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Old 2nd Dec 2020, 7:47 pm   #21
Radio Scotland
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Default Re: HX108-2 AM radio kit problem

Starting to think that the tuning cap is trying to pull the LO to low and its stalling hence the noise,

Messed about peaking it up and its a sensitive little radio when compared to some other radios i have
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Old 2nd Dec 2020, 8:42 pm   #22
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Default Re: HX108-2 AM radio kit problem

In my radio, I did not use any of the supplied 223 ceramic disc caps as most were way out of spec. Maybe the ones in the RF section are suspect and causing some sort of tracking mismatch/ instability ? I would try substituting those. Also the newer HX108-2 kits seem to have different tuning caps compared to the set I built.
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Old 2nd Dec 2020, 9:03 pm   #23
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Default Re: HX108-2 AM radio kit problem

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Originally Posted by Jolly 7 View Post
In my radio, I did not use any of the supplied 223 ceramic disc caps as most were way out of spec. Maybe the ones in the RF section are suspect and causing some sort of tracking mismatch/ instability ? I would try substituting those. Also the newer HX108-2 kits seem to have different tuning caps compared to the set I built.
I checked all the ceramic caps before installing them jolly and they all checked out ok, The tuning cap in mine was garbage, Think the vains were shorting as i was getting random noises whilst tuning through the band. I Substituted it for a better quality one,

The lowest marking on the dial is 530khz , But you can go lower as the tuning cap still has a bit of play in it before it end stops, Which makes me think the LO is being pulled to low hence the noise.

P.S The tracking is pretty much spot on

Jay
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Old 2nd Dec 2020, 9:31 pm   #24
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Default Re: HX108-2 AM radio kit problem

Looking at the schematic shown upthread, I note the absence of a padding-capacitor - which would im[ply to me that the tuning-capacitor would need to have different angle:capacitance profiles for the 2 sections.

In days-gone-past there was a Jackson Bros. tuning capacitor with one set of vanes giving 208pF the other set 176pF just for this reson.

You say you substituted a different capacitor - do you know if this had the right angle:capacitance profile for both the ferrite-rod signal-frequency stage and the local-oscillator?

If it's not tracking properly, you could find either the LO or the ferrite-rod end up tuned perilously close to the IF [or its 2nd harmonic] at which point interesting things are likely to happen.
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Old 2nd Dec 2020, 9:46 pm   #25
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Default Re: HX108-2 AM radio kit problem

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You say you substituted a different capacitor - do you know if this had the right angle:capacitance profile for both the ferrite-rod signal-frequency stage and the local-oscillator?

.
I don't no. It was just a tuning cap i had lying around, Youtuber shango066 does a long detailed video on this kit, And he seems to experience a similar problem at the bottom of the band, But he dismisses the noise as camera noise, The radio works great otherwise. Dial tracking seems pretty accurate




Cheers
Jay
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Old 9th Dec 2020, 10:04 pm   #26
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Default Re: HX108-2 AM radio kit problem

These all suffer with the same issue. I've made two and no matter what I've tried the problem persists. I think it's down to the PCB layout or the IF bandwidth is too wide in addition to the higher IF frequency. Fortunately only Spirit Radio on 549Khz is the only station that's readable here in Scotland just comes through with no instability but only just. I think you will have to live with it TBH.
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Old 10th Dec 2020, 12:05 am   #27
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Default Re: HX108-2 AM radio kit problem

Parasitic oscillations.

David
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Old 11th Dec 2020, 10:01 pm   #28
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Default Re: HX108-2 AM radio kit problem

100 nf across electrolytic between power rails ei7ka.
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Old 15th Dec 2020, 6:30 pm   #29
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Default Re: HX108-2 AM radio kit problem

Quote:
Originally Posted by murphyv310 View Post
These all suffer with the same issue. I've made two and no matter what I've tried the problem persists. I think it's down to the PCB layout or the IF bandwidth is too wide in addition to the higher IF frequency. Fortunately only Spirit Radio on 549Khz is the only station that's readable here in Scotland just comes through with no instability but only just. I think you will have to live with it TBH.
Yeah i just manage to get spirit radio a fraction lower and it starts to oscillate, And it could well be the way the radio is designed, Its annoying, But looks like i'll just need to put up with it,

I'll try the 100nf cap as suggested and report back,

Cheers
Jay
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Old 15th Dec 2020, 10:03 pm   #30
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Default Re: HX108-2 AM radio kit problem

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Quote:
Originally Posted by murphyv310 View Post
These all suffer with the

Yeah i just manage to get spirit radio a fraction lower and it starts to oscillate, And it could well be the way the radio is designed, Its annoying, But looks like i'll just need to put up with it,

I'll try the 100nf cap as suggested and report back,

Cheers
Jay

Can you receive Algeria on 531 kHz and Kossuth radio Hungary on 540 kHz on your set ?
Mine can and both stations are very strong right now. It's just after 9 PM.
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Old 15th Dec 2020, 10:14 pm   #31
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Default Re: HX108-2 AM radio kit problem

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Quote:
Originally Posted by Radio Scotland View Post
Quote:
Originally Posted by murphyv310 View Post
These all suffer with the

Yeah i just manage to get spirit radio a fraction lower and it starts to oscillate, And it could well be the way the radio is designed, Its annoying, But looks like i'll just need to put up with it,

I'll try the 100nf cap as suggested and report back,

Cheers
Jay

Can you receive Algeria on 531 kHz and Kossuth radio Hungary on 540 kHz on your set ?
Mine can and both stations are very strong right now. It's just after 9 PM.
Yeah i can receive a station just a fraction below spirit radio. Which must be radio hungary. Its on the point where the radio starts to oscillate,
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Old 16th Dec 2020, 6:02 pm   #32
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Default Re: HX108-2 AM radio kit problem

I note on the diagram in #6 that the first transistor has an 'G' suffix while all the others have 'H' suffixes. Is this actually carried through into the kit contents, do you get one 'G' and the rest 'H' or are they all just the same type?

