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Old 21st Oct 2021, 4:46 pm   #1
davidgem1406
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Default 7 Transistor Kit Radio Output Problem

I have 2 Chinese kit portable radios to build for a friend.

The first I have built up as far as the completed PCB, and then realised that there was a problem with the output stage.
This uses 2 transformers, an intermediate and an output. One is correct to the circuit whilst the other is not.

I have attached the circuit of the output stages.

The two transformers consist of 1 Primary and 1 tapped Secondary winding
You can see from the circuit that this is not correct, as the output transformer consists of one winding only, with 3 taps.

The configuration of the transformers are as follows:
1/. Primary of 6R + 6R and a Secondary of 2R
2/. Primary of 230R and a taped Secondary of 105R + 105R.

I have made 2 assumptions about this, that the intermediate transformer designated B6 is the Primary of 230R and Secondary of 105R + 105R
Whilst the output designated as B7 drives the speaker and that the diagram is incorrect.
I have no idea as to the positioning of these transformers as their information is all in Chinese, so my assumptions could actually be reversed.

Also, it occurs to me that a 2R secondary can not drive an 8R speaker?

Any help with this would be appreciated.

Thanks,
Regards Dave.
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Old 21st Oct 2021, 5:17 pm   #2
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Default Re: 7 Transistor Kit Radio Output Problem

Try this YouTube video at about 9-50 in. It explains the error in the diagram and what resistances to expect.
https://www.youtube.com/watch?v=XWts...nnel=shango066
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Old 21st Oct 2021, 5:39 pm   #3
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Default Re: 7 Transistor Kit Radio Output Problem

Quote:
Also, it occurs to me that a 2R secondary can not drive an 8R speaker?
That is the common fallacy of 'speaker impedance matching to an amplifier" an amplifier is but a voltage source for the speaker, the lower the impedance the better.
 
Old 21st Oct 2021, 8:48 pm   #4
Graham G3ZVT
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Default Re: 7 Transistor Kit Radio Output Problem

I don't know quite why, but I've just ordered one of these HX108-2 kits (£6.35).
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Old 22nd Oct 2021, 1:52 am   #5
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Default Re: 7 Transistor Kit Radio Output Problem

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
Also, it occurs to me that a 2R secondary can not drive an 8R speaker?
That is the common fallacy of 'speaker impedance matching to an amplifier" an amplifier is but a voltage source for the speaker, the lower the impedance the better.
The 2R secondary I suspect refers to the resistance of the secondary, rather than the output impedance of the amplifier. As such it will waste a fair amount of the potential power available from the amplifier since it is a significant fraction of the speaker impedance.

The impedance matching involved is that of transforming the speaker impedance to the optimum collector load for the output transistors.
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Old 22nd Oct 2021, 10:38 am   #6
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Default Re: 7 Transistor Kit Radio Output Problem

Google Lens will do the translating for you.
It's available for Android phones and you can save the translation to the phone.
I made two of these last year with no problems, they aren't wonderful and display instability at the low end of the band. They are really 6 transistors with one wired as the detector.
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Old 22nd Oct 2021, 11:12 am   #7
davidgem1406
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Default Re: 7 Transistor Kit Radio Output Problem

Thank you all for your input, that explains it all to me.
Snowman Al, thanks for the video link, that gave the answer about the transformers.
It actually confirms what I had already assumed about the placement of the 2 transformers.
The circuit diagram is in error.

I can now complete the build.

Many thanks to you all,
Regards Dave.
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Old 2nd Nov 2021, 5:41 pm   #8
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Default Re: 7 Transistor Kit Radio Output Problem

Build now completed and all working, not bad for what it is.

Regards Dave.
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Old 25th Dec 2021, 7:33 pm   #9
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Default Re: 7 Transistor Kit Radio Output Problem

After an eight week wait (post #4) the slow boat carrying my radio kit has finally arrived.

The delay was fortunate in a way as it allowed me to acquire this basic vintage signal generator, which itself became a project requiring some work including rewinding one of the coils. For good measure (pun?) I ordered a frequency counter kit which arrived from China in record time.

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First impression was the substantial package weighing in at over 150g.
The cabinet looks basic but is well made and beats many production radios of the 1960s that I have owned.

All the components were present and with no unexpected substitutions.

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I built it on Thursday evening. Any anomalies in the Chinese documentation were addressed in the many Web pages and Youtube videos devoted to this project and I would wholeheartedly recommend this kit to a beginner.

The performance easily matches production pocket radios with similar ferrite and speaker sizes.

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Alignment was a delight. I expected the RF alignment to be a compromise because of the very small amount of movement allowed of the coil former along the ferrite rod, but I felt the coil had hit the sweet spot and adjusting the trimmer at the top end of the band for a peak was all that was necessary.

Calling it a 7 transistor radio is a bit of a stretch, (nothing new there), as one is used as the detector diode (B-E junction)

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Currently listening to Radio Budapest on 540kHz
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Old 25th Dec 2021, 10:28 pm   #10
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Default Re: 7 Transistor Kit Radio Output Problem

I built one of these HX 108-2 kits almost four years ago but did not have a working signal generator at the time and my alignments may not have been spot on. Having now repaired my RF-1U, it would be great to align it properly.

