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 Post subject: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 3:45 am 
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Location: Orlando, FL
I built this kit back in college (VEC-1290K)but have never been able to get it to work right and was looking for some ideas. It has basically no range at all. I can get some small transistor type radios to play but only with the transmitter antennae draped over it, and the sound has a lot of static and low volume. I've tried moving the jumpers around and adjusting the trimmer caps per the instructions but can't seem to get it to radiate a signal more than a foot. If anyone has any experience with one of these please give me some insight.
Thanks.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 5:08 am 
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If everything is correct, they do work fairly well.

Mine did have a problem with the antenna tuning cap, worked good after replacement. The higher frequencies should transmit better than low.

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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 5:59 am 
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Location: Mountains of Mourne. Ireland.
26-page pdf instruction manual.
http://www.vectronics.com/Downloads/ind ... vectronics

Greg.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 11:43 am 
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Attachment:
Vectronics AM Transmittr Schematic.JPG
Vectronics AM Transmittr Schematic.JPG [ 97.74 KiB | Viewed 2326 times ]


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 1:07 pm 
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Have you checked to make sure that inductors L2 through L9 were all installed in the correct places, and that jumpers JMP1 through JMP5 are correct for your frequency of operation?

I would use a frequency counter and a 'scope to check the stages and to see that the modulation looks good.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Tue 30, 2019 3:37 pm 
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Location: Texas, U.S.A.
1. Since the transmitter's signal can be received at very close range, it indicates that the oscillator is able to oscillate. I endorse Mr. Cook's suggestion to check that all inductors are installed in the correct positions. It is good practice to double check that all components are installed in the correct positions and where polarized, with the correct orientation.

2. With the transmitter power turned on; pin 5 of the LM386, both ends of L1 and the collector of Q1 should measure quite close to 6 volts dc relative to the circuit ground (ground is everything connected to the symbol made up of 3 progressively shorter horizontal bars). You'll need a voltmeter of some sort - a multimeter can be used (most commonly found at auto supply stores or major general merchandise stores for less than $20).

3. Pin 6 of the LM386 should be around 12 volts dc.

4. With the transmitter power off, use the highest resistance range (for example R x 100k) of the multimeter to measure across C8 and across C11. The meter indication across both of these capacitors should be the same as if the meter leads are not connected to anything.

5. With the transmitter power off, use the lowest resistance range of the multimeter (for example R x 1) to measure
from the C8 end of L3 to the C11 end of L6. This reading should be less than 200 ohms.

6. Be sure the antenna wire or conductor is six feet long as specified in the instructions and not close to large metal objects.

7. If the line level output from a hi-fi or the earphone output from a computer, mp3 player or smartphone is to be used as the audio source, remove C2. This reduces the amplification of the LM386. The amplification with C2 in the circuit is only suitable for using a microphone as the input source.

Eric LaGess
WB5HDF


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 12:08 am 
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Location: Orlando, FL
I found my manual and did a component check to be sure everything was where it should be. I found one vagary between C9 and C10. The book calls for 100pF ceramic caps and I somehow have two caps with different markings. I have a note in the book on that line that doesn't make sense now but obviously was a "flag" of some sort when I built it.
Markings are as follows:

C10: 101K 1KV(or KW, hard to tell)
C9 : NPD 101J (this one is physically larger than the other as well)

These look like they are in some sort of a tank circuit with the 100pF adjustable cap.
Other than that all the other components are in their proper places.

infzqi, I'll run through your scenarios with my multi-meter and check back in later.
Thanks.

UPDATE:
1. Since the transmitter's signal can be received at very close range, it indicates that the oscillator is able to oscillate. I endorse Mr. Cook's suggestion to check that all inductors are installed in the correct positions. It is good practice to double check that all components are installed in the correct positions and where polarized, with the correct orientation.See above note on C9/C10, all else is normal.

2. With the transmitter power turned on; pin 5 of the LM386, both ends of L1 and the collector of Q1 should measure quite close to 6 volts dc relative to the circuit ground (ground is everything connected to the symbol made up of 3 progressively shorter horizontal bars). You'll need a voltmeter of some sort - a multimeter can be used (most commonly found at auto supply stores or major general merchandise stores for less than $20). I'm getting .3VDC using the shield of the audio input as ground.

3. Pin 6 of the LM386 should be around 12 volts dc. Getting 15VDC

4. With the transmitter power off, use the highest resistance range (for example R x 100k) of the multimeter to measure across C8 and across C11. The meter indication across both of these capacitors should be the same as if the meter leads are not connected to anything. Infinite resistance

5. With the transmitter power off, use the lowest resistance range of the multimeter (for example R x 1) to measure
from the C8 end of L3 to the C11 end of L6. This reading should be less than 200 ohms. Getting 6 ohms.

6. Be sure the antenna wire or conductor is six feet long as specified in the instructions and not close to large metal objects. Using 5ft supplied piece.

7. If the line level output from a hi-fi or the earphone output from a computer, mp3 player or smartphone is to be used as the audio source, remove C2. This reduces the amplification of the LM386. The amplification with C2 in the circuit is only suitable for using a microphone as the input source.

