Radio Tech kit

[Frankje]

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Hi all !
Influenced and motivated by Christoph, who builded the 3 console FM receiver, I rediscovered the Kosmos Radio Tech kit.
The X4000 kit, I found in a local toy shop around 1990.
I've put all parts in 2 small boxes for better storage.
A few years ago, I was lucky to be able to order all pieces of this radio tech kit by sending a mail to Kosmos, and they were so kind to send them.
But the thing was, I never succeeded to build a working FM radio
So, I took some time to build the UKW-FM super on page 123.
When powering on, only noise and faint music.
The trick was to tune the local oscillator, the RF circuit and the IF transformer.
And after half on hour of annoying my family members, the radio works with very good reception and clear sound.
Amazing how this radio kit performs

Are there any other members of this forum who builded these radios ?

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[oe1cpa]

Hello "Frankje",

yes the radio tech was working well - in both directions ! As radio and as FM transmitter :D :D

But the best results I had by adding an AFC loop. This is very simple on Radio Tech.
But you have to do very uncritical modification on the IF Module. You have to short-cut the
output capacitor (100nF) near the OUT pin (and use an external capacitor before the volume
poti.

When you short-cut the cap, you will get the DC part which reflects the tuning level. So, if the
frequency is tuned too low, the "center" of FM (DC part) is higher than average, if it is tuned
too high, the DC level is lower.

No you need to feed back the DC part to the tuning circuit. I did this with the 220k resistor.
On the tuning poti I used the 5k6 resistor. On the connection point of both, I set a capacitor
to GND (I think it was the 100n, but I have to check) and a further resistor to the tuning
input of the VHF module (value I also have to check, uncritical !) - I only used parts, which
are INCLUDED in Radio Tech, nothing extra !

This works well now. If the tuning knob is in capturing of a station, it automatically "jumps"
to the center ! Really great !

-----

By the way !
If you also want the extend the radio by a STEREO Decoder, RDS, .... you have to remove the
22nF capacitor on the IF module, above the 100nF output cap. This resistor short-cuts the high
frequency parts (above audio) against the IF internal + supply. You can simple de-solder the
part and keep it. Both modifications can be undone !! So do not worry !

Now the full MPX spectrum should be usable. RDS I have not tried yet, but I did a short experiment
with a stereo decoder module - it worked, but not HiFi - many years ago this experiment !

----

Greetings / 73
Christoph OE1CPA

[Frankje]

Hello Christoph
Thank you for the tech info.
I will try these modifications to the circuit when I find some time.
In the meanwhile, I have added an ON/OFF switch and a mini jack chassis on the X4000 panels.
This is much easier then pulling or adding wires to board.
Greetings

[Frankje]

Hi all,
I've recorded a MP3 of the FM radio circuit, tuning from 88 MHz to 108 MHz.
It takes a bit of fine tuning but the overall sound is quite acceptable ( it's not HIFI :wink: )
Click on this link : https://www.dropbox.com/s/6e87rpv6yfs34hm/2.mp3?dl=0
I've used an external FM antenna.
I will try to build an AFC circuit later
Greetings