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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Sun 04, 2019 12:34 am 
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Joined: Jul Wed 09, 2008 1:40 am
Posts: 315
Location: Orlando, FL
So evidently I'm still having issues with this. The new IC is installed and I'm getting about 1.25v on pin 5 of U2. And close to 11v on pin 6. I can't get any signal to a radio. I'm feeding it with a 13v adapter at this point. The IC does get hot under these conditions. I'm wondering if I got the power input polarity mixed up at some point and fried C1/ IC. The transistor does have 2N3904 printed on it.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Mon 05, 2019 3:00 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
Yes if U1 was faulty it could be keeping Q1 turned on which would pull down the pin 5 voltage of U2.

Since the IC's are socketed, I'd turn off the power and gently pull out U1.
Then apply power and check for correct power supply voltage at pin 16 of the U1 socket.
If the voltage is close to the unloaded voltage of the supply, check pin 6 of U2 for the supply voltage. If the voltage is okay, check U2 pin 5, should be about half of the supply voltage.
If these check okay, get about a 5k to 20 k ohm resistor and, while monitoring the voltage at L1, touch the resistor momentarily between U1 socket pins 10 and 16. The voltage at L1 should drop from around 6 or 7 volts to about the 1.25 volts that you saw previously.
If this checks okay, likely U1 is faulty.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Wed 07, 2019 12:49 am 
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Joined: Jul Wed 09, 2008 1:40 am
Posts: 315
Location: Orlando, FL
Remeasuring I'm getting the following with U1 removed:
U1, Pin 16=No voltage
U1, Pin 1=Supply voltage about 12v (?)
U2, Pin 6=Supply voltage about 12v
U2, Pin 5= 1.25v

The IC is not getting hot anymore. I can also hear something like an oscillator working/buzzing faintly with one ear close to it.
I'm leaning towards polarity reversal and I cooked something. Probably should order new e-caps, ICs, transistor and start over.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Wed 07, 2019 1:27 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
I had not checked the data for U1, the 4049, pin 16 is not the supply voltage pin.
Pin 1 is the Vdd pin, which you did check and the 12 V there should be nominal.

Possibly U2, the LM386, is oscillating. With C2 installed the gain of U2 is 200.
With C2 removed, the gain is reduced to 20, where unwanted oscillation of U2 is much less likely.
The gain of 20 is plenty for using audio input from mp3 players or Hi-Fi line output level.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Wed 07, 2019 1:58 am 
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Joined: Jul Wed 09, 2008 1:40 am
Posts: 315
Location: Orlando, FL
So I'll pull one leg up out for C2 to get it out of circuit and see what that gets me. So if I understand correctly I will end up with more voltage out of Pin 5 on U2 and thus more signal into the transistor. I am using an Iphone to drive this, not a hight level source like a speaker output.


Edit: Still only seeing about 1.25v on pin 5 without C2 in circuit.
According to the book Pin 5 should be around 6v. I've got to believe the issue centers around this IC.

Nothing on the radio neither.


Last edited by ff4312 on Aug Wed 07, 2019 3:00 am, edited 1 time in total.

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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Wed 07, 2019 2:43 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
...
Yes that is a good way to disconnect C2, unsolder only one lead.

Yes when using the iPhone, operating with C2 removed is the best way to have the transmitter set up.

Study of the LM386 data sheet reveals the following:

1. Adding a 50 uF electrolytic capacitor from pin 7 to pin 4 of U2, positive to pin 7, is best practice (on the underside of the circuit board).

2. Adding a ceramic capacitor of about 4700 pF to 10,000 pF (.0047 to .01 uF) directly from pin 6 to pin 4 of U2 (on the underside of the circuit board) is best practice. Especially in a transmitter circuit such as this.

I recommend adding these two capacitors.

Edit: Additional notes -
3. With the 12 to 13 V power supply, the standard LM386 can be used, don't have to use the -4 version.
The standard LM386 should be less expensive, could obtain several. They can be useful for other projects. Could get extra 4049's too.

4. If the power supply polarity had been inadvertently reversed at some point as you noted, the only parts that might have been damaged would be the IC's, U1 and U2.

5. Add a ceramic capacitor of 470 pF to 2000 pF between pin 2 and pin 4 of the LM386, on the bottom of the board. This is to greatly reduce unintended radio frequency input to the LM386. (but see note 6 too)

6. There really needs to be low pass audio filtering provided for the input to the LM386. This requires adding a 10 k ohm resistor in series on one side of C1 or the other, it does not matter which. The added capacitor between U2 pin 2 and 4 will need to be 1000 pF, it will act as RF bypass as noted previously and as part of the low pass audio filter.

