Construction Diary -- Part 1, The LED matrix/display

[Klaas Robers]

Andrew,

the square wave has another purpose as well. When you use a probe that attenuates by a factor of 10 (it has a built in resistor of 9 megaohm, with the 1 megaohm this gives a attenuation of 10 x) Should be adjusted for the high frequencies. They have some little screw in the probe itself or in a small box in the connector, that you can turn on.

When you do this while connecting it to the calibration output of the oscilloscope, you will see that the steep edges on the screen change in length. The horizontal part then gets a curved beginning, curved down or up. The correct setting is: no curve at all, so straight horizontal lines.

This is not needed when you use just a piece of coaxial cable or a probe of the type 1x.

[Klaas Robers]

Gary,

I have a simple DOS-based program "Torbase". it gives me short information about lots of transistors. Unhappily for you it is in Dutch, but all numbers and pinnings should be readable still. I will send it to you by e-mail. It goes much faster than googleing.

[gary]

Thanks Klaas, sounds very useful to me and others.

[Andrew Davie]

Today I went out and bought a BD139, snipped out the incorrect transistor and soldered in the replacement. If you'd ask me what was the absolute LAST thing I would expect to happen, it would be that the LEDs would still light up...

The LEDs still light up! I can't see a single thing different in how the circuitry "works" compared to the other transistor. Time to start suspecting my soldering joints, no?

I calibrated my oscilloscope using the .1V/1V square wave today. The gain was way out of whack on both channels. Somebody had a play with the adjustments, I guess. Could have been me... don't remember which knobs I played with when it first arrived. In any case, now that the gain is correctly set, I'm getting "good" readings (ie: they correspond to the multimeter readings) on the oscilloscope for the rectifier circuits.

Now that I've had a very good play looking at the signals from the club CD, I can just about recognise from the oscilloscope trace what image is actually being displayed. Most of the waveforms are very distinctive.

When I was on the hunt for a CD player with a line-out, I actually found two. I found out today that they have different 'polarity' -- that is, the line-out signal is inverted (wires are switched) compared to the other. This is kind of handy for me, but in any case I can always hook up the 'scope to make sure I have things right.

The first thing I learned is that the variable potentiometers were connected 'backwards'. That is, of the three tags on the potentiometer, i was previously unsure which was which -- were the circuit diagrams showing the pot "up" or "down"? The answer is, "down". From the diagram point of view, we're looking at the bottom of the pots. I discovered this by watching the waveform after it went through the pot, and noting that as I turned the contrast pot to the right (clockwise), the voltage range of the signal reduced. That's not what I wanted, so I switched things around so it looked OK.

Then I did some measurements at various places in the circuit.

The really interesting bit for me was checking the line pulse and frame pulse on the sync board. This was kind of tricky to get the oscilloscope to display (because, I think, of the very low frequency of these pulses). In any case, though I was not able to measure the actual rates (I didn't really feel like trying to count very close together dots), I could confirm that there were regular pulses on these two and they seemed to be in roughly the correct ratio.

I had a look at the video signal as it got to the video in of the LED driver board. Looked OK at that point. I noted that as I adjusted the contrast pot, this signal reduced/expanded. I also noted that when the pot was at 0, the line/frame pulse also stopped.

The rectified power on the sync board was 12.06V (sorry Klaas -- I really do know the difference between accuracy and precision and the lunacy of too many significant digits). I'll get used to saying "12V" eventually.

I measured the range of voltage across the brightness pot with my multimeter. It went from 0V to 2.6V. This, too, seems a very correct figure to me.

But my LEDs were still on. I measured 7.2V across the whole array. Adjusting the brightness had no effect on the LED array at all.

I tried with the select connected to ground, and to 12V -- no difference for either of these.

I measured and photographed the waveform across the LEDs. This appears to be a corrupted sine wave, no idea what is going on here.

Finally, I measured the voltage across the C/B/E of the transistor. Did I put it in backwards, perhaps? I put the writing such that it was facing me with the left pin closest to the LED matrix input.

From the "left" to "middle" measured 1.47V with no input.
With input from CD (ie: signal) this lifted to 1.52V
from the "left" to "right" measured 1.4V/1.52
from the "right" to "middle" measured 0V

So, that's it for another night. A few steps forward, some steps backwards. I'm learning a bit every day, but it's clear to me now that these boards and debugging them is not a learner's job. I'll get there in the end

[DrZarkov]

About a cheap, small and very good CD: Take an old CD-rom-drive from a computer! On the backside, where you connect the soundcard, you have line-out. The other problem is power, usually they only need +12 Volt (the yellow wire, black is earth), the IDE or SCSI-cable is of course not necessary for us. If you take a flat one from a notebook, you maybe can built it into your NBTV-monitor.

