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Old 18th Jan 2019, 7:52 pm   #1
Jolly 7
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Default HX108-2 kit radio- potential improvements/ mods ?

For those of us who have built this superhet radio kit, quite a few have been disappointed, either due to the poor circuit design, substandard components or both. The most common problem seems to be failure of the AF circuit.

Some improvements have been suggested to include additional resistors in the audio driver and output stages. I have marked these in blue. Can anyone please advise the correct values for these (marked in blue) and whether any other modifications would help.

I am keen to use a Germanium diode such as 1N34A, OA80 etc. instead of V4, a 9018H transistor that has been configured to work as a diode. How is it possible to alter the circuit for this purpose ? V4 is circled in red. I have tried a direct Ge diode substitution, but it does not work.

Finally can V5 to V7 be also replaced with Germanium transistors with further modifications ?

Thanks for any useful tips.
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Old 19th Jan 2019, 1:49 am   #2
Argus25
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Default Re: HX108-2 kit radio- potential improvements/ mods ?

There are quite a few things wrong with that radio design.

To try to illustrate these by example I have attached a circuit, of a vintage transistor radio, that has close as possible to your radio's circuit design, in that it has similar AGC arrangements, NPN transistors and transformer coupled audio stages.(it has germanium transistors and a 6V supply though)

In the case that silicon transistors are used there are no neutralization capacitors in the IF's because the silicons have insignificant collector base feedback capacitance and for a simple design there is no audio negative feedback, and the additional AGC diode (which comes into conduction on very strong signals) would not be there either, so just pretend those parts are not there. I marked them with the red stars.

As you can see it is customary to have small emitter resistors for the two output transistors, often these are in the range of 2.2 to 10 ohms. The audio driver transistor should have a base to ground (neg in this case) and base to filtered power supply resistor, as well as an emitter resistor, bypassed by a capacitor to recover the AC gain. This stabilizes the collector current for a wide range of transistor specimens with differing hfe's and over a wide temperature range. The second IF transistor in your radio is not ideally biased either.

Often, the AGC can be applied to either just the first IF transistor, or both.

One thing is, if you look around for transistor radios with silicon transistors, by the time this happened, the transformer coupling was on the way out in the audio, so there are not too many about. Running from a 3V supply, probably the germaniums are better anyway.

Here is a tip: Lets say you come across some transistor radio circuit that uses some polarity transistor, say the common pnp germaniums of early radios and its positive ground. If you want to see what the circuit looks like with npn transistors all you have to do is re-draw it with npn transistors, reverse the battery, reverse all diodes and reverse all electrolytic caps. Also if you go from a pnp germanium to an npn silicon circuit, you will have to lower the base to supply bias resistor values a little compared to the germanium case, and if there is a germanium bias diode in the output stage, it has to be silicon to get the extra bias voltage. Also again as noted, neutralization caps in the IF are not needed either with silicon transistors or later generation germaniums like the AF127 for example. But all radios with early generation transistors like the OC45, need the neutralization components in the IF's.
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Last edited by Argus25; 19th Jan 2019 at 1:57 am. Reason: typo
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Old 19th Jan 2019, 2:48 am   #3
Jolly 7
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Default Re: HX108-2 kit radio- potential improvements/ mods ?

Very informative and interesting. I don't recall seeing a radio circuit with 4 AF transistors before, but maybe this provides a higher audio output ? Or is it for tone ? Sony doesn't mention what the transistors or diodes or thermistor actually are. I guess this was meant to be a trade secret ? Also if I am not mistaken, the supply is actually 4.5V ?
You have provided a lot of ideas to work on and hopefully make a better HX108-2, so thanks once again.
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Old 19th Jan 2019, 6:16 am   #4
Argus25
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Default Re: HX108-2 kit radio- potential improvements/ mods ?

You can ignore the extra AF transistor in the circuit I posted, though the gain it provides is not unhelpful, it was a way of marketing a "7 transistor" radio vs a 6, though of course you can do this if you wire one of the transistors as a detector diode.

When the battery voltage is low at 3V, there is more of an issue as the batteries go flat, using silicon transistors vs germanium, because of the higher B-E voltage drops of the silicon types. I was quite surprised to find that "modern" , by that I mean year 2000 Japanese made dictaphones, using small cassette tapes, that ran from two AA cells, still used 2SB series germanium transistors in the audio stages for this reason as the supply voltage was low. It was the last commercial use of germanium transistors that I saw.

In the circuit I posted, D3 the bias diode for the output stage has a similar voltage drop to the B-E junctions (and same temperature coefficient) as the output transistor B-E junctions, I think it is actually one of the transistors wired as a diode.

If I were making small 6 transistor superhet radios to run from low voltages like 3 volts, I would go for AF127 transistors or similar for the OSC & IF's and a germanium detector diode, maybe an AC125 or AC126 as an audio driver transistor and a pair of AC128's as the output devices and the usual small audio transformers.

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Old 19th Jan 2019, 8:44 am   #5
Argus25
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Default Re: HX108-2 kit radio- potential improvements/ mods ?

I could have mentioned something else, about the audio driver transistor in most transistor radios.

The collector current could be in the range of 1 to 3mA or thereabouts. Since the intrinsic emitter resistance is about 25 divided by that current value, and the hfe of the transistor might typically be 40, and if the transistor has its emitter resistor fully bypassed to audio frequencies with a capacitor, then the input impedance of the driver stage is roughly the hfe x the intrinsic emitter resistance, the input impedance at the base of the driver transistor could be in the range of 1000 Ohms, or lower. This is a tad low to load the diode detector, though the volume control resistance helps, still it would be loaded with that at full volume setting.

In some radios you might see a resistor such as 4.7k to 10k in series with the coupling capacitor to the base of the audio driver transistor. The application of this is to reduce the load on the detector, which ideally in a transistor set, is not a lot lower than about 5k Ohms.

Therefore, one advantage of the extra audio pre-driver stage, because it runs a lower collector current than the stage that drives the driver transformer, is that its input impedance is higher and there is less load on the detector and it increases the gain.
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