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Old 30th Dec 2016, 10:38 am   #1
Neil Purling
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Default Two or three IF stage Chinese radio kits.

I have made several of the Chinese radio kits:
There may be two or three IF stages after the mixer. Most kits have just the two stages as in the S66-E in the photograph. What advantage is there in having the extra stage?

The S66-E sounds pretty bad even after I fitted a Japanese speaker from a broken pocket radio. To do that I had to hack a hole in the back of the case to accommodate the speaker bell.
I have another radio kit that is a TRF, with a TA7642 and a 3 transistor transformer-less audio section. It does not sound so 'tinny'.

I appreciate with the narrow plastic cases and low-profile speaker these radios will not have much bass. The sound of the S66-E makes me wonder if the audio stage is incorrectly biased, or if I did the wrong thing in peaking the IF stages with a signal generator.

I can post the circuit if you wish to offer advice on improvement of the audio..
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Old 30th Dec 2016, 11:08 am   #2
G8HQP Dave
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Default Re: Two or three IF stage chinese radio kits

Peaking the IF stages will reduce treble, if the bandwidth is too small.
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Old 30th Dec 2016, 1:44 pm   #3
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Default Re: Two or three IF stage chinese radio kits

As my green S66E has an earphone socket I plugged it into an 8" Celestion speaker.
There's still some degree of harshness, I don't know why.

What are the benefits of the extra IF stage in something like a HX108-2?
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Old 30th Dec 2016, 3:12 pm   #4
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Default Re: Two or three IF stage chinese radio kits

Harsh sound could be crossover distortion from badly matched or biassed output devices, or even poor detector design.

An extra IF stage should give better sensitivity and will be cheaper than a tuned RF stage. It won't help any with second channel interference though, unlike better RF selectivity.

What's the transformer for? Totem pole output driver?
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Old 30th Dec 2016, 6:01 pm   #5
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Default Re: Two or three IF stage Chinese radio kits.

Here is the entire circuit diagram. If you think there is anything obviously wrong in the audio stage, please point it out & offer any remedies. The transformer seems to be the driver.
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Old 30th Dec 2016, 6:26 pm   #6
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Default Re: Two or three IF stage Chinese radio kits.

That looks to me like it's got a single IF stage and VT3 is being used as a detector, though at first glance I can't get my head round the biasing.

Given that you've only got 3V of "HT" to play with, the totem-pole single-ended push-pull output stage is never going to be a big performer, once you realise that (assuming silicon transistors) you typically can't get lower than 0.6v per transistor in C-E drop even when biased hard on.

Can you hook a high-impedance earpiece to the wiper of the volume-control and check to see if the distortion's present there? And what's the voltage at the junction of R8 and R9?? It should be 1.5V if the output-pair are sensibly matched.
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Old 30th Dec 2016, 6:46 pm   #7
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Default Re: Two or three IF stage Chinese radio kits.

The voltage at the junction of R8 & R9 is 1.336V measured with a DMM under no-signal condition.
Don't better radios have a skeleton pre-set potentiometer to get the voltage at this point exactly right.
I don't mind a bit of modification to gain an improvement in tone.
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Old 31st Dec 2016, 10:36 am   #8
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Default Re: Two or three IF stage Chinese radio kits.

If the distortion is audio based, what about introducing negative feedback?
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Old 31st Dec 2016, 1:42 pm   #9
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Default Re: Two or three IF stage Chinese radio kits.

I would go with G6Tanuki suggestion, check the detector output is clean, then move onto the audio stages.
If you have a Oscilloscope, it should be easy to see where the distortion is coming from.
Frank
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Old 31st Dec 2016, 4:06 pm   #10
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Default Re: Two or three IF stage Chinese radio kits.

I agree the biasing of VT2 & VT3 looks a tad odd.
In the diagram R4 has an ‘icky’ just above it which to me means ‘read the notes’. The bottom line of text(?) on the diagram also happens to have ‘R4’ half way through. I wonder if R4 should be selected for best performance? I would temporarily substitute R4 with a 47K or 100K pot and try adjusting it for 0.5mA at ‘B’ (VT2 collector), or perhaps just for the most pleasing audio.

Mike
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Old 31st Dec 2016, 5:41 pm   #11
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Default Re: Two or three IF stage Chinese radio kits.

