A new 90-line Mirror Screw

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Re: A new 90-line Mirror Screw

Postby Klaas Robers » Thu Jan 16, 2025 8:37 pm

I have good experiences with DIN connectors. They give a solid contact and keep that contact. DIN = Deutsche Industrie Norm, so it is designed by the germans. That usually is a well engineered product. If you fear that they will be pulled out accidentally, there are DIN connectors with a screw ring. And of course they are compatible with the unscrewable ones. I have them with 8 pins, so I get 8 connections at once. "Made in Germany".

This in contrary to the SCART connectors. They are designed by the French. That kind of products look nice and pretty, but are in most cases unpracticle and/or unreliable. I have some in use and during time they get loose, give unreliable contact and I have to reinsert them now and then. However I think that the worst thing for them are the thick and stiff cables, with 19 wires and in many cases a braid around them. That induces too big forces on the connector with the result that during time the thin tinplated pins will loose their contact.

But we have a CTV monitor with SCART-connectors we have to use them and hope for the best.
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Re: A new 90-line Mirror Screw

Postby Panrock » Fri Jan 17, 2025 1:54 am

Will you be coming to the Convention, Klaas?

I've missed it the last few years. I'm now treating the provisional date of April 5th as something of a deadline to complete my project. Whether I can manage it remains to be seen.

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Re: A new 90-line Mirror Screw

Postby Klaas Robers » Fri Jan 17, 2025 4:21 am

No Steve, I will not be coming to the convention. It is too far travelling up and down for just a few hours of convention. And I have to cross the UK border twice, which is much more troublesome than it was a few years ago, when the UK was still part of the EU.

And about your 90 lines mirrorscrew: I wondered already if you can polish aluminium surfaces to a real mirror, and if those slats will keep that mirror surface over time. I fear that the rapid oxidation of aluminium will turn it very fast to dull. May be that you can spray a very thin protective layer on it as soon as you have polished them, but I don't know.......

Good luck!
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Re: A new 90-line Mirror Screw

Postby Panrock » Fri Jan 17, 2025 5:11 am

Klaas Robers wrote:And about your 90 lines mirrorscrew: I wondered already if you can polish aluminium surfaces to a real mirror, and if those slats will keep that mirror surface over time. I fear that the rapid oxidation of aluminium will turn it very fast to dull. May be that you can spray a very thin protective layer on it as soon as you have polished them, but I don't know.......

Good luck!

Thanks!

I have been advised that with a very high polish and the consequent reduction in "hills and dales" surface area on the microscopic scale, oxidation tarnishing is less likely. Of course aluminium acquires a thin coat of oxidation almost immediately (which doesn't make much difference). Having said that, yes, a thin coating of something like furniture polish (and buffed off) could help. When I am in the final stage of polishing, I will try this for comparison.

If all else fails, I would consider plating. Not in time for the Convention though. :)

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Re: A new 90-line Mirror Screw

Postby Panrock » Sat Jan 25, 2025 1:55 am

I should have the three stage re-polishing done by the end of this weekend. Each of the 90 slats is being polished with a rotary wheel individually. It's a slog and I can't say I am enjoying it. But the result looks promising. I am 'sealing' each one with Pledge furniture polish.

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Re: A new 90-line Mirror Screw

Postby Panrock » Mon Jan 27, 2025 4:13 am

I've completed the polishing as planned. The surface is highly reflective and smooth, but these are slim slats, only 1mm wide and they show up every tiny imperfection. It may well be I shall have to tolerate some reflection 'sparkling' from the surface. The limit could now be being set by the grain of the aluminium itself.

It remains to be seen how significant this will be for picture quality. What could be affected is the sharpness of the scanning spot at the mirror surface. But then the eye won't be focused on this plane; rather on a virtual plane as far behind the screw as the line-o-lights are in front.

