Sync Separator

[Andrew Davie]

Well, my televisor and I had a bit of a fight, and I got quite angry and we split up for a while.
But, after counselling, we've decided to give it another go and we're back together. I've agreed to be more tolerant, and she's agreed to stop smoking.

I ripped out the original power system because it was just loopy. I was feeding 22VDC into everything, with the 12V regulated going into the sync separator but the raw 22VDC going straight to the LED driver (NOT to just to the light box). That didn't seem right. In any case, the original was fed by a single AC in which I know now doesn't work - to rectify two voltages, must use two transformers. So, out came the original power stuff. Out with the old, in with the new.

With the new setup, I'm feeding either 9VAC or 12VAC into the rectifier and then the consequent voltage into the regulator. With 9VAC I get a post-regulator voltage of 11.2V which isn't terrible, but not quite 12V. With 12VAC I get a post-rectifier of about 17V and post-regulator voltage of 11.8V which is closer but I know there's ample voltage and the regulator must be trimming to about that. I have noticed NO heat from the regulator with the lower voltages - the heatsink is obviously more than sufficient. With that sorted/understood, I'm moving onto the sync separator diagnosis.

(added note: the sync separator and LED driver circuits are running just fine with the 9VAC inputs (11.2VDC).

I 'scoped the signal direct from CD player first and that looked OK. Then I scoped after the brightness potentiometer, and saw good-looking increase in height (voltage) of the signal as I adjusted it. Cuts out completely just before the dial is at "0" and increases nicely.

Next I set the sync circuit adjustment pot to zero, and scope leads on the output of that circuit. I adjusted the pot to see/understand the valid range of that, and found the setting where the output on the sync stage was consistent for all inputs and all brightnesses - except, of course, for 0 brightness. The resultant output is many +12V spikes, and the occasional wider -12V spike. I hope that's what I should see. I will go back through my original build soon and have a look at the notes there.

sync.jpg (203.75 KiB) Viewed 8280 times

Here's my interpretation - the image shows a grab with one of the (infrequent) larger -12V pulses, and two (frequent) +12V pulses. Obviously the pulse heights are 12V. The larger -12V pulse is about 1.2 ms long and I need to understand what that is. The smaller +12V pulses are spaced at 2.5 ms apart and about 60 us duration. Let's see - 2.5 ms is 400 Hz and divide by 32 to get a frame count gives 12.5Hz - right on. Will do some more analysis of that larger negative pulse after some reading to understand what it is.

So we're getting along a bit better. She's stopped smoking, and I'm not so angry.
My little DSO Shell oscilloscope is just fantastic for this work.

[Andrew Davie]

Here's a look at the regularity of the pulses.

sync2.jpg (198.54 KiB) Viewed 8279 times

OK, so that -12V pulse looks to be every 80 ms.
Um. That's 12.5 Hz. Looks like a frame pulse.
No idea how it's determining that from the signal but ... hmmm.

Now I'm confused, because in this trace it looks like the period of the positive pulses is 10 ms - and that's different.
I would have to infer a sampling issue here, and that we are missing most of the positive pulses in this view.
More testing.... and yes, that's what it is - the positive pulses are still at 2.5 ms frequency so the apparent 10 in the image here is deceptive!

So, we appear to have reasonably good line sync pulses and even a frame sync pulse.

[Andrew Davie]

See NBTV sync separator and video pre-amplifier.

sync3.jpg (47.73 KiB) Viewed 8277 times

So, line&clamp pulse should be 50 us. I measured about 60 us.
And the frame pulse 1.2 ms - bang on.
Looking good!

Probably the next thing to do is have a look at the signal-out which has presumably been stripped of the sync pulse... and which, by the above diagram, should be about 1.4V peak to peak.

[Andrew Davie]

Well, here's the output with the sync signal stripped off...

sync4.jpg (223.35 KiB) Viewed 8276 times

Obviously not 1.4V pp here in this example, but it's going to depend on the signal, of course. I'm reasonably confident this is OK.