BTW: Today I've got from the club-shop the same circuit-boards and my Nipkow-disc (good quality, interesting material. I've already cleaned the holes and cut it out). I will go back in this thread, and then we will see when I do arrive at the same problems. I'm living a bit too far away to borrow your osciloscope, but luckily Klaas is a "buurman" (neighbour) to me, as I live only 3 km from the dutch border.

[gary]

Today I went out and bought a BD139, snipped out the incorrect transistor and soldered in the replacement. If you'd ask me what was the absolute LAST thing I would expect to happen, it would be that the LEDs would still light up...

The LEDs still light up! I can't see a single thing different in how the circuitry "works" compared to the other transistor. Time to start suspecting my soldering joints, no?

...

Finally, I measured the voltage across the C/B/E of the transistor. Did I put it in backwards, perhaps? I put the writing such that it was facing me with the left pin closest to the LED matrix input.

From the "left" to "middle" measured 1.47V with no input.
With input from CD (ie: signal) this lifted to 1.52V
from the "left" to "right" measured 1.4V/1.52
from the "right" to "middle" measured 0V

So, that's it for another night. A few steps forward, some steps backwards. I'm learning a bit every day, but it's clear to me now that these boards and debugging them is not a learner's job. I'll get there in the end

Andrew, it *sounds* as if you have it in backwards:

[Andrew Davie]

One expected a certain amount of standardisation. Now I know I need to check these things. How inconvenient -- and why on earth don't they actually mark the pinout on the part itself? Backwards it is. I'll reverse it tonight, and try once again

[Steve Anderson]

Andrew,

Just a suggestion...if you really want to make a start in learning electronics may I suggest a book written by Horowitz & Hill, called 'The Art Of Electronics'. ISBN 0-521-37095-7 (Hard-back version)

It's not a dry tome and is actually quite readable. Although the last edition was published in 1990 (I think) the vast majority of the content is still applicable today, just the sections relating to microprocessors and recent digital developments are well and truely out of date.

It's certainly more than enough to get to grips with the electronics used in NBTV and the like. Even today I often use it as a reference. They've kept the maths to a minimum although some is essential.

I hope it helps, and I hope you can get a copy. I wish it were around when I was studying!

Steve A.

[Steve Anderson]

Andrew,

Here's a link to the book...

http://www.artofelectronics.com/

Steve A.

[Andrew Davie]

Just a suggestion...if you really want to make a start in learning electronics may I suggest a book written by Horowitz & Hill, called 'The Art Of Electronics'. ISBN 0-521-37095-7 (Hard-back version)

OK, Steve, I'll chase that one up. As good a time as any to mention the website www.abebooks.com -- a very handy worldwide interface to second hand booksellers worldwide. I've bought quite a few books through this site, and have been very happy every time. Highly recommended.

I note that I can get a new copy from a seller in the USA for US$31 or so. I might have one sent to my in-laws to bring over next time they visit. The cheapest copy in Australia is an absurd US$141

[Steve Anderson]

Andrew,

Yes, US$141 is absurd, but even at that price it is really well and truly worth it! You'll have no need for any other reference material except for recent developments since the last publication. (pdf files from the component suppliers cover that).

If anyone is wondering, no, I have no connection with Cambridge Unversity Press or the authors.

Steve A.

[Andrew Davie]

Andrew,

Yes, US$141 is absurd, but even at that price it is really well and truly worth it! You'll have no need for any other reference material except for recent developments since the last publication. (pdf files from the component suppliers cover that).

If anyone is wondering, no, I have no connection with Cambridge Unversity Press or the authors.

Steve A.

I've been doing a bit of searching, and note that the softcover version of this book (2nd edition) is available on eBay from India for 1350 rupees, including shipping. That's roughly US$32. The downsides are that the printing is black and white, and the cover is softcover. Neither of those really bother me. Other than that, it's supposed to be word for word the same.

Thought I'd throw up this information for some opinions, before purchasing.

[Andrew Davie]

One expected a certain amount of standardisation. Now I know I need to check these things. How inconvenient -- and why on earth don't they actually mark the pinout on the part itself? Backwards it is. I'll reverse it tonight, and try once again

I switched around the transistor, and the LEDs *still* remain fully lit (though there is a very slight variation when playing with the brightness knob). The waveform across the LED array looks basically the same... I'm really starting to get a bit frustrated with this lack of progress. There must be something obviously wrong but I can't see it. My first real question, though, is that given there's a (rough) sine wave showing at the LED... where the hell is this coming from?!! The power input is a ripple-free DC 12V, rectified at both ends of the circuits (that is, the power to the LEDs is rectified 11.8V ripple-free) and the power to the NBTV circuit boards is a ripply 17V, but internally rectified to 12V on those boards.