Your digital frequency display meter looks useful. How did you connect the signal generator and the meter to the radio ? Can you please provide some further details. Thanks.
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Old 25th Dec 2021, 11:37 pm   #11
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Default Re: 7 Transistor Kit Radio Output Problem

The frequency counter kit is available on ebay it is actually a poor clone of one published by "Wolf" Buscher (DL4YHF)
If you build one I recommend you include a pre-amp like this guy shows you
https://www.youtube.com/watch?v=0BxpMm6SLoE

I have simply connected it across the output of the sig gen.
Here it is again, this time with my cheap multi-meter which has a frequency range.
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Old 25th Dec 2021, 11:42 pm   #12
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Default Re: 7 Transistor Kit Radio Output Problem

Did you test any of the ceramic capacitors. I tend to do with these circuits as it seems like they are supplying rejects or out-of-tolerance components. What is acceptable tolerance? +/- 20%.

Can you receive Manx Radio on 1368 during the day or after dark? I do not know their erp.
Radio Caroline on 648 Khz is a good test of the sensitivity of a radio during the day. Essex to Yorkshire with 1KW.

I haven't got one of these kit radios at present. I think that it would be easier to tune one if the tuning wheel was a larger diameter.
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Old 26th Dec 2021, 12:20 am   #13
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Default Re: 7 Transistor Kit Radio Output Problem

Manx Radio was just about audible but just came up in QSB to a comfortable listening level as I type this. Confirmed on line, playing "Rambling Rose".

I didn't check the capacitors, only the resistors as some of the colour codes were difficult to discern. They were all reasonably OK.
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Old 26th Dec 2021, 1:51 am   #14
Jolly 7
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Default Re: 7 Transistor Kit Radio Output Problem

Quote:
Originally Posted by rambo1152 View Post
The frequency counter kit is available on ebay it is actually a poor clone of one published by "Wolf" Buscher (DL4YHF)
If you build one I recommend you include a pre-amp like this guy shows you
https://www.youtube.com/watch?v=0BxpMm6SLoE

I have simply connected it across the output of the sig gen.
Here it is again, this time with my cheap multi-meter which has a frequency range.
Thanks for the information.

I too have a multimeter that reads frequency, but it's a bit frustrating because it will not read the frequency from the RF output socket of my Heathkit signal generator. I tried coupling it to my test leads via 0.01 uf caps but the results were still the same. I have verified that the generator itself is working by holding a multiband portable radio close to it after tuning it to a select MW (or FM) frequency and then selecting an appropriate frequency range on the generator to produce a rushing noise or a whine on my radio. The generator's analog dial is not too accurate but it does come in pretty close to the digital readout of my radio.
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Old 26th Dec 2021, 10:37 am   #15
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Default Re: 7 Transistor Kit Radio Output Problem

A lot of multimeters with frequency ranges are only good for working over the audio frequency range and not a lot more. So it not being able to count the frequency from an RF sig gen is not surprising.

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Old 31st Dec 2021, 4:03 am   #16
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Default Re: 7 Transistor Kit Radio Output Problem

A bit OT perhaps but pertinent I hope.....

I gave up using a frequency counter for normal bench alignments - now I use a small portable battery radio (a Tecsun PL600). Punch the required frequency into the radio (no need to worry about 'band', it's general coverage) - switch to USB - tune the sig gen to the whistle - job done! For the LW and MW bands the ferrite antenna is good enough, for HF a short bit of wire in the aerial socket works. Another bonus - using the radio to listen for the local oscillator of the radio under test. This solved a problem in a Heathkit Mohican with a nominal IF of 455kHz - the high frequency band goes from 20 to 32MHz - it's easy to tune the LO coil at the low end but adjusting the trimmer at the high end was a nightmare - it turned out that the setting of the bandspread capacitor was critical. The instruction glibly say "adjust the bandspread capacitor to be fully open at the 'set' mark (the high frequency end) on the scale - there is no mechanical stop on the capacitor so it has to be done by eye. But by eye, because of the shape of the vanes the capacitance has actually gone past the minimum and started to increase again. It slowly dawned on me what the problem was by chasing the LO with the tranny - as the bandspread capacitor moved toward the 'set' mark, the LO frequency started to go back down! Adjusting the capacitor so that it gave the highest frequency at the 'set' mark solved the problem - now the LO HF trimmer cap has the range to tune the 30MHz mark to the sig gen.

I forgot to add that this Mohican was the American version where the bandspread capacitor only varies the LO frequency leaving the RF stages largely out of tune which is probably why there is an antenna tuning cap. The UK version tunes both the LO and the RF stages with a dual capacitor.

Happy New Year to everybody!
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Old 31st Dec 2021, 6:12 am   #17
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Default Re: 7 Transistor Kit Radio Output Problem

It was a very common practice (deceptive?) in the early days of transistor radios to use one transistor as a diode detector. Often the other junction was open.

I have a couple of "6" transistor radios in my stash that only use 5 transistors. The other one is isolated on the PC board. Yeah it "Has 6 transistors" , but only 5 are connected and working on these pocket radios. Deceptive advertising?
Very much in line with the Chinese kit radios that say "5 tube" and are really transistor radios.
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Old 31st Dec 2021, 9:32 pm   #18
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Default Re: 7 Transistor Kit Radio Output Problem

Quote:
Originally Posted by Neil Purling View Post
Radio Caroline on 648 Khz is a good test of the sensitivity of a radio during the day. Essex to Yorkshire with 1KW.
Actually they upped it to 4kW a month ago.
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Old 31st Dec 2021, 11:57 pm   #19
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Default Re: 7 Transistor Kit Radio Output Problem

That's only one "S" point!
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Old 1st Jan 2022, 9:11 pm   #20
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Default Re: 7 Transistor Kit Radio Output Problem

Just about receiving Caroline on 648kHz on the Chinese kit radio here, 3 miles N of Manchester.
The limiting factor appears to be the co-channel station that produces a 2Hz beat.
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