7. Change C4 to 1000 to 2200 pF ceramic. The present 0.1 uF capacitance of C4 is far in excess of what is required for RF bypass, its intended function.

8. Kits of assorted electrical values of the same type of part, like ceramic capacitors, resistors, electrolytic capacitors, transistors, etc. can be found on eBay and Amazon.

9. It would be informative to measure the ripple voltage of the 13 V supply. Many newer digital multimeters can measure the AC component of the dc voltage by putting the meter range selector on AC volts. Some older analog meters have an Output setting or jack that is used to measure AC on a dc voltage. Ideally, an oscilloscope would be used to look at the supply voltage. The less ripple AC voltage the better, probably less than a couple hundred millivolts will be tolerable.

10. You have already seen that the power supply voltage looks good at U1 pin 1 and at U2 pin 6. I would proceed by putting only U1 back in, putting the jumpers in the JMP 2 and JMP 5 positions, turn on and tune the radio receiver to a frequency around 1500 kHz that does not have a radio station on it with the volume turned up to make good noise, turn on the transmitter, place the receiver antenna within a couple of inches of U1, then slowly adjust C12 from one end of its range to the other. If the U1 oscillator is working correctly, at some point in the adjustment of C12 a drop in noise from the receiver should be apparent.

...


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Thu 08, 2019 11:55 pm 
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Joined: Jul Wed 09, 2008 1:40 am
Posts: 315
Location: Orlando, FL
I ordered a couple new ICs and some transistors today. I want to swap out both U1 and U2 and see if I can get pin 5 up to 6v or there abouts called for in the instructions. I want to rule out fried ICs. My first goal is to get it working in its "as designed" state. Once I get that far I can experiment with the modifications suggested. Question on that is how do I solder under the PC board? I didn't really leave any excess leads when I built the thing so there isn't much to "hold" onto.

I'll try out suggestion #10 when they get here, I just tried it with the current ICs and no dice.

Another question: If I were to "sacrifice" one of the current LM386s and bend pin 5 straight out so it doesn't go into the socket when seated, and then take a reading from it to ground would that tell us anything? My train of thought is if we get 6v then something downstream is causing the power drain, and if it stays the same then the IC is the problem. Or is that voltage determined by the load connected to it?
Let me know what you think,
Thanks,


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Fri 09, 2019 2:33 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
....
Yes - good thinking - inserting U2 with pin 5 out of the socket is okay with the LM386, because the required feedback resistor is internal to the IC.
If the lead is bent out only to the minimum angle required, in the wider part of the lead, it may be able to withstand more than one bending.

To solder to a component pad on the bottom side of the board, first the leads of the component(s) to be installed are cut and bent to be the required length and angles that will position the component as desired. About the last 0.1 inch of the component lead needs to be able to lay flat against the pad that it will be soldered to. Having the PCB held in a vise (gently) or the like helps. I prefer to apply a small drop of liquid flux to the pad to be soldered, and to use a small chisel tip on the iron. Melt a little solder on the tip. Then holding the component lead in about the desired position against the existing solder that is on the pad already, bring the tip in so that it contacts the new component lead and the existing solder on the pad. Should only take about one second of iron application.
Soldering two such bottom side component leads to one pad is trickier, have to hold both on at the same time. One method is to use some sort of adhesive to mount the components first so the leads are already resting on the component pad. A small piece of double sided tape is one possibility.

If it does turn out that there is around 6 volts dc at U2 pin 5 with pin 5 not inserted in the socket, then the ac (alternating current) voltage at pin 5 can be checked too while it is out of the socket, assuming use of a modern digital multimeter set to ac volts. With no audio signal into the transmitter, the ac voltage at pin 5 should be zero. If an audio source (iPhone for example) is then connected to the transmitter input, and the source audio volume level increased from zero to maximum, the pin 5 ac voltage should also go up from zero to around 3.5 volts at maximum. An non-dc blocking ac meter can be used, but a capacitor of at least 1 microfarad would need to be used in series between the meter and pin 5, positive side of capacitor to pin 5.

....


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Tue 13, 2019 3:59 pm 
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Joined: Jul Wed 09, 2008 1:40 am
Posts: 315
Location: Orlando, FL
So with new ICs installed (U1/U2) I’m getting the required voltages but still can’t get signal to a radio. I just picked up a scope so maybe someone can coach me on what to check and look for? I did look at the 13v in and it’s not a clean sine wave. It’s got some jagged spots.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Tue 20, 2019 5:39 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 340
Would you like to build my design? It sounds excellent.
Are you picking up oscillations or carrier wave from CD4049 on the radio? Check out for any faults in the pcb design.


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