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Re: A new 90-line Mirror Screw

Postby Panrock » Tue Feb 04, 2025 12:53 am

The past week has been spent reassembling the Mirror Screw, slat-by-slat. I had a good morning in the workshop today - and the job is at last finished. The complete screw has also been mounted at the top (again). It all fits together as it should. The re-polished slat edges look nice. It remains to be seen how much use they really are. Compared to its 120-line stainless steel predecessor, this is very much an 'economy' screw. The hope is it will be more practical to transport and exhibit though.

Next, to get a grip on the instability in the LED Driver Control Box. Then the whole system can be run up.

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Tue Feb 04, 2025 11:14 am

Panrock wrote:Next, to get a grip on the instability in the LED Driver Control Box. Steve O

Instability. A word any circuit guy never wants to have to utter. What instability? And how does it manifest itself? I do feel somewhat responsible here! Is it the same 'control box' as used with the 120-line version?

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Re: A new 90-line Mirror Screw

Postby Panrock » Wed Feb 05, 2025 12:52 am

Probably not instability. It's at times like this it feels so good to have the assistance of the esteemed circuit designer (no less) to hand. Needless to say, a big thank you! On the face of it, we have the following very simple symptoms:

Take the red, green and blue DC bias 'sits', each controlled by a pot. (the gain pots are at zero). Each sit pot is connected in its channel as shown by VR102 on the attached diagram. The current in each channel is monitored on meter M101 on the diagram (threefold).

If you:

...advance the red sit - the green and blue currents also increase, but by less.

...advance the green sit - the red and blue currents also increase, but by less.

...advance the blue sit - the red and green currents do NOT also increase. So this control works correctly.

This must be pretty elementary, but it has me puzzled! As a first step, I'll check the stiffness of the -12v rails.

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Wed Feb 05, 2025 12:14 pm

This is odd, the only thing I can see that the three channels (RGB) have in common is the power supply rails and the ground/0V. I'm assuming the +12V/-12V supplies are regulated by someting like 7812/7912 regulators [1] and the 0V is of low impedance (thick cables/wires). Check there are no solder splashes and the like, esp. where Veroboard (if used) tracks are cut.

There could be some interaction if the nominal +65V is poorly regulated, but I really doubt that. It really shouldn't need regulation.

If +1V is applied to one channel the appropriate gain pot should control that channels LEDs from 'off' to full brilliance without affecting the others, check with a meter or 'scope. Turn the gain pots on the other two channels down to zero when doing this. But check anyway that there's no 'leakage' from one channel to another with no input to the other channels without the +1V applied.

Trace the circuit out (as built, very carefully), it'll probably turn out to be a 'silly' somewhere.

I just noticed the date on the circuit diagram, some 12/13 years ago, is that really possible? I would have thought perhaps 3 or 4 years!

Steve A.

[1] Not absolutely required, more a suggestion, and as this is DC-coupled, ensres DC stability. The 7812 may need a small heatsink, the 7912 probably no heatsink.
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Re: A new 90-line Mirror Screw

Postby Panrock » Thu Feb 06, 2025 12:46 am

Thanks as ever, Steve.

Panrock wrote: As a first step, I'll check the stiffness of the -12v rail.

Steve Anderson wrote:This is odd, the only thing I can see that the three channels (RGB) have in common is the power supply rails and the ground/0V. I'm assuming the +12V/-12V supplies are regulated by someting like 7812/7912 regulators [1] and the 0V is of low impedance (thick cables/wires). Check there are no solder splashes and the like, esp. where Veroboard (if used) tracks are cut.


The -ve rail has now been verified as 'stiff'. Yes, each is regulated with a 7812/7912.

But Steve A has made a pertinent final comment above. As you will see further down...

Steve Anderson wrote:There could be some interaction if the nominal +65V is poorly regulated, but I really doubt that. It really shouldn't need regulation.

