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Old 3rd Jan 2019, 11:29 pm   #101
Jolly 7
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Default Re: More Chinese AM radio kits

As the driver and output transformers are highly unreliable, I am thinking of making and fitting a 3 volt amplifier in place of them. This could either be IC or transistor based. I have a couple of LM386Ns somewhere, so they might do the trick.
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Old 4th Jan 2019, 10:00 am   #102
Neil Purling
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Default Re: More Chinese AM radio kits

There's a HX108-2A. This uses the TDA2822 8-pin IC. I also have ordered one of these.

I note that the HX102 received Kingstown Radio well in Hull. This is located in the Hull Royal Infirmary, the transmitter being a merely 45 watts.
At night the sky wave brings in other transmissions almost on top of it. One of which being Manx Radio.
I haven't tried this radio at night yet. I am waiting for the CR123 batteries I ordered to arrive.

m0cemdave: That speaker further reinforces my suspicion that these kits are assembled from QC rejects.
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Old 5th Jan 2019, 11:57 am   #103
Jolly 7
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Would it be correct to say that transistor V1 is acting both as an oscillator and a mixer in the HX108-2 ?
Some superhet radios have separate oscillator and mixer transistors if I am not mistaken ?
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Old 6th Jan 2019, 8:08 am   #104
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Default Re: More Chinese AM radio kits

The first transistor is always a self-oscillating mixer in all of these kit radios. Unless it uses a IC of course.
Nothing new has arrived since Christmas/New Year. Still waiting for more bits & a total of eight kits.
I already had several HX-6B kits, so I assembled one of those yesterday. I made a mistake around V3 which I need to sort out.
It is oscillating, I know this after testing with another radio.
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Old 6th Jan 2019, 12:06 pm   #105
Jolly 7
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Default Re: More Chinese AM radio kits

Thanks Neil. I am trying to build a superhet without a circuit diagram on an old unused radio PCB, so it helps to know. So far I have figured out where all the coils and the first transistor go.
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Old 7th Jan 2019, 2:11 pm   #106
Neil Purling
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Default Re: More Chinese AM radio kits

The six transistor radio is very deaf, compared to the 7 transistor HX108-2. The S66E, which is the 6 transistor radio has audio that sounds like the speaker has sand in the voice coil. It is not the OEM speaker, but a 50mm plastic-coned water resistant driver.
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Old 7th Jan 2019, 11:35 pm   #107
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Default Re: More Chinese AM radio kits

In spite of only two IF stages in the S66E, someone set the IF to 475 kHz and reported very good results. I haven't tried building the kit myself yet.

The HX108-2 uses the fourth transistor configured as a detector.
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Old 8th Jan 2019, 8:11 pm   #108
Neil Purling
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Default Re: More Chinese AM radio kits

The plastic coned speaker I used lasted all of two weeks before it sounded like Lee Marvin performing Wandrin Star with a sore throat. I put the bog-standard speaker back.
The HX108-2 is worth getting, but there is so little precision in the tuning that it is impossible to find Manx Radio at around 1370 Khz above Kingstown Radio on 1350 Khz.
There's all sorts of stuff that appears after dark on the sky wave.
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Old 11th Jan 2019, 9:47 am   #109
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Default Re: More Chinese AM radio kits

I didn't like the dial window of the HX108. At the high frequency end of the band it was rendered opaque by the strip of adhesive.
So I got busy with the scissors & re-used part of a HX-6B dial. It is not pretty, but who cares anyway?
No new kits have arrived yet.
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Old 22nd Jan 2019, 1:20 pm   #110
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Default Re: More Chinese AM radio kits

Can anyone that's built the HX108-2 please tell me what the current draw should be as mines drawing around 17.5ma which I think is a little high.
It works but doesn't pick up any stations.
regards poppydog
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Old 22nd Jan 2019, 2:26 pm   #111
Jolly 7
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Default Re: More Chinese AM radio kits

