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Old 12th Dec 2018, 5:20 pm   #21
Neil Purling
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Default Re: More Chinese AM radio kits

I am determined to make that last KX168 and have it work. The way the coil colours are marked seems to be totally wrong for it to stand any chance of functioning if you make the radio as the sheet says.
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Old 14th Dec 2018, 4:58 pm   #22
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At last! some luck with this S66D. I believe that it is very much like the HX-6B, except there is provision for an earphone jack (which I have not fitted).
I peaked the white & black cores with an injector at around 460 Khz, observing the amplitude of the waveform arriving on the base of VT3. I had to take the volume control off

You see that there is that mark by R4. Long ago someone who could read Chinese said that it referred to adjustment of the value, but on what conditions? Can't remember.
As you can see R4 is listed as 30K, I fitted 51K.

The best improvement I observed was after throwing the supplied speaker in the bin & fitting an 8 ohm 0.2W 50mm plastic cone speaker that is 11mm deep. It is the sort of component that you'd fit in a shower radio.
I do have an old Japanese pocket radio speaker, but i'd have to molest the back of the case to accommodate the magnet bell.
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Old 14th Dec 2018, 11:44 pm   #23
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Default Re: More Chinese AM radio kits

That circuit is typical of AM radio designs of the 1960s & 70s - easy to understand unlike modern radio designs!! What it performs like in today's conditions I wouldn't know, but it should at least receive strong local stations without problems.
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Old 15th Dec 2018, 12:49 am   #24
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This is a general question in relation to these superhet radio kits. All have an oscillator coil, but the S666D has as few as two IFTs. The usual number of IFTs should be three. Therefore, in theory, radios with fewer IF stages will not perform as well as those with all three IFTs. Is that a correct conclusion to make or can the circuit design compensate for this ?

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Old 15th Dec 2018, 1:29 am   #25
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Default Re: More Chinese AM radio kits

Jolly 7: You need to look around with the forum search. There are threads that will tell you of the experiences the Forum members have had with them. I first got one of these radio kits around this time last year.

I believe that this circuit is a particularly basic one. The output stage is not push-pull.
It is what is referred to as totem-pole. I do not know that that means in terms of audio quality or the power output.
The radio is best for local stations as there isn't the extra gain stage in the IF strip to drive the audio stage fully.
That is my understanding anyway. Shango066 on You Tube built examples of the six and seven transistor kits and took them out to a DX spot as well as a Arvin that had the full IF strip and a RF amplifier. I wish I knew which of his videos had the comparison between the six transistor HX-6B and the seven transistor HX108-2.
BTW: The S66E is the one I just got done making. The circuit is exactly the same as the HX-6B. Only the case and maybe the PCB are different. A lot of the AM only radio kits are to the same design, though some do have a true push-pull output stage.
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Old 15th Dec 2018, 3:22 am   #26
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Default Re: More Chinese AM radio kits

There is one thing to watch out for on all these transistor radios that use the red slug oscillator coils.

There are many of these red top coils that look identical from the outside and have the same application in MW band AM transistor radios. Wouldn't you know it, there are two types. One has the phase of the windings reversed. So if it is put in a radio pcb or wired up for the other version, there are no oscillations.

A lot of radio kits or other circuits using these coils get built and the constructor doesn't know why the the kit won't work and the transistor often gets blamed.

The suppliers for these coils, eBay or elsewhere never know which version they have. The only way is to test them. I had not known this until recently, having used these many times in the past, even in my childhood making AM radios, until I built a circuit with two VFO's using these coils and I got a bunch of them from a few suppliers. It was a big surprise to me that there where two types and it explained why I had read about troubles with kits using these coils.
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Old 15th Dec 2018, 7:11 am   #27
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In the examples of the radio PCB's they are drilled for the coil, usually 3 terminals one side & two on the other.
I have not seen these Chinese 455/470 Khz coils sold separately, or in sets.

Are you thinking of the Japanese Toko coils, or has that company ceased making them?
I remember using them and Mullard 'Lockfit' transistors for the IF strip of a AM radio we built on stripboard in College around 1984. So you know where I got the desire to build radios from.
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Old 15th Dec 2018, 1:11 pm   #28
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I have at least two different examples of the red slug oscillator coils. One was purchased in Japan and is Toko or Mitsumi ( will need to dig it out of my parts box to say which exactly). I desoldered the second oscillator coil from a junk IC-based radio hoping to use it for my projects, only to find that it has a primary coil winding and no secondary coil at all. I never knew that such variations existed and am not sure how it actually works.

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Old 15th Dec 2018, 4:45 pm   #29
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Default Re: More Chinese AM radio kits

You can buy that KXA168 radio ready built!
I have been browsing again and one listing had a variety of prices, starting with the kit only & adding things like a battery, solder, soldering iron and finally a ready-built radio.
It is over 8.00 UKP, wheras the DIY kit is 2.60UKP.
Maybe I should have done that after those 3 failures.
The total sum I spent on the 3 failures is around equal to what I spent on the Soviet 'Almaz' radio I recently did.
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Old 15th Dec 2018, 10:44 pm   #30
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Quote:
Originally Posted by Neil Purling View Post
Are you thinking of the Japanese Toko coils, or has that company ceased making them?
Toko RCL is just one of the many manufacturers of these coils. They are usually very good quality. Most now come out of China. For many years Jaycar sold a coil kit with the Red slug osc coil and three IF coils white, yellow & black and the small V/C and ferrite rod & coil to make AM radios.

Now though with the plethora of these coils about, there are variations of the red slug ones that look the same but are not.