We are looking at an effect here where the LEDs are minimally illuminated, so this won't be a factor. However, I've now noticed the secondary LED illumination in keeping with the meter readings. when the other channels follow the first (as this is advanced) does not occur. Therefore this must be purely a monitoring issue.

(Later) And so it has proved! Although the bulk of the circuitry in this 'box' was built on a specially-made printed circuit 'mother' board, the meter drivers were the exception - these were made up on Veroboard as a sub-assembly. Running the point of a Stanley knife blade along the lined gaps in the vero has evidently cleared the 'leak' (wherever it was) and all is now well.

I don't usually get this lucky! Thanks again for the help.

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Re: A new 90-line Mirror Screw

Postby Steve Anderson » Thu Feb 06, 2025 11:44 am

OK. glad all is well now. However it shows just how important PCB/board 'hygiene' is, even in fairly low-impedence circuits like this. Always clean off solder flux residue with a solvent after assembly. Fingerprints can also be a source of leakage currents in high impedance circuits. Cleaning inhibits corrosion of thin copper tracks from various chemicals and human-sourced contaminants.

A blast in an ultrasonic bath (Arklone) followed by a sprayed-on conformal coating is the best for consumer electronics. But this is going a bit far for the sort of stuff we do here. But with Arklone you need to be careful as it dissolves some components (particularly plastics) as well as getting rid of the contaminents.

I use a locally sourced cleaner called 'Ultracore', applied with a stiff brush it gets rid of most rubbish. Like most of these cleaners it is highly flammable, especially the vapours, so be careful. A litre lasts me years.

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Re: A new 90-line Mirror Screw

Postby Panrock » Sun Feb 09, 2025 3:50 am

I had the mirror screw running today. It whizzes round very serenely at 25 frames per second and the WC-01 locks to it. The simple DC motor and driver transistor are keeping cool and the screw maintains a steady speed. The raster looks to be of satisfactory quality. One thing that is noticeable is the "picture" (so far just a slat setting waveform) is quite a bit dimmer than on the pucker 120-line screw. This is because the scanning spot, which is where the thinner slats intersect the reflected lines-o-light, is much smaller in area. Still, in this regard it is still usable.

However, it's looking I could be in big trouble.

The setting up of the timing delays on Karen's Timing Corrector is not working as anticipated. Although it's possible to select a given picture line, it's not clear the delays are being adjusted as they should be. (Also the Frame Phasing on the WC-01 is not functioning... however this is likely due to the absence of frame syncs on the test 625-line square wave as supplied by Karen's unit).

I now fear that despite my earlier blithe assurances, there is something about Karen's unit that means it will only work either with 60-lines or 120-lines, but nothing else in between!

The Manual is 'out there'. Can anybody who really knows his oats with digital stuff "fish me out of it" here? I'm feeling really guilty... sorry! :oops: :oops: :oops:

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Re: A new 90-line Mirror Screw

Postby acl » Sun Feb 09, 2025 6:24 am

World User Manual 2.3.pdf
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A new 90-line Mirror Screw

Postby acl » Sun Feb 09, 2025 6:28 am

A timing corrector for 60/120 line mirror screw
Karen Orton

Many members are familiar with my 'NBTV timing corrector' - a device which counts lines and imposes pre-defined delays on each line so that some image artefacts caused by mechanical errors are cancelled. The resulting improvement can be striking. For some time now I have been considering a timing corrector for higher line rates and bandwidths, specifically, to correct for small mechanical errors in Steve Ostler's mirror screw monitor. This is a 25 frames per second 60 line screw with the capability of 120 lines. The bandwidth requirement for these standards far exceeds the capabilities of my usual PIC-based approaches.