If you are getting hiss from the speaker and are able to change the volume, it means your AF stages should be fine. Does your radio beat against another one nearby ?
I have had to make modifications to the PCB and add a couple of components to make mine work properly. The first one I made went dead due to blown preamp/ output transistors as a result of excessive current in the AF stages. This is due to defective circuit design.
I will attempt to post some photos of my mods and current consumption readings later today.
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Old 22nd Jan 2019, 2:53 pm   #112
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Default Re: More Chinese AM radio kits

Thanks for the reply jolly 7, ive got no beat from another radio or any tone from sig gen so I don't think the osc is running, I was sent different transistors in my kit so its not helped matters. I did have some interference from a led light coming out of speaker but that has disappeared now. I will post some pics after tea tonight in the hope that any eagle eyed members can see a fault that I've missed. There is some specs regarding current that I didn't see earlier on the sheet so will look at them tonight as well.
regards poppydog
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Old 22nd Jan 2019, 6:22 pm   #113
Jolly 7
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Default Re: More Chinese AM radio kits

My first kit never oscillated either. It's no surprise with the largely substandard components included with it. For example the 0.02 uF caps could measure anywhere from 0.01 to 0.04 uF.

For the second kit, I used my own polyester caps and transistors. Either 9018G or 2N3904 could be used as the local oscillator/ mixer. For V2 and V3 I used 2N3904s, which provide high gain similar to the 9018Hs that are in the circuit diagram, but not supplied with the kit.

For V4, either 2N3904 or a BC548B should work, paying attention to pinouts. I substituted this with a vintage Ferranti silicon diode...and it worked too Germanium doides like OA70 don't work here unless the circuit is modified...I am yet to try this out myself.

For V5 to V7 I used S8050s. The driver and output transformers vary wildly in their resistance readings from kit to kit, but did work.

I've attached a photo showing all the five points that need to be bridged and the points that should be cut to allow additional components for the AF stage, as indicated.
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Old 22nd Jan 2019, 9:01 pm   #114
Jolly 7
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Default Re: More Chinese AM radio kits

Quote:
Originally Posted by poppydog View Post
Can anyone that's built the HX108-2 please tell me what the current draw should be
I measured the total current draw on mine. It's roughly 20 mA with no station and rises to 40 mA and above with strong stations.
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Old 22nd Jan 2019, 9:16 pm   #115
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Default Re: More Chinese AM radio kits

I thought I would put these pics up on here in the hope that someone may pick up on something that I have missed regarding assembly etc, I have checked and double checked my work. I am not going to spend too much time on this however I am willing to spend a couple hours more on it just in case it turns out to be something silly. There were also a few out of spec components in this kit as well, I think you get what you pay for, though I was impressed with the way the HX-6B turned out that I built last year.

Regards
Poppydog
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Old 22nd Jan 2019, 9:53 pm   #116
Jolly 7
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Default Re: More Chinese AM radio kits

If I can see correctly, one of the antenna wire connections looks transposed to me. Please try as suggested and see if the radio starts oscillating.
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Old 22nd Jan 2019, 11:00 pm   #117
poppydog
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Default Re: More Chinese AM radio kits

Thanks for the reply regarding the current, there is a mention on the sheet about 20ma but I'm not that fluent in Chinese.

Some times you cant see for looking, I will move that aerial wire tomorrow. Glad I posted pics now.
regards poppydog
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Old 23rd Jan 2019, 1:59 pm   #118
Jolly 7
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Default Re: More Chinese AM radio kits

No worries. If that doesn't work it might be useful to check continuity of the two antenna coils on a multimeter in the event of any breaks that might not be apparent.
I take it you are happy that all five testpoints are bridged ?
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Old 23rd Jan 2019, 8:45 pm   #119
poppydog
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Default Re: More Chinese AM radio kits

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
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Old 23rd Jan 2019, 8:58 pm   #120
Jolly 7
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Default Re: More Chinese AM radio kits

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.
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