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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
Tully Bascombe
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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
Avid_Nerdlinger
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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
Argus25
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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.

Last edited by Argus25; 21st May 2019 at 10:17 pm.
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Old Yesterday, 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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