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Help needed with first Televisor

PostPosted: Mon Jan 28, 2019 6:45 am
by sebket
Hello everybody,

as this is my first post in this forum, let me introduce myself briefly.
My name is Sebastian and I live in southwestern Germany. At work I am an IT administrator, but in the hobby shack I spend my time with audio, video and telecommunications, preferably with technology from yesterday :D

I am currently building my first mechanical NBTV Televisior and have made good progress so far (I will create a seperate post with the building process, including some pictures, as soon as the device is finished).
I am just left with the following two problems:

1.) It is very hard to get a stable frame sync. The PLL tries its best to get the image stable. The motor accelerates, but then overshoots (the image as it should be is visible for a brief moment). Then the PLL tries to lower the speed, but overshoots again and the speed gets too low. Then it will accelerate again, and overshoot again. With very subtle adjustments of the potentiometer that goes to the motor control PCB and with breaking the motor a bit with the hand, it is possible to get a stable image. But...
2.) there seems to be a problem with line sync as well. The last 4 to 5 (aproximately) lines that should be on the left hand side of the image appear on the right side! This means that the first line that should be on the very right is somewhere in the middle of the image, where the 5th line should be. The whole image is shifted to the left, one could also say.

Now I don't really know what could be the cause of that strange behaviour. Maybe I should mention that I am building a Televisior that's a bit larger than the one described in the handbook, using the club's 15 inch stainless steel disc. My motor is an industrial grade pancake (also called disc rotor) DC motor. I figured it should be a perfect match for a Televisor as it has a very wide voltage range (starts turning already at 3 Volts) and has no cogging like other DC motors do. When the motor has the right speed (means image is in sync), it draws around 300 mA.
I have built the motor control circuit with the values given in the handbook. The only thing I've changed is I omited the 10 Ohm resistor in the positive lead to the motor. With the resistor, the motor didn't get up to the speed required and the resistor got quite hot, despite it being one with 12 Watts of dissipation.

So, if anybody could give me a hint on where to start looking for the cause of my two problems, that'd be great! :)

Re: Help needed with first Televisor

PostPosted: Tue Jan 29, 2019 10:01 pm
by Andrew Davie
Welcome!

To answer #2, this is normal. There are two types of "lock' - line lock, and frame lock.

The first, line-lock, is where your disc is essentially spinning at exactly the correct speed for the signal, and you do indeed see each line "stable", side by side, and forming an image. But the synchronisation of "line 1" (i.e., the first hole) with the start of the frame is not there. All that getting the disc to the right speed does is make sure that each line is displayed correctly, and the next line starts at exactly the right time according to the signal.

But what you don't have there is any information tying the first line to the signal. In other words, you have 32 possible positions of "line lock" - your picture can be split in half incorrectly 31 of those 32 possible configurations - based on chance. If you're lucky, then you get the first line starting when the frame starts, and that's when you get "frame lock".

If you actually have "line lock" operational, then you can get "frame lock" by slowing down the disc with your fingers pinching the edge, and it will "roll" through the various line lock positions (those potential 31 bad positions). If you are careful, you can "shift" the split frame so that it eventually locks on the correct line at the start of each frame, thus giving you "frame lock".

Frame lock is kind of a "holy grail" of mechanical TV, and has been somewhat difficult to achieve. The timing holes on the nipkow disc are 31 in number (usually), and the missing hole is designed to give a signal for frame locking. That's one way to do it. I did it a different way, having just a single hole for frame lock, and none for line lock - and used a microprocessor to get the timing right.

Good luck, and I hope you're having lots of fun!

Re: Help needed with first Televisor

PostPosted: Tue Jan 29, 2019 10:03 pm
by Andrew Davie
I'll also add; if you are always getting a stable position with 5 lines off to the side, then solving your problem is as simple as moving the "masked out" timing hole around by 5 positions!

Re: Help needed with first Televisor

PostPosted: Tue Jan 29, 2019 11:46 pm
by sebket
Hi Andrew,

thanks a lot for your reply!

Line lock is no problem if we let aside the issue I am having with the motor always revving up and down around the correct speed until I tame it with working the control pot on the motor PCB and slowing down the disc a little manually.
This will also make your suggested method of slowing down the disc by pinching it between the fingers to get correct frame lock virtually impossible, as this would cause the system again to lose line lock and have me fiddle with the machine for minutes to get line lock again.

I will check however if the error is consistent, meaning frame lock is always off the same amount of lines. Then I will follow your advice and move the masked out hole around 5 positions.


Does anybody have any advice regarding #1 in my initial post?
I will check the quality of the sync signals reaching the PLL from the sync seperator and from the opto sensor with the scope, perhaps the problem lies there, even if I don't think so because as soon as line lock is achieved, it is rock stable.
I think it might be an issue with some of the values on the motor PCB, as they are specified for a small DC motor like they're used in cassette tape recorders. As I've mentioned before, my motor is a bit bigger and probably draws quite some current while accelerating. Which components would make sense to start tweaking?

Re: Help needed with first Televisor

PostPosted: Thu Jan 31, 2019 4:27 am
by Klaas Robers
Sebastian, you did not tell which circuit you use for synchronisation of the disc. I assume the circuit in the NBTV handbook.

speedctrlCKT3.gif
speedctrlCKT3.gif (6.07 KiB) Viewed 2881 times

Important is the resistors R3 and R4. The ratio defines in how far the 4046 influences the motorspeed. This is now 1 : 1. But it might be that it should be less. So try to change this part to this circuit:

speedctrlCKT.gif
speedctrlCKT.gif (5.41 KiB) Viewed 2881 times

With the potentiometer of 220k you may change the ratio. You may even mahe the influence of the 4046 zero if you remove the jumper "Close Loop". In that situation the disc should run the correct speed. So first see that the disc gives a slowly upwards or downwards moving picture using the speed control potentiometer. Then close the loop and adjust the damping.

