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Old 5th May 2021, 12:55 am   #1
WilliamTK1974
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Unhappy HK-6B transistor radio- nothing but birdies

Hello everyone,

In case you're not familiar, the HK-6B transistor radio is an AM (MW) only, six-transistor DIY radio kit. They're still available, and appear to come in around $10US including shipping. No ICs on this board. Just straight transistors. I've had one for a bit and decided to have a go at getting it together over the last couple of evenings.

There's an English language website, https://www.usefulcomponents.com/mai..._superhet.html
This site breaks the instructions down into English since the sheet that comes with the radio is Chinese.

Once I got it together, it worked on the first try. It was kind of weak, so I start working through the instructions for someone who has no test equipment. I got through the simple adjustments, and decided I wanted to have a go at the slightly more complicated adjustments for someone who didn't have test equipment.

The instructions say to start at the low end of the dial, but we don't have any local stations down there that I can pick up, so I went up to the higher end, where we have some in the 1400 range that come in clear. Well, looking back, I may have gotten confused. I read his instructions to mean that I was supposed to adjust the Red trimmer slug at the high end, when it appears I was supposed to only do that if I had a low-end station. At the high end, I was supposed to adjust the OSC trimmer on the tuning capacitor.

So, I did it wrong. I barely turned the red slug, when the signal dropped out and I'm now getting nothing but birdies. I've been using a plastic knife cut down to make a plastic screwdriver, so no metal is involved, and as far as I can tell, I haven't cracked the slug. But I don't know how high or low it was before I touched it.

Nothing seems to bring the radio back. Just birdies. I was quite proud that my soldering skills were up to the task, but now I'm afraid I've ruined it.

How can I get it back to operation?

Thank you,
-William
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Old 5th May 2021, 8:49 am   #2
AdrianH
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Default Re: HK-6B transistor radio- nothing but birdies

I am not sure if this would work for you or not, there are a lot of if's, but worth a try. If you know what the IF frequency is of your kit and you have another Longwave/medium wave radio available and you also know what the IF frequency of that one is.

So for example if your Kit, and another radio both use 470KHz IF's.

The Local oscillator is typically on the High side of the wanted frequency, so switch the working radio over to Long wave, if it tunes 1100 Meters to 1900 meters that is 272KHz to 157KHz. Then the local oscillator will be tuning 470 KHz above this and track between 742 KHz and 627 KHz. So this could give you a simple Carrier oscillator near the bottom end of the medium Wave band.

So if the LW dial is set to 1900 Meters or 157KHz you could get a carrier on the medium wave band at 627 KHz or 478 Meters.

If you place the radios close to each other there should be enough leakage of the osc from the working radio to be picked up by another.
Something to try with two working radios first if you have them to get the idea.

Adrian
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Last edited by AdrianH; 5th May 2021 at 8:56 am.
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Old 5th May 2021, 2:55 pm   #3
WilliamTK1974
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Default Re: HK-6B transistor radio- nothing but birdies

Thank you for your response.

I knew that something like this was possible, but didn't know the details. I remember reading that the Nazis during WWII would use something similar to this in order to detect when people were using illegal radios, so people started making crystal-type radios using pencil stubs and blued steel razor blades.

Since this radio isn't a treasured family heirloom, I decided to have a closer look inside of the red coil. I've never looked in one before. So, I desoldered it from the board and took it apart Inside, I saw the slug and the windings, and noticed that one of the wires that crosses under another wire was broken. It was almost unnoticeable. They must use garbage grade components on these kits because you can read other stories from people who have pre-tested the components before beginning assembly and found them to be out of spec right out of the box.

That said, I'll probably build another one. It was satisfying to have it working on the first try, even for a short time. The soldering practice is a good thing. I also have an HX108-2 kit waiting, and have heard that it's a better performer.

Thank you,
-William
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Old 5th May 2021, 7:52 pm   #4
Cruisin Marine
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Default Re: HK-6B transistor radio- nothing but birdies

Try to get hold of some PLASTIC knitting needles to use as trimming tools, easy to file down, and they save many a core from cracking.
The old Pye plastic trimming tools were essentially just that.
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Old 6th May 2021, 1:53 am   #5
WilliamTK1974
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Default Re: HK-6B transistor radio- nothing but birdies

Quote:
Originally Posted by AdrianH View Post
The Local oscillator is typically on the High side of the wanted frequency, so switch the working radio over to Long wave, if it tunes 1100 Meters to 1900 meters that is 272KHz to 157KHz. Then the local oscillator will be tuning 470 KHz above this and track between 742 KHz and 627 KHz. So this could give you a simple Carrier oscillator near the bottom end of the medium Wave band.

So if the LW dial is set to 1900 Meters or 157KHz you could get a carrier on the medium wave band at 627 KHz or 478 Meters.

If you place the radios close to each other there should be enough leakage of the osc from the working radio to be picked up by another.
Something to try with two working radios first if you have them to get the idea.

Adrian
Ok, I had a go at this earlier today and have a question. I used a Selena Vega B212 on LW and a GE transistor AM/FM for the AM. The Vega's dial isn't the most precise, but I think it's around 1900m. I'm not sure what the GE's dial said except that it was on the lower end of the band. There was the typical random AM static, then I got a loud-ish, hissy sort of silence at one point, if that makes sense. Is that the carrier tone that I would need to be listening for if I were going to use something like that to adjust a radio on the low end of the dial?

Thank you,
-William
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Old 6th May 2021, 8:13 am   #6
AdrianH
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Default Re: HK-6B transistor radio- nothing but birdies

Hello William;
Yes I think that sounds like it could be the carrier of the local oscillator, I know it would be better if there was a way to put some form of modulation on it, but it could be a way to get a wanted signal. If there is an AGC line on the receiver under test then use of a DVM on the line can help in making tuning adjustments as you tune for max AGC action.

I had a play with two battery valve portables I had at this end yesterday and it worked reasonably well, probably a bit less screening on the valve sets. The frequency range is very limited in what you can achieve this way and a lot depends on the IF frequency, the lower it is the less chance you can get the correct osc frequency to overlap into the MW section before you have to start looking for 2 x osc and 3 x osc etc.

It could be worth while seeing if the bidding site has any basic RF signal generators, with internal modulation. Or study a few circuits and see if you can use the oscillator sections of that kit radio in some way to build a bit of gear for yourself.

A word of caution, collecting test gear for the 'could be useful' situations can become as much of a hobby as the radios themselves.

Adrian
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