Generally they all have the 5 pins. The tuned winding is the same and the tap on that is normally, for all of them, in the same place closer to one end of the tuned coil. However, the phase of the other coil (connected to the other two pins )is reversed. So if you drop these versions into a pre-existing pcb that was not designed for that, the L/O will not oscillate. But they work fine if you reverse the connections.

I don't know what proportions or numbers of the coils out there are like this. All I know is when I bought a range of these from different sellers, some were reversed compared to others, which is a real trap. One of the phasings is probably a common sort, the other may be rarer, but they are still out there.

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Old 15th Dec 2018, 11:31 pm   #31
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Toko RCL is just one of the many manufacturers of these coils. They are usually very good quality. Most now come out of China. For many years Jaycar sold a coil kit with the Red slug osc coil and three IF coils white, yellow & black and the small V/C and ferrite rod & coil to make AM radios.

Now though with the plethora of these coils about, there are variations of the red slug ones that look the same but are not.

Generally they all have the 5 pins. The tuned winding is the same and the tap on that is normally, for all of them, in the same place closer to one end of the tuned coil. However, the phase of the other coil (connected to the other two pins )is reversed. So if you drop these versions into a pre-existing pcb that was not designed for that, the L/O will not oscillate. But they work fine if you reverse the connections.

I don't know what proportions or numbers of the coils out there are like this. All I know is when I bought a range of these from different sellers, some were reversed compared to others, which is a real trap. One of the phasings is probably a common sort, the other may be rarer, but they are still out there.[/QUOTE]

So, if a certain oscillator coil does not work as in Figure A due to winding phase issues, it might work if reversed as in Figure B ? I have taken figure A as an example from an actual circuit diagram. Maybe a BC548B or other suitable transistor could be used as the local oscillator ?
Apologies for the poor diagram.
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Old 16th Dec 2018, 12:34 am   #32
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Quote:
Originally Posted by Jolly 7 View Post
So, if a certain oscillator coil does not work as in Figure A due to winding phase issues, it might work if reversed as in Figure B ? I have taken figure A as an example from an actual circuit diagram. Maybe a BC548B or other suitable transistor could be used as the local oscillator ?
Apologies for the poor diagram.
Yes that's right, if the specimen coil doesn't oscillate just reverse the coil connections in the collector circuit.

Practically any RF signal transistor will work in the local oscillator, even the early germanium RF types with high collector to base feedback capacitances.

In the case of the IF coils of course, at least using modern silicon transistors (or good germaniums with a low feedback capacitance AF124 etc) no neutralization is required.



So if it happened that the phase of the winding of the secondary of the 455kHz IF transformers, happened to be reversed compared to another transformer specimen, nobody would know and they would work just fine.

The winding phase of the IF coils is only important if it is a vintage transistor set, say with something like OC45's, where there is a feedback or neutralisation capacitor which takes it signal from the base of the next stage or IF transformer secondary.


By the way, the circuit in post 22 has a poor arrangement for the bias for the transistor driving the primary of the audio driver transformer. The collector current would vary widely depending on the transistor's individual hfe and be unstable to temperature, It needs an emitter resistor to stabilize it , bypassed at audio frequencies with a capacitor and a base to ground resistor as well. It is an example of very poor design.

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Old 16th Dec 2018, 9:49 am   #33
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Default Re: More Chinese AM radio kits

The better seven transistor HX108-2 has a variant with a audio stage that just uses a TDA2822 IC.
They are also found in your accessory 2.1 PC speakers. On only 3 volts they aren't going to make much power, but we really only want 0.25W to move a better speaker that may just give some more bass.
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Old 17th Dec 2018, 10:23 am   #34
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In the HX-6B, 9018-2 and S66E & others the driver is a 9014.
Are there any effective improvements I can make that might have a positive effect on the sound output?
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Old 18th Dec 2018, 6:08 pm   #35
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I have looked at the HX-6B and HX108-2 on Shango066's YouTube channel.
The extra IF stage of the HX108 does make quite a difference in the signal strength arriving at the detector. I just don't know what order of magnitude it represents.
The audio quality didn't seem that good. There's probably some room for change by trying different speakers.
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Old 19th Dec 2018, 5:53 pm   #36
Neil Purling
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I am proceeding with the last KX168 in a different way.
I have assembled the audio stages. I inserted the transistors first, checking they were the right way & I had plenty room to use my pliers as a heat-sink on the leads.

I do not presently have anything that gives 1Khz to test my work.
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Old 19th Dec 2018, 6:45 pm   #37
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I am sure some of us would be keen to learn how to properly align a superhet radio including kit radios.. It would be great if you could guide us with some more details as to what signal frequencies should be used and how. Thanks.
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Old 19th Dec 2018, 7:04 pm   #38
Neil Purling
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This is the signal generator I have: I have seen exactly the same thing under several names. It came from a Forum member.
The lowest frequency it gives is 165Khz.
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Old 19th Dec 2018, 7:36 pm   #39
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That looks neat. I have a Heathkit signal generator but haven't really used it yet. Some time ago I also built a 455 kHz minispot signal generator but not sure how to use it to get the most out of it.

A bit off topic, but here is a Chinese pocket radio. Three of these were given to me as a gift but none really work. I am not sure what circuit design this is as there are no oscillator coils or transformers. I can see four transistors and possibly a diode.
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Old 19th Dec 2018, 11:38 pm   #40
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I was referring to this I built some time ago. It was fairly easy to make.

http://www.siliconchip.com.au/Issue/...r?res=nonflash
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