I suspect some subtle part substition again. Looks like it's going to be a difficult debugging time ahead

[gary]

One expected a certain amount of standardisation. Now I know I need to check these things. How inconvenient -- and why on earth don't they actually mark the pinout on the part itself? Backwards it is. I'll reverse it tonight, and try once again

I switched around the transistor, and the LEDs *still* remain fully lit (though there is a very slight variation when playing with the brightness knob). The waveform across the LED array looks basically the same... I'm really starting to get a bit frustrated with this lack of progress. There must be something obviously wrong but I can't see it. My first real question, though, is that given there's a (rough) sine wave showing at the LED... where the hell is this coming from?!! The power input is a ripple-free DC 12V, rectified at both ends of the circuits (that is, the power to the LEDs is rectified 11.8V ripple-free) and the power to the NBTV circuit boards is a ripply 17V, but internally rectified to 12V on those boards.

I suspect some subtle part substition again. Looks like it's going to be a difficult debugging time ahead

Andrew,
Can you tell us what the magnitude and frequency of the wave form across the LED array is please.

Can you also measure (this is important) what the signal at the base pin of the transistor is (magnitude and frequency if any).

Is there any input to the LED driver when you see this? If so what is it?.

This could be mains hum being picked up and amplified through grounding errors so that is worth checking.

The other possibility is that one of the op-amps is oscillating so also check on the output of each op-amp.

Welcome to the wonderful world of electronics! At least you will have learnt how much there is to learn at the end of it.

[Andrew Davie]

Can you tell us what the magnitude and frequency of the wave form across the LED array is please.

Can you also measure (this is important) what the signal at the base pin of the transistor is (magnitude and frequency if any).

This could be mains hum being picked up and amplified through grounding errors so that is worth checking.

The other possibility is that one of the op-amps is oscillating so also check on the output of each op-amp.

Welcome to the wonderful world of electronics! At least you will have learnt how much there is to learn at the end of it.

After my last post, I disconnected all wires, examined all joints carefully, noticed a couple that appeared to be less than desirable, and re-did those ones. Then I reconnected everything, and proceeded to do the measurements. Things have changed, and the LEDs are (mostly) off -- though I don't really know that this is a good thing.

I did try NOT connecting the power to the NBTV boards, so the LED matrix was going through the NBTV boards' ground -- and lighting up. But I didn't do the waveform on this as it was before your question. NOw I can't get the LED to light up at all.

Looking at the input signal, this still looks OK at the pot -- it increases as I rotate the knob. It is recognisable NBTV type waveforms.

But what is very very strange is when I look at this signal as it gets to the LED driver board. Where the "video 1.4V in" wire comes from the sync board, I connected the red lead of the oscilloscope. I connected the black to a nearby "ground" pin on the same board. And the video signal has "shifted" by +6V. That is, I see a NBTV signal, but it's way "up" on my screen. I don't know what is happening here.

When I measured same pins with multimeter, I get 6V too, so I know it's nothing to do with the oscilloscope settings. Very strange.

I am wondering why all of the grounds on the circuit diagram are marked as "ground", except for one which is marked "power ground". I have assumed there is no difference.

I've not yet got around to measuring voltage at the transistor, because my LEDs are no longer operational and I have a 6V offset of my video signal.

I'm almost tempted to order a new set of NBTV boards and start completely from scratch. Frustrating.

One new spanner in the works is an oddity I'm seeing on the oscilloscope signal when i have nothing at all connected -- just plain oscilloscope leads unconnected. I see a very small sine wave -- about 0.1V @ 20ms (=50Hz, our power frequency). If I ground the leads across me, the signal jumps to about 10x size. I'm a living amplifier.

If I bring the leads nearish to the powered-up circuits, it also increases in size. It's quite bizarre, I can't figure out where/why this is happening. Sometimes I notice it, sometimes I don't. Sometimes I see a definite sine-wave ripple in the NBTV signal I'm measuring, sometimes not.

One other thing I noticed -- I measured the AC voltage across the 15V tag of the transformer itself. It wasn't a sine wave, but only a semi-reasonable approximation of one. Flat top, sloped (but not curved) sides. More like a kind of triangle wave with the tops cut off.