At one point I considered using a DSP (TMS320F2808). In the end however, I decided upon a combination of hardware for implementation of controlled delays, and a PIC for supervision and to manage the various interfaces. It is perhaps not the best decision - the chip count for the project went into double figures - something I have come to regard as a bad sign. However, much of the complexity is simply down to the need for three channels of timing correction (Steve's monitor is a colour system). The instrument I eventually built uses two 100x160mm circuit boards: a horribly complicated board (HCB) bearing analogue-to-digital converters (ADCs), first-in-first-out (FIFO) memories, digital-to-analogue conversion (DAC); and a much simpler PIC board (SPB).

The ADCs are of the 'flash' type and have a resolution of only six bits, which is okay provided a gamma close to 0.5 is used. The existing mirror screw monitor uses a gamma of 1.0 throughout, and this presented a slight problem. Fortunately, the Aurora World Converter can be set to output a gamma of 0.45 thereby presenting a signal suitable for direct input to the ADCs. Restoration to a gamma of 1.0 must then be performed on the reconstructed outputs, and for this I used a piecewise linear circuit based on diodes. This gave very satisfactory results, despite being discrete and having no adjustable components. I am doubtful however that this circuit would work so well over the full range of component spread. Ideally, a third board carrying analogue input and output processing would have been included but there was simply no more room left in the case! Such a board could use high speed op amps to mitigate the effects of component spread. More importantly, more attention to analogue processing might avoid the situation where the input gamma and output gamma are different. In respect of the latter, I regard the current design as a compromise.

Actual timing correction is carried out using FIFO memories. The PIC has the ability to suspend input to the memories from the ADCs, and to suspend output to the DACs (which in the process, will be forced to black level). This control is sufficient to time shift individual scan lines. In addition, the PIC must charge each colour channel's FIFOs with a 'head' of samples at the start of each frame, to reflect the spacing of the line-of-lights. Other duties of the PIC are to monitor button presses for setting adjustment, and to sense the setting of three potentiometers using the PIC's internal low speed ADC.

Actual adjustment is performed by a hand controller on a lead. This plugs into the font panel and allows the instrument to be sited some distance from the viewing position. While editing, it is arranged that the currently selected scan line flashes so as to make its identification easy. An internal pattern generator furnishes a 625 line compatible signal bearing a vertical bar. The intention is that this signal is fed into the Aurora World Converter so that the bar is displayed on the mirror screw monitor. Adjustment can then proceed with the aim of getting the right side of this vertical bar (which is almost central in the image) free of jaggedness. At my suggestion, Steve built a new line-of-light to use three separate rows of red green and blue LEDs rather than his previous colour mixing arrangement. Colour convergence is now achieved using small additional fixed delays to individual colour channels. These delays are specified by front panel potentiometers as opposed to an internal digital representation. This is to allow easy adjustment of the convergence following minor repositioning of the line-of-light, or following set-up at a new venue.

Klaas's IC610 was used in the 625 line pattern generator. It proved easy to find a signal (that on pin 10) which could be used to generate a vertical bar. The 2.5MHz clock required by this part is supplied by the PIC. Coincidentally, the resolution of the timing corrector is based on this frequency, leading to timing steps of 400 nanoseconds. Such steps represent: 13 seconds of arc of screw rotation; 0.06% of a scan line, or 5% of a square pixel* at 60 lines; and 0.12% of a scan line, or 20% of a square pixel* at 120 lines. The 2.5MHz sampling rate allows for a video bandwidth of 800kHz, which is wider than that of either standard by a good margin.

An internal serial port was a late addition to the circuit. This is only a matter of three components and may permit reading or writing of the settings by a computer. Future firmware enhancements may include auto-repeat on the adjustment buttons (it currently takes eighty presses to move between the adjustment limits). Also to be added is a means to shift all settings along by one line (in either direction). This latter feature is in anticipation of some small alteration to a system, resulting in the timing corrector changing its 'opinion' on which line is line zero. Such a change would instantly invalidate all of the settings - a situation that can be almost completely recovered using the mentioned shift facility.

* Based on the assumption of 80 square pixels per line at 60 lines, and 160 square pixels per line at 120 lines
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