Robot Research Model 70 SSTV Slow Scan TV working

Forum for discussion of SSTV topics. Slow Scan television (SSTV) is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or colour.

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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Tue Jan 07, 2020 2:13 pm

Associated photos (1 of 1).

Click the image to get the full-resolution version. That works with my browser, Chrome. Then right-click that image and click "Save Image As..".


Steve A.

That's all folks...
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Tue Jan 07, 2020 3:14 pm

Note: I've posted 8 times here today, depending your browser/screen resolution/settings you may miss some...scroll backwards quite a way...basically all those dated the same as this posting...depending on your time-zone. Europe and the UK should be the same date the 7th Jan, but early in the morning, the US it will be the previous day, 6th Jan, late evening, Australia & NZ afternoon of the 7th.

I also have the same sort of information on the Robot 60 & 61 SSTV viewfinders and the Robot 80A SSTV camera, though not scanned as yet. Anyone interested in those?

Steve A.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Tue Jan 07, 2020 7:44 pm

Thank you Steve, I have looked through the circuits and read the texts. It is almost as I was expecting, only the PLL for horizontal synchronisation is new to me. I will look at that more carefully.

I have a booklet about SSTV by Grant Dixon, John Wood and Mike Wooding (anno 1987). I have known Grant Dixon very well and I have been in contact with him very frequently. Grant was also important in NBTV, Jim Wood too. So SSTV and NBTV are very closely related. In this booklet I have read that there were versions until 70D. I think: 70, 70A, 70B, 70C and 70D. The D-version had provisions for double resolution, and a few more, that will need some more circuitry and, as I have seen, a few extra switches on the front.

It more or less compares to my "Heathkit-like" SSTV-monitor, so no need to buy it. But still thanks for the insight.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Tue Jan 07, 2020 9:10 pm

No problem, if I have the info I'm willing to share it.

Quite honestly we can do a better job ourselves, but there is some 40 years of development since these devices first appeared, so the challenges aren't quite as daunting as they faced...and they had to make a profit, we don't...though it would be nice!

Steve A.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Harry Dalek » Tue Jan 07, 2020 11:08 pm

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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Harry Dalek » Tue Jan 07, 2020 11:39 pm

I also came across these photos the ic lay out is interesting i didn't think it was that complex ! i am not great at this type of circuit but it looks like a memory to me ? so it had a memory board for replay ? i can't see any where on the panel for this ..has me wondering if this is a scan converter if so i don't know why it came with the robot TV...
edit ... hang on i just noticed the model 400 on the last page he must of got the documents mixed up so that is a scan converter
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Robot-Research-Model-70-SSTV-Slow-Scan-Television.jpg
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Robot-Research-Model-70-SSTV-Slow-Scan-Television-_57 (7).jpg
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Wed Jan 08, 2020 2:42 am

The last three photos are from the documentation of the ROBOT 400 digital scan converter. This IS a memory and a lot of surrounding circuitry. I think this man has put the documentation of the ROBOT 70 monitor and the ROBOT 400 digital scanconverter together in one binder and overlooked that the second part is not at all from the monitor.

Almost all that surrounding circuitry Steve is going to catch into two microcontrollers. That is the ongoing miniaturisation of modern integrated cicuits.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Harry Dalek » Wed Jan 08, 2020 8:04 am

Klaas Robers wrote:The last three photos are from the documentation of the ROBOT 400 digital scan converter. This IS a memory and a lot of surrounding circuitry. I think this man has put the documentation of the ROBOT 70 monitor and the ROBOT 400 digital scanconverter together in one binder and overlooked that the second part is not at all from the monitor.

Almost all that surrounding circuitry Steve is going to catch into two microcontrollers. That is the ongoing miniaturisation of modern integrated cicuits.



Robot 400 scan converter for sure so your dead right Klaas

I noticed that Richard Lab guy has 2 of these robot 70's but hes used one to convert to display for hes image tube project


youtu.be/HBPk6av2hCY


youtu.be/zcUuoyxSJLY
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Wed Jan 08, 2020 10:21 pm

Have you seen how high the intgernal resistance is of the HV generator of this monitor? Let me explain:

When there is a lot of "white" in the picture, the beam current will be large and the voltage (12 kV) will get down. With a lower accelerator voltage the electrons hit the screen at a lower velocity, but what is more: they travel slower through the tube. And then they will be deflected more, wider. So the picture on the screen is larger.

When there is a dark picture, the beam current is low and the voltage will rise. The electrons in the beam travel faster and will hit the screen earlier, when they are not yet deflected enough. So the picture will be smaller.

