dead rx circuit

[MichaelR]

Have you made sure that the loudspeaker is connected across 5 and 8 and is not grounded.

Check the voltage powering the amp is at least 3V, it is specified to need at least 3V to work

Mike

[Neil Breward]

I still think there's something odd about that circuit. However, if you get loud signals that must mean that T3 is acting as a leaky-grid (leaky-base?) detector. Not sure that's the most efficient way of doing it, but if it works . . .

If the volume pot is correctly wired, you should still be able to turn the volume down to zero. Check the wiring again. The slider should go via the 2.2k stopper resistor to the TDA7052. One end of the track should go to chassis, the other end via the 220n cap to T3 collector.

If the circuit is how you received it, with the 'reversed' volume control, then I'd suspect other possible drawing errors might be present. For example, that 150p cap across the 56k bias resistor to T1. With a 100n decoupler on the coil side, that means that the 150p cap is effectively connected between T2 collector and chassis 'earth', adding to the load on T2 and by-passing some of the RF signal to ground, especially at higher frequencies. As you need to reduce stray capacitance at the output in an untuned RF amp of this type, this just doesn't make much sense. That's why I was thinking a detector diode has been 'lost' somewhere, and this 150p should be the feed to it.

Cheers,
Neil

[Mike Brett]

Few more observations.
Speaker is accross pin 5 and 8 and not grounded.
Voltage on pins of TDA with sound from speaker and earth from pot not connected.
Pin 1 3.3v
Pin 2 2.5v
Pin 3 0v
Pin 4 1.1v
Pin 5 1.54v
Pin 6 0v
Pin 7 0v
Pin 8 1.66v
Voltage on pins of TDA with earth from pot to chassis hence no sound from speaker.
Pin 1 2.91v
Pin 2 1.33v
Pin 3 0v
Pin 4 1.1v
Pin 5 0.82v
Pin 6 0v
Pin 7 0v
Pin 8 2.2b
I tried the wiring the pot in the configuration you said but there was no change.
Still sound without earthed pot.
No sound with earthed pot.
MIke

[MichaelR]

In your second no volume state the voltage on pin1 has gone below 3V , the device will not work.

Check that you have the correct value resistor connected to pin1 and also that the battery voltage has not dropped significantly.

In your second condition the device could be drawing more current and pulling down the voltage below 3V either due to a too high value resistor at pin1 or the regulation of your battery is poor.

Just asuggestion

Mike

[jimmc101]

There is something very wrong with the voltages around the TDA7052
See data sheet here http://www.datasheetcatalog.org/data...7052_CNV_2.pdf.

The second set of readings show (2.2 -0.82)VDC across the speaker giving a current of 172.5mA no wonder the supply voltage is sagging.

The input of the TDA7052 is differential (pins 2 & 3) and should be at the same DC voltage i.e. 0v.
The input bias current is quoted as 300nA max, so with a 24.2k source resistance (22k volume + 2.2k stopper) the maximum voltage on pin 2 should be no more than 7.26mV.
For there to be 1.33v on this pin something must be pumping current into these resistors.
Either the wiring is wrong or the TDA7052 is faulty.

Try connecting pin 2 to ground via the 2.2k resistor with nothing else connected to this pin.
If pin 2 now measures more than a few millivolts wrt pins 3 & 6 or the difference between pins 5 & 8 is more than 100mV then I'm afraid the TDA7052 is faulty.
(I would disconnect the speaker while doing this to prevent the large current drain if the fault persists.)

Jim

[Skywave]

Can I ask an obvious Q., please?
The TDA7052: was in new / unused when you fitted it or was it a reclaim / salvaged item?

Al. / Skywave.

[kalee20]

Originally Posted by MichaelR

Detection looks as though it occurs in T3.. surely

Well, with 150pF from Q2 collector bypassing RF on Q2's collector to the 100nF capacitor and thence to 0V, I would have thought the design aim was to have AF available by this point. But I could be wrong!

As others have said, it looks as though a diode should exist at this point somehow, maybe reflexing detected AF through T1 and T2.

[jimmc101]

OK, it's raining and I'm bored so...

470R & 150pF break at 2.26MHz so no problem for MW reception.

Running a simulator on T3 shows it will demodulate AM. see figs.
(I offset the AM generator to clear the axis on the display)

I'm a bit worried about the 47n caps, I wonder if the second one should be after the 2.2k.
However 6.8k & 47n break at 500Hz. Even 2.2k & 47n give 1.5kHz.
Sounds like some experimentation may be in order once the RX is working properly.

On a practical note, is the 100n between pins 1 & 3 of the TDA7052 close to the device, I would be worried if the path between the pins (via the cap) was more than about an inch.

Jim

[kalee20]

Originally Posted by jimmc101

470R & 150pF break at 2.26MHz so no problem for MW reception

I agree - so there could be plenty of RF at T2 collector, which T3 will demodulate. I should've done the sums!

But then, what is the purpose of the 150pF capacitor?