When I found the video output point in the tangle of wiring on the back of the board, I realised it was disconnected from the next stage - so that's why my LEDs weren't pulsing properly! Very old board - I guess the soldering has deteriorated over the years with mechanical stress and poor storage. Good to find another fault.

A note on the 'scope behaviour here - I was confused because when I went down below 0.5V on the V/DIV, the trace flatlined. Only when I switched from DC to AC did the signal "come back" at those lower voltage divisions. Somebody's going to explain that to me but I'm guessing something to do with a floating voltage/reference point...

[Andrew Davie]

Upon resoldering the afore-mentioned "loose wire" which was the output from the sync separator into the input of the LED driver, my lights started pulsing again! So, the LED driver looks like it doesn't need any repair/review - and the LEDs are now being driven with their own 22V DC supply, so that means that the two supplies and the wiring for all of that now appear to be correctly setup. WITH a common ground Yay!

I might come back and revisit the current-limiting on the LED array - although it is blindingly bright I would like to just make sure I understand how it's done, what the current value(s) are, and how bright the LEDs are compared to their capability.

Funny story - turned it on and the LEDs didn't come on - so I thought "hmmm, maybe a loose wire" so picked up and looked at the array from the front. Turns out the CD player had just finished the test track and was seeking back to the start. I happened to turn the televisor on just after the track finished, and then look at the LEDs JUST as the CD player started playing the track again. Still have after-images a minute later. Don't do that again!

Right, so the LEDs are being driven correctly - with a correct contrast setting (that is, when nothing is playing I set the contrast so that the LEDs are just all off) then, with a track playing the brightness dial to 0 has all LEDs off but a slight turn and they light up and start pulsing (dimly). Full turn and they're dazzling, and still pulsing. Fantastic.

Time to move over to motor control.

[Klaas Robers]

Andrew, it is not correct to set the brightness to just "on" of the LED cluster. In most circuits the bottom of the sync is kept constant, not the black level (which is not defined in NBTV). you need an NBTV video- black signal to set the brightness. Then the bottoms of the sync are (far) below black.

What do you have as input signals? I know that on NBTV CD number 1 there is a track with video black. That was put onto that CD especially with this purpose.

[Andrew Davie]

Andrew, it is not correct to set the brightness to just "on" of the LED cluster. In most circuits the bottom of the sync is kept constant, not the black level (which is not defined in NBTV). you need an NBTV video- black signal to set the brightness. Then the bottoms of the sync are (far) below black.

What do you have as input signals? I know that on NBTV CD number 1 there is a track with video black. That was put onto that CD especially with this purpose.

OK, noted - thanks. The issue for me is I want the LED array OFF when there's no signal. I'll get to that later. I am just using a couple of old CDs with stuff I was playing around with in 2007. So, I will dig up the test cards and stuff - I must have that CD somewhere - and set the black per your advice.

Meanwhile, here's a clip I just did with manual speed control (pinching the disk) - the picture has come up pretty well.


youtu.be/BKPcWkIna9k

This is from my smartphone and it's a bit wobbly and blurry. Of course the large black band is just an artefact of the camera looking at a flickery object.

One thing - with the 9VAC input, I was getting wide vertical black banding through the picture. I had no idea what was causing that, but intuitively tried upping the voltage first - now 12VAC input - and the bands have disappeared. I'm really pleased with this disc. It was made by Peter Yanczer and when I first got it I was a bit disappointed and said so. Well, I clearly didn't know what I was talking about and I regret that.

[Klaas Robers]

Andrew, the banding was caused by the fact that the voltage stabilizers got a too low input voltage to function properly. The input voltage should stay at least 3 volts above the wanted output voltage. So if you want a DC voltage of 12 volt, the unstabilized input voltage should stay above 15 volts. There it may ripple as much as it wants, but not below 15 volts. With your oscilloscope you can check that easilly. You can check the stabilized output voltage. That should be a smooth horizontal line, with no rippling at all in it. In the input voltage there should be less than 1 volt ripple. If it is more, the value of the capacitor is too low. My rule of thumb was always 4700 uF per ampère output current. So for 100 mA it should be 470 uF or more.

Good luck.!