Mindsets Televisor

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Mindsets Televisor

Postby mikew8760 » Sat Aug 07, 2021 12:53 am

I was recently given a Mindsets Televisor kit by a friend. He is interested in Baird's work and thought this would be a good visual aid, having seen the "Grand Illusions" Youtube demo. My involvement was 'simply' to put the kit together!
Despite a lot of fiddling I have never been able to obtain a stable image, as expected. The only image which can be synced is the on-board "Test Card". I have some knowledge of electronics, and it seems that my kit has a fault. I've sent a couple of emails to Mindsets who gave replies, but no offer of any useful information.
Hence my problem is to try and find some info on this kit to assist troubleshooting. I've reverse engineered part of the circuit board, but am disinclined to spend hours to do the the rest. I wonder if anyone who has this kit would be willing and able to produce 'scope waveforms at important points for comparison?
On a slightly different tack, can anyone explain the purpose of the missing line on the Nipkow disk? I get the impression it's for frame sync, but cannot find anything in the circuitry that might achieve this; no doubt I'm looking in the wrong place!
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Re: Mindsets Televisor

Postby Dave Moll » Sat Aug 07, 2021 10:30 pm

This may be a case of "teaching grandmothers to suck eggs", but have you checked that the polarity of your input signal is correct? Many audio sources invert the signal they output - which makes no difference for audio (as long as both channels of stereo are the same polarity), but means that NBTV is trying to interpret video as synch and vice versa.

You are correct that the "missing synch" pulse is for frame synchronisation, though I'm not sufficiently familiar with the circuitry of the Mindsets unit to know how this is handled.
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Re: Mindsets Televisor

Postby mikew8760 » Sun Aug 08, 2021 1:26 am

Hi, many thanks for your response. I left out some of the boring detail of the problems I had with the kit, some of it due to my lack of knowledge in NBTV.
The 'video' signal polarity escaped me completely, until I got a reply from Mindsets; the accompanying CD does say it contains both upright (?) polarity and inverted(?) polarity. Would have saved me some faffing about with an inverter for the signal from my CD player if this was explained in their booklet.
My feeling that my kit has some sort of fault is based to some extent on the "Grand Illisions" U-tube video. As I mentioned, the only image which can be synced is the Test Card, which must be generated by the on-board PIC. Scoping, for example, the output from 4046 PC2 is a 'jumble' with signals played back from the CD, and never locks. As an aside I made a little signal generator using a PIC and a DAC to avoid using a signal from a CD player. (Also used an iphone to play mp3 tracks) This was able give a more stable image, but experiments both with and without the 'missing line' made no difference. Hence my query about frame sync.
I shall pursue the problem a little further as I'm reluctant to consign the whole lot to the spare parts bin... at least for now.
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Mindsets Televisor

Postby acl » Mon Aug 09, 2021 7:59 am

Hi there,

Many of our members have built this unit and impressed with it's performance. The only problem with mine was the positioning of the disc. Klaas Robers is the club guru on this unit and may be able to help out.

Regards Chris Lewis
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Re: Mindsets Televisor

Postby mikew8760 » Wed Aug 11, 2021 12:20 am

Thanks for the comment; I can only hope he may have some useful suggestions.
I agree about the Nipkow disc. Mine is about 2mm off centre and has an awkward curl. I don't think either of these is any real problem; the eccentricity perhaps causes the line sync to have a slight modulating ripple and the curl causes it to foul the pcb if pushed too far on to the motor spindle. Also, the disc soon parted company from the plastic hub as the double-sided sticky tape was not up to the job.
Regarding frame sync, I'm coming to the conclusion that there is none, nor none needed. The missing segment on the disc simply causes the output from the 4046 to 'hiccup' , but again this seems not to be a real problem.

Hopefully there may be some further suggestions along the way.
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Re: Mindsets Televisor

Postby mikew8760 » Thu Aug 12, 2021 2:30 am

That would be helpful, if he is listening!
There is one thing, amongst many, about the voltage "signal" I've seen at the drain/motor terminal. I am surprised to see wildly and seemingly random fluctuations, with low level ripple and the occasional large spike. I presume this is due to arcing of the brushes in the motor, but at only 6v supply it seems odd. This assumes a 'normal' low volage DC brushed motor, and I can't think of any alternative. Perhaps someone could put me straight?
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Re: Mindsets Televisor

Postby Klaas Robers » Fri Aug 13, 2021 9:01 pm

Firstly see that the "monitor" functions correctly on the built in test signal. See that you bend the disc such that it is almost flat. The disc when running should not touch the PCB and the paper "stroboscope" (sync) disc should keep clear from the opto sensor in the PCB.

If the "centerhole is not in the center, then the disc is faulty. I think that you should have a better disc if you complain about it. The edge of the disc should be constant within about one mm. Otherwise you will also have problems with the lines of the NBTV picture.

Again: concentrate on the built in test picture. Only if that is Ok try to look at the pictures of the CD.

The output of the CD-player gives you a few extra problems that you can solve only after the built-in test picture displays correctly. The problems are:
1. the polarity of the video signal,
2. the amplitude (strength) of the video signal,
3. the low frequency response of the CD-player.

Ad 1: the Mindset CD has the video signal in two polarities, try them both en use the one that works. Preferably use an old CD-player used in an audio sound system. They useally have a "Line output". Use that output. Originally this output should have a defined polarity and amplitude. This should solve all three problems.