If none of them are marked differently to the others there is likely to be some variance in the hfe values for each, so maybe just try swapping the first transistor for one or two of the others to see if that makes a difference?
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Old 16th Dec 2020, 6:55 pm   #33
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Default Re: HX108-2 AM radio kit problem

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Originally Posted by SiriusHardware View Post
I note on the diagram in #6 that the first transistor has an 'G' suffix while all the others have 'H' suffixes. Is this actually carried through into the kit contents, do you get one 'G' and the rest 'H' or are they all just the same type?

If none of them are marked differently to the others there is likely to be some variance in the hfe values for each, so maybe just try swapping the first transistor for one or two of the others to see if that makes a difference?
Argus 25 (no longer a member) touched on the potentially negative impact of the variability of transistor hfes in a previous post. In the kit I built some two years ago, there were no 9018H transistors, all were 9018Gs. I used 2N3904s for V2 to V4 instead, as the gain is higher than 9018G. My set works well but still produces a few squeals after dusk. These mostly do not impact on the reception of the stations I occasionally listen to. The worst squeal I can hear is around 930 kHz. This may not be a coincidence, given that the circuit IF is 465 kHz.
The more recent kits may now be supplied with 9018Hs.
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Old 16th Dec 2020, 6:56 pm   #34
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Default Re: HX108-2 AM radio kit problem

Quote:
Originally Posted by SiriusHardware View Post
I note on the diagram in #6 that the first transistor has an 'G' suffix while all the others have 'H' suffixes. Is this actually carried through into the kit contents, do you get one 'G' and the rest 'H' or are they all just the same type?

If none of them are marked differently to the others there is likely to be some variance in the hfe values for each, so maybe just try swapping the first transistor for one or two of the others to see if that makes a difference?
From memory they are all ''G'' suffix. Which seems to be common for these kits. There was quite a variation in gain when i tested them,
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Old 28th Dec 2020, 10:51 pm   #35
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Default Re: HX108-2 AM radio kit problem

My HX-108 have same motorboat oscillation problem....it is very sensitive but the audio is low quality..
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Old 1st Jan 2021, 4:14 pm   #36
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Default Re: HX108-2 AM radio kit problem

Is the thing over-sensitive or is the PCB badly designed?
I was just wondering if it would be stable if the same radio components were built up on strip-board instead.
They still sound appalling.
Maybe they also need better driver & output transformers as well as well-matched output transistors?
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Old 7th Jan 2021, 11:48 am   #37
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Default Re: HX108-2 AM radio kit problem

Hi.
I think for the price and the purpose of these kits as a learning aid, we really cannot be too critical.
Many years ago we had the likes of the excellent Philips EE8/20, amongst others. These days learning aids like this have all but gone, the Chinese Kits are not ideal as there is no explanation as to the circuit nor decent construction details but they are better than the zero learning aids made in the UK.
They do work but not what we expect as more experienced hobbyists. So I don't think we should be so critical of them.
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Old 7th Jan 2021, 2:39 pm   #38
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Default Re: HX108-2 AM radio kit problem

I have made a couple of these kits and spent some time 'playing' with them. I also have aligned them with an RF signal generator. In addition, a couple of friends have also made them.

So, my thoughts are they really are excellent little kits. I did the usual recommendation and tested all the parts prior to building, and one of mine has really excellent performance. (apart from the obvious audio limitation of a tiny speaker) It easily out performs my Tecsun radio for pulling in stations and is great fun for DXing. I have received more stations with this set than many others.

However - what I have found is that the supplied transistors vary considerably with performance. It has been suggested in the past that they may be 'rejects' and I would tend to agree. One of my friends builds just wouldn't work at all; no amount of messing and checking could get it to oscillate properly. After swapping out the oscillator/mixer transistor it burst into life, despite the original testing OK. I have put a number of the supplied transistors on my Peak semiconductor analyser and found considerable variations - hfe that vary from 18 to 265, leakage currents which vary between 0 and 0.7mA, and B-E voltage drops varying between 0.45 & 0.9V

Clearly not ideal when trying to get a set to work! I would suggest swapping at least the oscillator transistor and maybe the 1st IF one and see if that helps.
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Old 15th Feb 2021, 10:57 pm   #39
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Default Re: HX108-2 AM radio kit problem

Just a quick update on this, I haven't had a chance to work on the radio since posting this thread, But i recently dug out the first one i built and it doesn't have the same problem, (Could have sworn it did) Although it does have the mandatory scratchy tuning cap
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Old 17th Feb 2021, 1:10 am   #40
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Default Re: HX108-2 AM radio kit problem

Can you actually get a better quality tuning capacitor & what values should be used?
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