-circuits, a sprinkling of Rs and Cs... (Roberts used it in their RIC1 radio).

Probably the closesat I ever did to your TRF was using a 2N3819 buffer feeding a pair of uA703 RF amplifiers, and a full-wave diode detector. Three signal-frequency tuned-circuits made it selective-enough without needing regeneration. I even arranged for the DC component of the rectified signal to feed the gate of the 2N3819 for AGC.
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