I'm using a 12VDC 700maH rated supply, tip positive but its reading 16VDC. Is the IC cooked?


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 2:31 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
1. From your description of C9 and C10, they should be correct, 100 pF NP0.

2. The 0.3 V reading on pin 5 and L1 indicates a fault, yes sounds like the LM386 could be blown or a bad solder joint.

3. Edited to correct maximum voltage. The standard LM386 is rated for 15V absolute maximum supply at pin 6. Yes, the power supply at 16V is too much. Is the LM386 hot to the fingertip?

4. Infinite resistance is good across C8 and C11.

5. 6 ohms should be good for resistance from C8 end of L3 to the C11 end of L6

6. 5 foot antenna is probably within acceptable limits, 6 feet would be better but first thing is to get the transmitter working.

WB5HDF


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 3:21 am 
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Location: Orlando, FL
Negative, its not getting hot. I found a 9V supply to use here on out. Solder joints look good. I guess I should order a new chip then?


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 4:05 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
Another possibility is that the transistor Q1 could be shorted. With the power off and one end of L1 disconnected, use the ohm meter to measure between ground and the collector of Q1 (the terminal that goes to L1). The meter should show some thousands of ohms or more. If it is only hundreds of ohms or less, the transistor is likely shorted.

If the transistor looks okay, it is likely that the LM386 is the fault.

WB5HDF


Last edited by infzqi on Jul Wed 31, 2019 6:31 pm, edited 1 time in total.

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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 4:35 am 
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Location: Sanford Fla 32771 (USA)
Quote:
built this kit back in college (VEC-1290K)but have never been able to get it to work right and was looking for some ideas.
Since you never got it to work right, maybe transistor is installed wrong, easy to do. Another thing is the ant has to be tuned with the jumpers and tunning cap as stated in the instruction sheet.

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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 6:20 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
1. Yes it never hurts to check the transistor orientation again. If the sole fault was Q1 installed backwards, (collector and emitter swapped), the dc voltage as measured in item 2, above, would be around half of the supply voltage instead of the 0.3 V measured.

2. Another test that can be informative is to test the switching function of Q1 with the power on. While monitoring the dc voltage at L1, short the base and emitter of Q1. If the voltage at L1 goes to around half of the power supply voltage, it would indicate a fault with U1. If the voltage at L1 stays at 0.3 V, it would indicate a fault with Q1 or U2. Test Q1 with ohm meter for possible short as described previously.

WB5HDF


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 4:48 pm 
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Location: Orlando, FL
Which chip should I order if the remaining tests prove it’s the culprit?
Digikey part#s:

296-43960-5-ND
296-44414-5-ND
296-43959-5-ND


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 6:10 pm 
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Location: Texas, U.S.A.
The Digi-Key part 296-43960-5-ND would be preferred because it is the LM386N-4, rated 22 V absolute maximum supply voltage. That would permit use of the 16 volt supply. U1, the 4049, is usually rated 20 V absolute maximum supply and 18 V operating.

Edit: For the ohmmeter test of Q1 to be valid, one end of L1 needs to be disconnected from the circuit.

WB5HDF


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 8:00 pm 
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Joined: Jul Wed 09, 2008 1:40 am
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Location: Orlando, FL
Would the uprated part/voltage provide a better signal? The interior walls of my house are block so that would be desirable. I noticed the max output ratings are different between the three. How would this effect sound quality?


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Jul Wed 31, 2019 8:38 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
Yes higher supply voltage should increase the radio frequency output and give stronger radiated signal for the same antenna.

The LM386-4 will definitely produce less audio distortion at pin 5 output power levels over 400 milliwatts than the other versions.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Thu 01, 2019 12:22 am 
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Joined: Jul Wed 09, 2008 1:40 am
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Location: Orlando, FL
With L1 disconnected I'm getting 12K Ohms on the collector end of Q1, grounding the same as I was before. The orientation of Q1 on the board matches the footprint that's printed on it. You'd have to try really hard to put it in backwards.
I think I'm going to try a new chip and go from there.
I'll report back in a few days and let everyone know how I make out.
Thanks for all the awesome help from everyone!


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Thu 01, 2019 1:43 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
I like to use IC sockets in home projects that won't be subjected to harsh environments. Digi-Key A400-ND is a good Aries brand socket.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Thu 01, 2019 1:51 am 
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Nice thing about this design is it uses IC sockets. Should be a quick swap once I get the chip.


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 Post subject: Re: Vectronics AM Transmitter, Need assistance
PostPosted: Aug Thu 01, 2019 2:30 pm 
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Location: Utica, NY 13502 (USA)
ff4312 wrote:
The orientation of Q1 on the board matches the footprint that's printed on it. You'd have to try really hard to put it in backwards.
It's strange that transistor Q1 isn't shown in the parts list although, later in the manual, there is a drawing of it with 2N3904 printed on it. Be sure what you have is, indeed, a 2N3904. If a sub was used, the pin-out may be different.

Dave


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