Can anyone read the text and tell me exactly what is important about R4?
I have uncovered a 9018-2 kit elsewhere. The circuit & component values are the same. The only differences between the S66-E, HX6-B and 9018-2 is the printing of the PCB and case styles.
The reference to R4 may say: "When the amplifier gain is low, you can change the resistance R4, the sound will improve".
It does not say anything beyond that but I assume that the next values above 30K will start to cut the collector current. I must have got some particularly high gain transistors and the signal needs attenuating.
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Old 31st Dec 2016, 8:08 pm   #12
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Default Re: Two or three IF stage Chinese radio kits.

I think R4 is 'adjust on test'. VT3 provides a bias negative feedback loop, which also biases VT2. VT2 has its emitter connected to 0V so is well turned on - well, if 0.5mA counts! VT3 has a big resistor in its emitter (the volume control) so has very low quiescent current. It is essentially an infinite impedance detector. So this set has one IF stage, not the two or three of the thread title.

The audio circuit is rather crude so good results are not to be expected.

My guess is that C5 (22nF?) is too high in capacitance, and so causing slew rate limiting. Try 10nF or even 4.7nF.
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Old 31st Dec 2016, 11:11 pm   #13
Neil Purling
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Default Re: Two or three IF stage Chinese radio kits.

I put a 47K resistor on the print side after removing the 30K component.
There is no space for a skeleton potentiometer & I think that really the crudity of the radio does not leave that much room for improvement.
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Old 1st Jan 2017, 2:04 am   #14
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Default Re: Two or three IF stage Chinese radio kits.

I'd check the current on VT4 is in the 2-5mA range suggested- too high and the TX may start to saturate which won't help! R5 will adjust- it's only controlled by the gain of VT4 and the current through R5. For the output stage, I think a pair of AC176s might help, they'll have a bit more headroom for signal. Raising the values of R7, R9 bias resistors to compensate for the lower Vbe will be necessary.

Could be fun to play with, just to squeeze performance it was never designed for out of it!

Possibly the best mod would be to increase the supply voltage and add some low value emitter resistors in the output stage. The feed tor the front end can be kept down by increasing R6.
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Old 1st Jan 2017, 4:36 pm   #15
Neil Purling
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Default Re: Two or three IF stage Chinese radio kits.

Could you use Russian MP41 transistors in the o/p stage? I have a bagful of them I bought for my Selga 404.
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Old 1st Jan 2017, 5:39 pm   #16
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Default Re: Two or three IF stage Chinese radio kits.

Are they P-N-P or N-P-N ?? This circuit needs N-P-N transistors.

In a circuit like this the lower 'fully-biased-on' C-E voltage drop of Germanium transistors might squeeze an extra few milliwatts out of it.
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Old 1st Jan 2017, 10:53 pm   #17
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Default Re: Two or three IF stage Chinese radio kits.

PNP, so no use there.
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Old 2nd Jan 2017, 10:21 am   #18
Neil Purling
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Default Re: Two or three IF stage Chinese radio kits.

I think it would be a good idea to build up a new kit incorporating every possible improvement.
Pre-set pots to get the current through each stage exactly right. Maybe even a different driver transformer & output transistors?
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Old 3rd Jan 2017, 4:10 pm   #19
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Default Re: Two or three IF stage Chinese radio kits.

Hi Niel,

Looks like you are still having fun with AM transistor radios!

Couple of points:

As suggested, put a 100k pot temporarily in place of R4, adjust till the current at VT2 collector is 0.5mA, as marked on the schematics.
Then take out the pot, measure it, and put in a fixed resistor with the value (close to) the measured resistance of the pot.

You did not get the half supply voltage at the point of R8, R9, VT5, VT6, C9, as the beta values of the final transistors (VT5, VT6) are not the same.
Get a handful of generic small signal Si transistors, like 2N3904, 2N222, 2N4401, or the Asian equivalent of a 9013 transistor, check the Hfe with a DMM, and pick two that are closely matched.
http://www.alldatasheet.com/view.jsp?Searchword=9013

The tinny sound is mostly the result of the small size phase splitting transformer, there is not much you can do about that.
Decreasing the value of C7 will add a bit more treble to the output.

The demodulator stage (VT3) is also a rather inefficient design. On the other hand, you do not want to overdrive the audio stage that already has enough amplification for that "bad" demodulator.

As you mentioned, best would be to build a radio with the components of this radio on a breadboard and experiment with various circuit modifications at the different stages.

Good luck, Peter
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Old 3rd Jan 2017, 5:28 pm   #20
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Default Re: Two or three IF stage Chinese radio kits.

I have just realised that R3, R4 etc. provide not just bias but AGC too. Strong signals could result in the AM detector being biased back too far so significant clipping might be seen.
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