Ad 2: unless audio sound, which may have all amplitudes (strengths), a video signal has a well defined optimal amplitude: 1 volt peak to peak. If you have an oscilloscope, look at that video signal and adjust the "volume" to at least 1 volt peak to peak. Quite some portable CD-players cannot reach this amplitude from their ear phone output. Adjusting potentiometer R7 on the PCB may also help you, but the n you loose the optimal setting for the test signal. Using the oscilloscope gives you also information about the polarity of the output signal. It should have sync pulses going negative.

Ad 3: Audio does not go lower than 20 Hz. NBTV video should go as low as 2 Hz. The old line output goes this low, ear phone outputs don't. However this does not affect the functioning of the picture display, 20 Hz should do. But the fidelity of the pictures is best with the 2 Hz output. So try different CD-payers. A CD-player good enough for sound may not be good enough for NBTV video.

Good luck, you may need it.
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Re: Mindsets Televisor

Postby smeezekitty » Sat Aug 14, 2021 7:03 am

mikew8760 wrote:I presume this is due to arcing of the brushes in the motor, but at only 6v supply it seems odd. This assumes a 'normal' low volage DC brushed motor, and I can't think of any alternative. Perhaps someone could put me straight?


Motor windings are inductive so when the contact is broken there can be spikes that are many times the input voltage
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Re: Mindsets Televisor

Postby mikew8760 » Sat Aug 14, 2021 8:25 am

Where did my draft go?

Does this work?

[img][img]
https://www.dropbox.com/s/n4om97wke3h4r ... 1.jpg?dl=0

[/img][/img]
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Re: Mindsets Televisor

Postby Klaas Robers » Sat Aug 14, 2021 9:05 pm

Yes, this works. I see in red your sync pattern as detected by the opto sensor on the PCB, and I see in blue the sync pattern after the limitter. I see that the first pattern (red) is modulated in amplitude and that at a certain point on the detected red signal is is too weak to trigger the limitter. Experiment with the position, distance of the disc to the opto sensor, on the motor spindle to make the amplitude of the red pattern as large as possible. The distance influences that amplitude. When optimal the "flat part" that is now in the blue trace disappears and correct synchronisation will suddenly occur.

Indeed this is the first thing that should work properly. I think this happens because the disc is wobbeling and the distance to the opto sensor in the PCB is far from optimal.

Good luck!! And show me a better pattern.

By the way: do you have this schematic diagram of the circuit on the PCB?

Schema.gif
Schema.gif (16.01 KiB) Viewed 3136 times
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Re: Mindsets Televisor

Postby mikew8760 » Sun Aug 15, 2021 6:50 am

Thanks for the info.I did find the schematic on this forum and it's been really useful, thanks. Perhaps I should repeat that I'm no newcomer to electronics, but have next to no knowledge of mechanical NBTV! Before complaining to Mindsets I shall have another attempt to flatten the disk. I deduce that the variations in the opto sensor output is purely due to its misalignment.
But one item that puzzles me is how frame synchronisation is achieved by this circuitry. The segmented disc has one missing segment, and this can be seen in the 'scope trace. I've read that Baird supposedly used the blank line for frame sync, but there seems to be no mechanism here to utilise this. I'd love to be enlightened!
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Re: Mindsets Televisor

Postby theo » Sun Aug 15, 2021 8:29 am

At first Baird was using a black frame for it’s sync, later came the (neg) syn pulses. And as you stated it’s verry difficult to see, but your disk is missing 1 puls p/frame and also the sync from the video signal has a missing sync. Possibly the 4046 – here used only as comparator – gives a extra long output to the r/c network, as long as both vertical missing sync are not in fase. Otherwise you are so wright in your conclusion, that it’s only a H sync comparator. I never build this circuitry and made my own design. Worst case; the picture will slowly go upwards or downwards.
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Re: Mindsets Televisor

Postby theo » Sun Aug 15, 2021 9:45 pm

Ps: also see my web-site or Youtube for the explanation of sync signals under NBTV video. https://www.youtube.com/watch?v=K0QTcikiwHQ or https://www.theofaberalmere.nl
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Re: Mindsets Televisor

Postby mikew8760 » Mon Aug 16, 2021 1:06 am

Hi, many thanks Theo for the input. Re: the frame sync., the 4046 PC2 outputs a fairly complex waveform which is fed into a simple low pass filter, as you can see in the schematic. The voltage applied to the FET gate is a fairly smooth, undulating signal. I guess if there was no motor or air, or any other drag on the disc this voltage would remain constant. But then there is the feedback circuit from the FET drain/motor connection to the second filter. I've not bothered to calculate or measure its time constant, but would be fairly long with the 22uF capacitor involved. Not sure of the detail, but it works, with the 10k pot allowing a slight phase shift for frame adjustment.
I shall be interested to look at your links.
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Re: Mindsets Televisor

Postby Klaas Robers » Tue Aug 17, 2021 7:25 pm

The frame sync is done by a "missing linesync pulse". This is far after Baird, the NBTV association suggested this in about 1970. And it works for the 4046, the disc will not synchronise if the missing sync pulse in the video is misalligned in time with the missing sync pulse in the opto sensor output. However the circuit tends to synchronise as well with a frame sync misallignment of one line, positive or negative. But the circuit is so simple..... The advantage of the missing line sync for the frame sync is that it occupies no extra video line information. 32 lines is already so few lines.

The Mindset disc synchronises rather fast, in about one second. My larger aluminium disc needs much more. But firstly see that your opto sensor works properly. I see that you have a kind of oscilloscope, so this will work.
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