Quote:
Originally Posted by Tully Bascombe
Keep us posted with your progress on these 2 radios, especially the ZX921. Mine whistles, howls and motorboats something shocking.
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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.