Good luck ! You will need it.

In the end you might decide to replace the jumper with a pushbutton that interrupts when you push it. Then you can adjust the speed with your other hand until the "correct" picture is in the viewing window. Then you release the button and your picture is framed and lined.

Re: Help needed with first Televisor

PostPosted: Thu Jan 31, 2019 5:45 pm
by sebket
Hello Klaas,

thanks a lot for your reply and the good advices.

Yes, I am using the sync circuit from the handbook.

As expected, the sync signals are not the problem. They look all right on the oscilloscope.

I am running the whole machine from an adjustable bench power supply at 12 Volts.
I've noticed that it is easy to get the disc syncing when I reduce the voltage to around 10 Volts.
As soon as it synced, I slowly raise the voltage again to 12 Volts and everything is fine.

I've mentioned before that I had to omit the 10 Ohm resistor in the positive lead of the motor because the voltage drop was too high: the motor didn't get up to the speed required and the resistor got quite hot.
So I figured I might try to insert a resistor of a lower value, say, 6 Ohms, and see if that works. The thought behind that is that I've observed that a lower supply voltage got the thing syncing, so feeding the motor less Volts by inserting a resistor of the correct value might do the trick,

However, I think your method is by far more elegant and promising.
So as soon as I'll find some time to disappear in the workshop I will modify the circuit according to your schematics.
For the "close loop" jumper I guess I'll use a toggle switch.

Thanks again, I will report as soon as I have news.

Perfection has been achieved!

PostPosted: Sun Feb 03, 2019 6:15 am
by sebket
Klaas, your advice to make the damping factor adjustable by replacing the two 100k resistor with a 220k pot did the trick!
I didn't even have to put the jumper/switch to interrupt the loop. Just by manipulating the speed pot and the damping pot, it is possible to get the Televisor syncing within moments.
I even think your modification to the circuit in the handbook is so useful that it should be mentioned there in a later edition.

Attached you will find a few photographs of the Televisor's display: perfect frame and line sync!

Thanks again everyone for the help! The next step now is building a nice case for the thing. Woodworking is not my speciality to say the least, but sooner or later I might come up with something I can dare to show you :lol:

IMG_20190202_192224.jpg
IMG_20190202_192224.jpg (801.98 KiB) Viewed 2823 times


IMG_20190202_191425.jpg

Re: Help needed with first Televisor

PostPosted: Sun Feb 03, 2019 9:40 am
by Andrew Davie
That's awesome quality - well done!

Re: Help needed with first Televisor

PostPosted: Mon Feb 04, 2019 10:41 pm
by sebket
Thank you!
:)

Help needed with first Televisor

PostPosted: Wed Feb 06, 2019 10:25 pm
by acl
Well done Sebastian.

Attached some ideas for producing a case.

Regards Chris Lewis

Re: Help needed with first Televisor

PostPosted: Thu Feb 07, 2019 10:36 am
by Harry Dalek
Is that a`camera to the right ?...great work BTW

Re: Help needed with first Televisor

PostPosted: Thu Feb 07, 2019 7:35 pm
by sebket
Thanks everyone for the kind words :)

The work on the enclosure is in progress.
I am using 10mm MDF and had the pieces cut to the right sizes at the local hardware store.
The enclosure is going to be 50cm wide, 50cm tall and 15cm deep.

With the stormy weather predicted for the weekend, I am hoping to get the chance to disappear in my basement workshop for a few hours and get the enclosure done.
I'm just not sure yet about the finish of the device. Somewhere on the forum I've seen a Televisor with an Art Deco themed styling. I guess I'll opt for something like that.

:?:
A question for the admin/mods: I made a few pictures during the build of the Televisor and will make some more while constructing the enclosure. Would the forum be interested in a seperate thread about the building process? If so, shall I open a thread directly in the subforum for construction diaries or better elsewhere?

Re: Help needed with first Televisor

PostPosted: Thu Feb 07, 2019 9:50 pm
by acl
Sebastian,

Well done.

I would be interested in the construction photos as we may need these for a future handbook update. the hardware stores can supply predrilled blocks used to assemble kitchen units and you can get edge protectors as MDF tends to split when hit on the edge. You may consider a magnifying lens off and old 35mm slide viewer to increase the size of the image.

Regards Chris Lewis

Re: Help needed with first Televisor

PostPosted: Thu Feb 07, 2019 10:12 pm
by Andrew Davie
sebket wrote: :?:
A question for the admin/mods: I made a few pictures during the build of the Televisor and will make some more while constructing the enclosure. Would the forum be interested in a seperate thread about the building process? If so, shall I open a thread directly in the subforum for construction diaries or better elsewhere?


This is up to you, but certainly we encourage separate "build threads". Place it where you think best; probably construction diaries - if needs be we can move it. Lots of photos, please :)

Re: Help needed with first Televisor

PostPosted: Sun Feb 17, 2019 10:05 pm
by sebket
It took a little longer than I thought, but here it is, the building thread:

https://www.taswegian.com/NBTV/forum/viewtopic.php?f=12&t=2711

If you have any questions, please feel free to ask :)