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 Post subject: Vectronics 1290K, more trouble than it is worth? Help!
PostPosted: May Fri 07, 2021 12:47 am 
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Joined: May Fri 07, 2021 12:29 am
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Hello all! Much has been learned and absorbed about the VEC1290K kit and since the price point is very reasonable I purchased one while I have a bit of downtime lining up restorations and quoting.
It arrived a few days ago, an easy build but it is not behaving like it should.

Having reviewed much on this forum and some other websites, I have gone through the troubleshooting, made modications, changed chips and caps, even the transistor to no avail. WHat I think is that I am "overthinking" and not looking at something that might be obvious.

Power up with a filtered 12v line I get sporadic oscillation. The Rx Explorer I am using to pin-point the out frequency and power level seems to bounce around and ocassionally locks on a frequency, power jumps up, then it just dies. It might come back. Sometimes not. No consistency.
I am stumped, maybe to those who have had similar issues and to those who have helped out with suggestions, you may have some ideas?

My target freq. is 1680, so the jumpers are set accordingly.

Many thanks!
TM


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 Post subject: Re: Vectronics 1290K, more trouble than it is worth? Help!
PostPosted: May Fri 07, 2021 4:35 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 298
I don't know whether this would help, but check the chapter on Instability at the link below.

https://www.vintage-radio.com/reviews/vec1290k.html


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 Post subject: Re: Vectronics 1290K, more trouble than it is worth? Help!
PostPosted: May Fri 07, 2021 1:25 pm 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 725
Location: Equinunk PA 18417
If those tips don't help,

what do you mean by "sporadic oscillation"? Do you mean the unit's broadcasted signal starts and stops, or its frequency changes randomly, or there is sporadic noise appearing in the received signal?

Do you have a digital multi meter? Do you have an oscilloscope?

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 Post subject: Re: Vectronics 1290K, more trouble than it is worth? Help!
PostPosted: May Fri 07, 2021 1:53 pm 
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Joined: May Fri 07, 2021 12:29 am
Posts: 2
richfair wrote:
If those tips don't help,

what do you mean by "sporadic oscillation"? Do you mean the unit's broadcasted signal starts and stops, or its frequency changes randomly, or there is sporadic noise appearing in the received signal?

Do you have a digital multi meter? Do you have an oscilloscope?


Hi;
When I say sporadic, perhaps a bit vague. The analyzer will show no output between 500-1800 khz ( I set it a bit wider than the broadcast band), then suddenly it starts, sometimes around 1200 khz, sometimes 1500 khz, it varies. Perhaps the operative word here is "transmits"; It might very well be oscillating the whole time but sporadic output.

Yes, I have a meter and scope.
TM


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 Post subject: Re: Vectronics 1290K, more trouble than it is worth? Help!
PostPosted: May Fri 07, 2021 5:24 pm 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 725
Location: Equinunk PA 18417
Glad you have a 'scope. Using your x10 probe, check for a square wave on one end of R3, specifically the end that connects to the nearby IC's pin 5. That is the output of the rf oscillator. There should always be a signal present, which would change in frequency depending on your jumpers and adjustments, but should never disappear. Also look for a similar signal on R4, the end that connects to the nearby IC's pins 9 and 10. That is also the end pointing away from the 2n3904 transistor. It should always have a nearly-identical square wave.

edit: the square wave will be roughly between 1 volt and 11 volts, give or take some. With the x10 probe dividing that by 10, use an appropriate scope channel gain, something like 0.2 or 0.5 volts per division.

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