When there is insufficient capacitor in the HV output, the high voltage will vary on the level of the video signal and the width of the picture will vary with the brighness of the video on that height of the picture. You can see that best on the left edge of the picture. The small black bar at the left varies in width with the brightness of the lines. A larger capacitor in the 12 kV (!) will smooth that but the breathing of the picture size will stay.

This was a common problem in CRT television, that was solved by well designed HV generator circuits, and by some "overscan". The edges of the real picture were hidden behind the mask of the picture tube. When the breathing is slow enough people do not observe it as a technical flaw.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Sun Mar 05, 2023 10:16 pm

This week I got a ROBOT 70A monitor for free. It looked that everything worked correctly, but today, when I connected my SSTV signal source to the monitor, I see that the FM-demodulator is not working Ok. It looks as if the 2400Hz resonator came down to a frequency somewhere between 1500 and 2300 Hz. The grey-scale is at its brightest in the middle of the picture, so grey-scale rendering is very bad. I must repair that.

I think that I am going to check the resistor values and capacitor values in the 2400Hz resonator first. For a lower resonating frequency the capacitors should get a higher capacitance, which sounds strange to me, and the resistors should also got a higher value, which looks to be reasonable. We'll see....
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Mon Mar 06, 2023 10:23 pm

The SSTV-monitor that I have got is a ROBOT 70A type. The documentation of the ROBOT model 70 has some extra pages which explain that the model 70A has got extra a "tuning-LED". This LED is connected to an extra resonator circuit on 1200 Hz. In this way you can tune your SSB radio to get the sync onto 1200 Hz. Then the LED is flashing a 15Hz (or 16,666 Hz) rithm.

The circuit diagram is slightly different from the diagram that Steve uploaded in this thread. It shows that there is also a model 70B. As far as I can see this model 70B has an extra switch (on the front?). If you flip that switch, the monitor changes into an oscilloscope that displays the video signal. Then the vertical deflection coil is switched to an extra amplifier that is fed with the video signal. However no extra information is given about that amplifier.

I can understand that the oscilloscope function, I would call it a video wave form monitor, is a useful extention. Especially because the signal path is DC-coupled from the FM-detector onwards, this is a better way to tune your SSB-radio correctly for receiving SSTV.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby kd2bd » Tue Mar 07, 2023 2:43 am

Klaas,

I have a Robot 70 monitor that suffers from the same greyscale shift you observed in yours. I came to the conclusion that every 1/4w non-precision resistor in the monitor has drifted high in resistance with age. The only solution is to replace the defective resistors. I haven't done that with mine, yet, as it is a lot of work.

Another thought I had was to replace the existing circuit board in the monitor with an improved design of my own. Even more work than just replacing the resistors... but I don't know if the improved results would be worth the time and effort involved...

73 de John, KD2BD
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Tue Mar 07, 2023 9:04 pm

Hi John,

thank you for reacting on my message, it gave me more confidence that I have to look at the resistors.

I have seen the A&B (Allen and Bradley) resistors and I didn't already trust them. My Heathkit SB-101 line originally was also built with these resistors and I experienced quite some problems with them. So long ago I replaced them all by Philips metal film resistors. That is indeed quite some work, but from then on I never had problems with the sets. Look on QRZ.com to see the line.

I read now and then that people standard exchange all the capacitors in older sets, but I never had problems with them.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Tue Mar 14, 2023 6:11 pm

A lot of your decision making is based on, "Do you want to preserve the device as much as it was originally built, or upgrade it with modern components and/or designs?" i.e. a replacement of most of the internals with modern circuits and components? In fact, in this case adding a regulated HV power supply for the CRT? (That's not for the faint-hearted).

In my mind it would be ideal to have two Model 70s, one 'restored' to as was just by replacing out-of-tolerance/faulty components, the other with all new internals. But the same CRT/Case/ and operation as designed, such that it appears externally no different to the original, but should perform somewhat better than the original. An ideal situation which is unlikely to happen.

If ever I were to happen across two Model 70s, that's exactly what I would do. Most unlikely though, sadly.

Steve A.

P.S. Back home now in Thailand from Laos....for a while...weeks...
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Wed Mar 15, 2023 4:34 am

I understand your point, but I go for an inbetween solution. My ROBOT 70A works too good to replace the electronics completely. So far the only problem is the discriminator of which the high resonator came down in frequency. I am going to check the resistors and replace only the ones that became out of spec. I will show you the behaviour later this week.

And yes, it is possible that I encounter more of these problems. We will see.

Klaas
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