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Old 1st May 2019, 8:29 am   #141
Tully Bascombe
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Default Re: More Chinese AM radio kits

Keep us posted with your progress on these 2 radios, especially the ZX921. Mine whistles, howls and motorboats something shocking.

There aren't any kits with long wave as far as I know
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Old 1st May 2019, 9:13 am   #142
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Default Re: More Chinese AM radio kits

If I can get these radios working properly I might try squeezing another coil on the ferrite rod and switching in a LW band, or maybe just modify one and convert it to LW. Actually the X-921 (ebay item 172841262851) photo has a long rod and two coils...
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Old 19th May 2019, 4:44 pm   #143
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Default Re: More Chinese AM radio kits

The HX-6B is actually a HX207, the ebay listing was a bit misleading. Anyway it seems to work up to a point, the dial frequencies seem to be way out compared to station broadcast frequencies but it's hard to say definitively as the MW reception here is very poor. I like the LED.

X-921 is not so good as I'm also getting squeals and farts out of it. It does pick up some stations at the right frequency but there's echo and Smash-makes-mash alien voice effects and a lot of microphony. Again the poor reception on the very top of GB makes it hard to test effectively.

Next I'll try converting HX207 to LW. I'm thinking of just soldering a cap on either side of the tuning variable capacitor to see what happens but I've got some suitable enamelled thin copper wire I could use to add some windings to the antenna coil.

I've also just ordered another one... HX108-2.
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Old 21st May 2019, 10:32 am   #144
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Default Re: More Chinese AM radio kits

I never actually managed to get hold of an HX-6B, appears to have been discontinued and replaced by the S66E. Likewise the HX207.

There's something fundamentally wrong with the design of the ZX921 causing the instability, whistling, howling etc. I wonder if someone can come up with fix as the radio itself has potential.
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Old 21st May 2019, 1:03 pm   #145
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Default Re: More Chinese AM radio kits

Quote:
Originally Posted by Tully Bascombe View Post
Keep us posted with your progress on these 2 radios, especially the ZX921. Mine whistles, howls and motorboats something shocking.
I find this thread quite remarkable in that clearly a lot of these cheap transistor kits have design and functionality issues.

Mostly, it appears, that the stages, specifically the IF stages, are unstable and are oscillating. And the likely reason for this is that it is a combination of poor design, not allowing for the wide range of transistor parameters like hfe, and silicon transistors with an excessive cut-off frequency (fT) for the 455kHz application they are placed in.

Yet everybody acts as though they are helpless bystanders, with no obvious effort to test, investigate, repair and/or modify the designs to correct these dilemmas.

All it requires is a decent scope and some familiarity with single conversion Superhet designs and there are dozens of folks on this forum who could do it or offer advice how to solve these problems too.

There is probably little point in building these kits if its pot luck and you just sit there wondering when they don't perform properly , whistle and "motorboat".

Maybe its because they are just so cheap that nobody cares, just buy another kit if one version doesn't work. But it is missing the point; that a technical challenge has been thrown in your face. So do you rise to the occasion, or just ignore it ?

Although this advice is probably not welcome, so don't sling mud in my direction, I'd suggest:

Be prepared to read up and learn more about the simple transistor superhet radio design, invest in a scope and signal generator, so that you can test, diagnose and modify these radios, so that you can get them to work at least basically well, or it is a total waste of time and money buying them even though they are as cheap as chips from China.
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Old 21st May 2019, 4:00 pm   #146
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Default Re: More Chinese AM radio kits

I got HX207 to tune to Radio 4 on 198kHz by adding a 82pf to the osc side of the tuning cap and a 1nF measuring 910pF to the antenna side. This HX207 was also a bit of a howler before I modified it but it is worse now. I can stop the howling by turning the volume down but that makes it too quiet to be useful. The same tuning cap appears to be used on X921, probably also the HX108-2 on order so I might try this on them at some point.

"Mostly, it appears, that the stages, specifically the IF stages, are unstable and are oscillating." - isn't it more likely the audio stage?
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Old 21st May 2019, 5:58 pm   #147
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Default Re: More Chinese AM radio kits

OK so you have all tempted me to get one and have a go at sorting it out when it hoots
It is advertised as 'superheterodyne radio teaching kit DIY training', I guess the training bit is getting it to work after building. It is coming by slow sampan so will take a while, I am going to build it in stages from back to front, testing each stage as I go. It can't be worse than the radio I have on the bench right now, works OK except FM where it picks up two stations at once but the funny thing is, it is impossible to detect the local oscillator when it has tuned into a station. Now this is going to be fun but should put me in the right mood for when the ZX921 arrives
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Old 21st May 2019, 10:02 pm   #148
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Default Re: More Chinese AM radio kits

Quote:
Originally Posted by Avid_Nerdlinger View Post

"Mostly, it appears, that the stages, specifically the IF stages, are unstable and are oscillating." - isn't it more likely the audio stage?
Possibly, it would take the audio stage to be oscillating at radio frequencies and be hetrodyning with either the received carrier or the IF signal. Its easy to find out, for that just temporarily connect a capacitor such as a 1000pF (probably 100pF would do) across the collector to base of the transistors in the audio stages. That will kill any RF response and tell you the answer there. If that is it just reduce the values if the audio sounds muffled, until it is clear. Of course with a scope it would be dead easy to see if there were oscillations coming out of the audio amp.