[MichaelR]

With the tuning coil and resistor Bias for T1 ?

[jimmc101]

I've found the original here http://users.belgacom.net/hamradio/homebrew.htm

Looking at the method of construction I suspect that the 150pF may be required to stop it 'taking off' at VHF.
Why it isn't direct from collector to ground I don't know.
The article does say that the performance drops off above 3MHz.

The reactance of the 100n capacitor (from the bottom end of the coils to ground) is 1.6 ohm at 1MHz so I don't think the 150pF (across the 56K) provides any significant feedback.

I now wonder if the unconventional connection of the 22K volume control is a way of getting an approximation to a logarithmic response from a linear pot. I think it may also act as an unintentional tone control.

Jim

[Mike Brett]

In an answer to your question about theTDA, yes it was new , purchased from Sycom last week, but I will do the tests to check it out. The 100n cap from pin 1 goes to the groundplan at present.
But if you think it should be more direct I will solder it direct to pin 3. I have plenty to work on for now but it will be Saterday before I can give it another go so will keep you informed then.
With all your efforts I am sure I will get this beasty going.
Mike

[jimmc101]

I didn't realise you had a ground plane, as long as the track from pin 1 is short then that will be fine.
Are you using a double sided PCB or 'Dead Bug' construction?

From my previous post ...
I now wonder if the unconventional connection of the 22K volume control is a way of getting an approximation to a logarithmic response from a linear pot. I think it may also act as an unintentional tone control.

Having a bit a a senior moment there.
The way it is connected gives a large change per degree at low volumes and a small change a high volumes, the opposite to a log pot.

Jim

[MichaelR]

Originally Posted by jimmc101

Why it isn't direct from collector to ground I don't know.
The article does say that the performance drops off above 3MHz.

The reactance of the 100n capacitor (from the bottom end of the coils to ground) is 1.6 ohm at 1MHz so I don't think the 150pF (across the 56K) provides any significant feedback.

Jim

Jim , if you consider T1 and T2 as a single unit the feedback is supplying DC bias back to T1. It is the simple basic negative feedback connection (as T3 has) to give stability to the operating point of T1 and hence T2.

As for the capacitor the combination with the resistor does provide a low pass filter maybe there are some low level harmonics being generated in the T1 and T2 combination.

Mike

[jimmc101]

Sorry Mike (R) I think we are talking at cross purposes.

I agree that the 56K stabilises the bias for T1 &T2, however with the 100n being a virtual short circuit I cannot see the advantage of connecting the 150pF to the 100n rather than directly to ground.
Any AC feedback via the 150pF would be attenuated in the ratio 150 to 100,000 (or 100,150 for the pedants ).
Since the voltage gain of T1/T2 is approximately 30, any AC feedback is insignificant.

I wonder if with the original 'Birds Nest' construction this was done for mechanical convenience.

Jim

[Mike Brett]

The circuit is built on PCB material dead bug style so that I can make full use of the groundplan. I have been using this method for a long time now and try to keep the leads short and as tidy as possible. I always mount any I.C.s in holders that are soldered to stripboard then cemented to the pcb. Making sure of course that the pins do not foul the pcb. Saterday I will get stuck in and try again.
Mike

[Mike Brett]

I connected pin 2 to ground via 2K2 resistor and voltage at pin 2 was .43 volts.
Pins 5 and 8 where almost the same at 1.921v and 1.917. I used a current supply with a limiter on it for these tests but at no time has the current limiter operated.
Do you still think the TDA is faulty only it does kick out a good volume. If only I could turn it down. Apart from these problems I am quite impressed with the reception considering its such a simple circuit. It seems to by resonable selective with just the crude arial I am using at present.
Mike

[Mike Brett]

There is a voltage of 2.54v between the earth pin on the pot and the chassis, is this correct. The voltage does not alter if I turn the volume control.
Mike

[Sideband]

....so it would appear that the voltage is coming down the volume control from the chip. I don't think there should be any DC at the input of the chip as this would cause the volume control to be very crackly. Not sure if this would work but try connecting (say) a 10uF electrolytic between the top of the volume control and the chip (positive end to pin 2). This will at least block any DC from getting back into the pot but I don't know if the chip will work that way.


Rich.

[jimmc101]

430mV across 2.2k is just under 200uA, the spec for the TDA7052 is 300nA max so I'm afraid it's dead.

The TDA7052 should not work at all with 2.54V on pin 2.
The input stage is almost certainly a PNP long tail pair and with 2.5V difference between the inputs (pins 2 & 3) it should be driven hard into saturation.
It sounds very much as if the internal device connected to pin 2 has suffered a base emitter short allowing significant current to flow out of pin 2.

If you want to soldier on with the device you've got, try Richard's idea with the 10uF cap, it's a bodge but without buying a new chip you've got nothing to loose.
(If you do get a new chip, it must have a DC path from pin 2 to ground so remove the cap.)

I still think the volume adjustment would be less course with the top end and wiper of the pot swapped.
(Of course cap is then from wiper to pin 2.)

Jim