NBTV-oscilloscope circuit problems

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NBTV-oscilloscope circuit problems

Postby aussie_bloke » Fri Jul 16, 2021 1:03 pm

G'day all.

Back in 2012 I built a NBTV-oscilloscope circuit using the schematic from John Lawrence GW3JGA's site http://www.gw3jga.net/jl-notebook-082 but couldn't get it to work properly, was briefly getting a distorted silhouette picture at best at the time, but then it stopped displaying anything altogether and I got a small shaking smeary rough looking raster. I've revived the project recently and have been trying to work out what's wrong with it. I first traced over my wiring, found one mistake and fixed it which made no difference. I tried swapping chips to see if any were faulty, none are.

Then I did signal tracing and I found the NBTV signal turns to garbage after the input transistor as at the emitter output I get what looks like a carrier wave, and it goes downhill from there as most of the signals throughout the circuitry are garbage all the way except for video output Z which looks pretty much like the input signal. The weird thing is occasionally I was getting a rough field/line sawtooths and sync pulses but then it turns to garbage again. I was told to try connecting pins 1, 8, 15 to ground instead of -5V, that did nothing.

I am perplexed to what might be happening with my circuit as I've copied it to the T except I used a PN2907 transistor instead of a BC212 because I didn't have one and for the power supply I made a standard dual +-5V power supply using 7805 and 7905 regulators, bridge and filter caps and hooked to a centre tapped 10-0-10V transformer instead of the DC-DC converter because I didn't have one. I am wondering if my choice of power supply might be inducing noise into the circuit hence the carrier wave looking signal at the transistor emitter???

Anyhow attached are photos of my circuit, transformer and signals displayed on the scope. Here is a link to my latest video log of this project https://www.youtube.com/watch?v=RSJfvLQQqvU .

Any help on getting this circuit operational would be much appreciated.
Attachments
nbtvscope.jpg
signals.jpg
circuit.jpg
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Re: NBTV-oscilloscope circuit problems

Postby Steve Anderson » Fri Jul 16, 2021 3:56 pm

First, the good news...ICs in sockets...that helps a lot...so do the 'scope screen-shots...

It seems that something is unstable (oscillating)...so here I go again...I don't see any supply bypass capacitors anywhere, not one...unless you've hidden them on the underside of the board.

Stock up on a few hundred 100nF/50V disc-ceramic capacitors,(e-bay etc..) and throw them around circuits like confetti...say one for every two chips...the circuit shows only two, C7 and C8. The regulators should have one on the input and output in addition (a total of four for the regulators alone) right on the pins. It's also not a bad idea to add an electrolytic cap on the +5V and -5V outputs, say 47uF/10V. Just one each is enough per board, observe polarity. Photo shows a 1000V version of the disc-ceramic caps, but the 50V versions will look much the same.
100nF 50V.jpg
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Next...BIG FAT Earth/Ground/0V lines...within reason. A more compact layout may also help with future builds.

My apologies for repeating myself again...

Steve A.
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Re: NBTV-oscilloscope circuit problems

Postby Harry Dalek » Fri Jul 16, 2021 4:14 pm

Steve Anderson wrote:First, the good news...ICs in sockets...that helps a lot...so do the 'scope screen-shots...

It seems that something is unstable (oscillating)...so here I go again...I don't see any supply bypass capacitors anywhere, not one...unless you've hidden them on the underside of the board.

Stock up on a few hundred 100nF/50V disc-ceramic capacitors,(e-bay etc..) and throw them around circuits like confetti...say one for every two chips...the circuit shows only two, C7 and C8. The regulators should have one on the input and output in addition (a total of four) right on the pins. It's also not a bad idea to add an electrolytic cap on the +5V and -5V outputs, say 47uF/10V. Just one each is enough per board, observe polarity. Photo shows 1000V versions, but the 50V versions will look much the same.
100nF 50V.jpg


Next...BIG FAT Earth/Ground/0V lines...within reason. A more compact layout may also help with future builds.

My apologies for repeating myself again...

Steve A.


Decoupling caps always help yep but if you look at the circuit Steve the mono's are not grounded ? where most or every one i have ever seen are these are on the negative supply rail do you think its a mistake the negative 5 volts on the mono's and should be on ground pin 8 at least and at the timing cap's !
I think the dual 5 volt supply was meant more of the opamp's and the mono's should be run from ground and positive 5 volt rail ?
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Re: NBTV-oscilloscope circuit problems

Postby Steve Anderson » Fri Jul 16, 2021 4:37 pm

Harry Dalek wrote:Decoupling caps always help yep but if you look at the circuit Steve the mono's are not grounded ? where most or every one i have ever seen are these are on the negative supply rail do you think its a mistake the negative 5 volts on the mono's and should be on ground pin 8 at least and at the timing cap's !
I think the dual 5 volt supply was meant more of the opamp's and the mono's should be run from ground and positive 5 volt rail ?

It might not look it, but it's perfectly legit, if not particularly elegant. In your head re-label the -5V to 0V, the 0V to +5V, and the +5V to +10V. or print it and write down the updated voltages. Then it should become clear. This is only possible with the 4000 CMOS series with these voltages.

The whole circuit could be sat at a few kV, positive or negative, and it wouldn't care. Much the same as the CRT grid-drive circuits you've built in the past, the transistors are only rated for (maybe) 160V, but the supply to them is several hundred or a kV or more volts negative.

Steve A.

I don't see why it uses split supply lines (even though I prefer them), a single +12V would have done the job, the output ramps would be centred around +6V, but that can be dealt with easily downstream.

I note it's originally called an 'Oscilloscope Converter', keep in mind if the scope only has AC-coupled inputs ('hobbyist' and older tube/valve scopes in particular) you'll end up with very non-linear sweeps, especially in the frame waveform.. Plus it'll need a Z-input, usually on the rear - when fitted (not often) to modulate the CRT grid/cathode circuitry. That too is invariably AC-coupled leading to smearing of the video and other artifacts...

Looking at it further, TR1 and its associated components could be dumped with a few minor changes...

Just noticed another hiccup in the circuit...pin 4 of IC2a should be connected to -5V not 0V, though I doubt it'll get rid of the oscillation...but you never know...
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Re: NBTV-oscilloscope circuit problems

Postby Steve Anderson » Sat Jul 17, 2021 1:40 pm

If I may suggest this...follows is my version of NBTVA sync-detection/extraction...it appeared in Vol 35 No. 4 (July 2010) of the newsletter...it doesn't generate the ramps but it works with line rates from below 300Hz to over 500Hz. It could be simplified if really needed. The ramp generators would be a simple downstream add-on. There are no monostables to 'tweak and adjust' to get the timing correct.

No need to contact Jeremy for the pre-programmed micro, we can do that via this forum...

Right-click filenames or image, "Save As", "Save Image As', etc, for better quality...

Steve A.
Jul 2010_04.gif

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Jul 2010_06.gif


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