Motorboating, if its truly that is often just a common impedance in the power supply feed to the audio output stage and its driver stage, meaning not enough power supply rail decoupling between the two. It can also be due to a high resistance power switch or poor filtering capacitor across the battery.

Whistling can be a sign of IF instability or oscillation. In this case to tame that down you cannot add base to collector capacitance as it will make it even more unstable. One good trick is to add a resistor in series with the transistor's collector, say 47 to 100R. This will suppress any HF & VHF oscillations. A 100R series base resistor can also work too.

An IF can also oscillate if there is a feedback pathway due to poor filtering of the AGC line. If there is just an electrolytic cap only, filtering the agc line, then try connecting a 0.1uF ceramic cap across it.

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Old 22nd May 2019, 4:18 pm   #149
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Default Re: More Chinese AM radio kits

Thanks for that clear description Hugo, it's most helpful especially the simple method to suppress any RF by shunting a low value cap across collector/base of the audio transistors.

Regards,
Symon
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Old 6th Jun 2019, 1:22 pm   #150
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Default Re: More Chinese AM radio kits

Can someone recommend one of these kits that is half decent?

I dug out an old QST article from Feb 1968 from my PDF archive which stuffs a converter front end on an off the shelf AM BC radio and adds a BFO to the IF. Fancy having a go at it and don't want to sacrifice something better!
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Old 6th Jun 2019, 2:03 pm   #151
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Default Re: More Chinese AM radio kits

I suspect none of these are anything like a consumer product. I've only tried hx207 & zx921 so far (just started a hx108) but neither worked half as well as a shop-bought one. The cheapest on ebay are about 4, it wouldn't cost much if anything more to get an old second hand tranny needing repair off ebay which would probably suit your needs much better.
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Old 10th Jun 2019, 2:43 pm   #152
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Default Re: More Chinese AM radio kits

That said, this hx108-2 I just finished is quite impressive, needed a bit of aligning but tunes to distant stations without any of the oscillations I was getting with the other two. If you are still looking for one this is probably the best available for 4 off ebay. I'd still recommend a second hand consumer product for anything you plan to make good use of though.
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Old 11th Jun 2019, 4:37 pm   #153
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Default Re: More Chinese AM radio kits

I looked over the listings for the radio kits on the E-place. It seems like there's three different versions of this kit.
One version has a more conventional output stage using an input and output transformer, another has an input transformer coupling some form of a complimentary pair and coupling capacitor to speaker and the third doesn't use a transformer at all! All direct coupling!
It's good fun, going back to basics!
Best regards, Dave, US radcoll1.
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Old 29th Jul 2019, 10:09 am   #154
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Default Re: More Chinese AM radio kits

I've been following the ups and downs of the contributors to this thread with great interest, and have succumbed to the temptation to buy one of these kits and have a go myself.
There is no type number, but it's like the one in the pic.
I await it's arrival with great anticipation (and some trepidation,too!)
The "Chenglish" instructions supplied are a foretaste of what lies in store, I suspect Tony.
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Old 31st Jul 2019, 10:29 am   #155
Tully Bascombe
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Default Re: More Chinese AM radio kits

The photo is rather small but it looks like the elusive HX-6B. When I ordered one from Banggood I got an S66E instead.
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Old 3rd Aug 2019, 3:21 pm   #156
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Default Re: More Chinese AM radio kits

The photo was taken directly off E*** and mine is claimed to be a "similar model" to the illustration, so it could be a quite different (in detail) version.
The vendor of my one also shews the Chenglish assembly instructions. on the page with all the details of the set on it. Nowhere is a specific model number given, though.
I've a feeling this is going to be fun!
Can't wait! Tony.
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Old 13th Aug 2019, 10:55 am   #157
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Default Re: More Chinese AM radio kits

Well, the package arrived yesterday, having been ordered on the first of the month, which I thought was very quick from China, and the model is stated to be a HS-6B on the otherwise completely-written-in-Chinese assembly instructions.
The parts seem to be of reasonable quality, but I shall electrically check everything before assembling it.
Luckily, the instructions contain a circuit diagram, and a sketch of the P.C. board, so assembly/fault finding shouldn't be any problem.
Hope to do this sometime in the next few days. If it works ok, I shall probably have a go at an additional long-wave mod.
Looks like fun, folks!... Tony.
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Old 13th Aug 2019, 11:56 am   #158
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Default Re: More Chinese AM radio kits

I'm so glad to find other enthusiasts who collect these kits. To date I have around 16 various types.

My reason for collecting them goes back some 50+ years ago when I saw an advert in our local Saturday newspaper and in the small adds on the back pages a local shop, Teleradio, in Upper Edmonton (Nth London) was selling a basic transistor radio kit, even with a plastic case !!!

However, Mum put her foot down, as: "We didn't have enough spare money to waste on toys".

Little did I know, Dad must have had one of his 'wins' on the 'Horses' and, 8 weeks later, on my birthday, I was presented with the very transistor radio...……..the rest is history .
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Old 13th Aug 2019, 3:53 pm   #159
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Default Re: More Chinese AM radio kits

Tony, you can get the Chinese translated into English using Google Translate on a smartphone in camera mode. Or you can upload a photo of them to Google Drive, open using Google Docs and then translate the text.

The translation might not be so useful though, I attach one I did for HX207 as an example.

Andrew
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Old 13th Aug 2019, 6:58 pm   #160
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Default Re: More Chinese AM radio kits

Wow. "Hands on welding" is inolved!
Rob
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