Chinese Radio Kit.

[Murphyv310]

Hi.
I bought a couple of these AM radio kits off Ebay for the oscillator coils and IF transformers. I got the itch and I built one up this afternoon. Fiddly with my eyesight to be honest, the diagram is wrong in a couple of places which I've scanned and I corrected. Worked first time, a little unstable at the lower end of the band, alignment not bad with only a little adjustment required, quite sensitive and tuning sharp too, audio not too good as the speaker is poor.

[Mike Watterson]

I've bought a few of those too for parts. There about 3 other variations of old AM radio updated to Silicon. It seems like an old Germanium design slightly updated for Silicon. Can't be beat for value. The kit with the 70 kHz SMD superhet IC for FM and the modern version of MK484/ZN414 works well and the trick is to lightly tin PCB and tack the two corners, then simply press down and reflow several leads at a time. I meant to do an article a couple of years ago. My grandson built the FM/AM kit when he was about 10 with a little help.

The first time I got a Chinese schematic I used the Chinese of the resistor code colours to decipher which colour of ferrite on the can was the correct position, as the LO, IF and final IF are different.

[Crackle]

Another radio boasting 7 transistors when 6 and a diode will do the job.

Mike

[radioman]

There seems to be a few versions of this particular radio.
I ordered one in 2015 (where does the time go?) for the same reasons as Trevor - however, when mine arrived I found out that it used 7mm coils with unusual colours instead of the expected 10mm types... so had to spend a bit of time decoding the instruction sheet.
Worked better that I expected although I've had to change the tuning capacitor in mine as it had gone 'noisy'.

Andy

[Murphyv310]

Hi.
In fact Caroline was receivable late in the evening yesterday. Although it works well enough it's really let down by the speaker. I'll fit a headphone socket I think. For the price it's OK and yes Mike it is 6 active transistors. I might try a 1N60 later instead.

[Murphyv310]

Andy.
Yes it's very similar indeed, looks to have a slightly better layout and the power and speaker wires go through the board. I see you had to use hot melt to retain the speaker, the retaining lugs break off the case easily.

[Mike Watterson]

Technically the 7th detector is cunningly wired to give a lower on resistance than a silicon diode, but the turn on will still be 0.6 to 0.63 approximately. So an 1N60 will work better. The Silicon transistors are now cheaper, which is why they do it!

The 1.25V to 1.3V approximate regulator for the receiver HT is a curious feature. I must compare with D1 and D2 open, over a 1.8V to 3.2 supply range (2 x Alkaline cells to end of life). Other than that it's like a silicon version of a germanium pocket set used for 1960s Caroline and Luxembourg

[Murphyv310]

Hi
Looking closely at the circuit a germanium Diode won't work as it stands and in fact doesn't. Its bias is too high. I replaced the transistor with a 1SJ150, and this works better with less distortion. I've added a 3.5mm mono switchable socket, the radio actually sounds good on an extension speaker.

[Mike Watterson]

You'd have to have no bias and rewire how the AGC is feed.

[Murphyv310]

It's not really worth, it's fiddly as it is without making it more difficult, I struggle enough with eyesight that has deteriorated over the last few years.

Here is the fully completed set with the wires from the aerial coil tidy.