More Chinese AM radio kits

[poppydog]

I moved the ferrite wire to its correct position in the hope that it would burst into life but no such luck, however whilst injecting a tone into the base of v4 its kickstarted it into life. Not quite sure why this has happened so do I replace it or leave it like it is? I can hear a faint station so im feeling a little more optimistic. Current has now risen to around 19ma.
The transistor line up I was sent comprised 4 of 9018G (no h spec as stated on sheet) and 3 of 945G for the driver and the output.
regards poppydog

[Jolly 7]

My suggestion would be to use 2N3904 for both V2 and V3. Leaving 9018G in for V1 and V4 will probably be ok. The C945s for V5 to V7 should be ok too, but as mentioned earlier V5 needs an additional 10ohm resistor from emitter to ground. A 5.6 ohm resistor is also needed from the junction of emitters V6 and V7 to ground.

[Jolly 7]

Here is my completed radio. It is not perfect because there is some squeal/ motorboating around 900 kHz which I can't seem to get rid of, but then my signal generator is fixed at 455 kHz and the radio's IF is 465 kHz, so that might be the cause.
Overall I'm still quite happy with it and this set will probably outperform many portable AM radios on the market due to the low noise floor and very good sensitivity.

[Argus25]

Reading through this whole thread is a reminder that when building any superhet radio, be it a kit or a self designed one, when it is completed, all parts of the system need to be tested & checked.

Firstly the supply current to the audio & driver stages and the whole radio. At least one check with a 1kHz sine wave and audio into a dummy load, to check that the audio stages are basically distortion free and have around the correct output level.

Then a sweep of the IF (ideally with a signal generator and scope) to ensure the IF is stable, has about the correct bandwidth & gain.(if not the bypass and grounding arrangements and AGC system checked and modified if required) Then a check of the local oscillator level and frequency range. Then an overall radio sensitivity check.

There are a lot of variables which could sabotage the operation and performance of the radio kit, so these checks, on a newly built unit, are definitely required. If they are not done you end up with all the reports of bad performance or failure to operate, many a case it will just be one unsuitable part or component or something simple. Without the systematic tests, the faults will remain unsolved and elusive. A lot of the time you can "get lucky" and everything will appear to be ok first pop, but as shown by the remarks on this thread, it is a bit hit & miss.

[poppydog]

Mines all sorted as well, for some reason the 9018g that was used as the detector diode wouldn’t detect unless it was prompted with my signal generator. I tried a silicon diode in place of it but that wouldn’t work so I tried a BC377 and that worked fine. One of the electrolytic caps tested awful so I stuck one of my own in. Took quite a while to get all the stations to come in and to get them in their right places on the dial but its done and its picking up everything very well now including Absolute (fading in and out)whose transmitters were turned off near me a while back now. For less than a fiver delivered its been a good learning curve. 5 mins with a meter or a tester, testing components are in spec is recommended prior to installation of components.

[regenfreak]

I built mine a few months ago and it was dead. I think the output transformer was defective. So I tossed it away.

Then I bought another more expensive and bigger superhet kit (about £16)from a Ham dealer in Thailand. It worked right away although there were a number of errors in the thai/English instruction and over current problem.

I ordered two weeks ago the same £5 Chinese superhet kit for the second time from a different dealer. This time it arrived quick and it has much clear and better instruction. I intend to use the oscillator and IF cans for IC NE612AN mixer/oscillator experiment.

I have also bought the Chinese 2P3 kit for £20. It is a very nice kit with very good instruction. Well-worth the money.

[GeoffK]

I decided to have a go at making one of these radios, bought from a Singapore site as it was under £5 including postage and arrived quite quickly. It went together reasonably well, the red plastic aerial ferrite holder needed one screw hole countersinking as the screw wouldn’t go into the tuner even after the edge was bevelled as it was hitting a couple of the solder joints and wouldn’t lay flat. I tried a diode instead of the 9018G transistor as the detector, a BA219 as it was the only one I had, a 1N916 should also work here. Because the agc is taken from the anode of the diode via the last IF secondary the cathode connects to the volume control end, unlike a conventional pocket radio with npn transistors where the cathode connects to the last IF transformer.

The radio has only been temporary put together along with a temporary speaker to try it, and it worked straight away. Only the oscillator trimmer, the one nearest the edge of the case on the tuner, needed some adjustment to get the stations in roughly the right place, this will be improved once the dial is fitted. It still needs some final putting together but I am tempted to leave the speaker as it sounds very good, it was quite a revelation as I wasn’t really expecting it to work all that well if at all. The quiescent current is around 20mA so would like to get this down a bit, possibly a larger value of base bias resistor or the output transistors emitter resistor added.

The 9018G transistors give plenty of gain in the IF stage, the IF transformers are spot on. The only components I used from my own parts was a 100uF capacitor instead of the one of the two supplied, and the 0.01uF mixer input coupling capacitor as, like others here, it was the only part missing. Orange sleeving was used on the resistor leads but they are so thin the sleeving wouldn’t go around the curved part. The gaps in the printed board for current measuring had wire from the offcuts soldered across rather than just solder to bridge them.

[poppydog]

I had trouble with the ferrite/tuning cap mount, mine was cured with the stanley knife, also the vol wheel kept coming loose but a blob of nail varnish seems to have stopped that. Whilst I'm fairly happy with mine I find that small pocket radios are terrible for listening to music on.

regards poppydog

[GeoffK]

Whilst I'm fairly happy with mine I find that small pocket radios are terrible for listening to music on.

For the size of the speaker I quite like listening to music. Sometimes it is the lack of full freqency that makes music easier to listen to. Speech can also be clearer with a small speaker.

[GeoffK]

Finished the radio, a piece of white gauze was dyed black with printing ink, and after drying put in front of speaker to make a grill instead of leaving the speaker cone exposed. The wires are all to long so these might be shorted. The aerial ferrite is held in place with a piece of sticky tape, the speaker needs this or glue.

[Jolly 7]

The speaker cone protection is a very good idea. I might do this myself with some very light black fabric I have lying around. We sometimes forget that dust, humidity and smoke can eventually ruin our prized electronic possessions.