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 » Wed Mar 22, 2023 6:21 pm

Some here may remember what I called "The Brute". It was initially going to be a combined NBTV and SSTV display monitor. Its original metalwork and much of the display power supply has been sitting on a shelf here for well over a decade gathering dust. The Aluminium (I'm British) chassis has also lost its original bright sheen.

My original designs for the core electronics I have reviewed and will be scrapped. They really are 'old hat' now. Though I won't be using anything that's not available for others to copy or modify for their own use. I'll be using a 5ADP1 5" CRT for the NBTV side of things, and a 5ADP7 for the SSTV usage. Now, the long-persistence 5ADP7 may vanish if I also add a SSTV-to-high-rate scan convertor, in much the same manner as the SSTV-625 scan convertor I did some years ago. Its output format may or may not be standard 625 - it probably will be, decision yet to be made.

I'll also delete the valve/tube audio for a simple and low-consumption LM386 or similar. It's a shame the LM380 has virtually vanished...it was 'just right' for so many applications. Such is progress...

This isn't going to happen overnight, I have other projects to do that pay, sadly this will not...

I will start a new thread for "The Brute, MkII".

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

Postby Harry Dalek » Wed Mar 22, 2023 8:10 pm

I Recall the Brute Steve ,i am Glad to hear the case at least will be reused and it still exists .
Think i now understand your interest in the project 625 line monitor it would come in handy here if you do the SSTV NBTV to 625line converter /
I suppose you could still go with the P7 CRT if you go down that road .
All interesting to me i loved the Bute project i will watch for sure
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Thu Mar 23, 2023 11:47 am

Looking back, the original 'Brute' was somewhat impractical, it was large, heavy and ate up a large portion of desk/workbench space. I'll still be constrained by the CRTs dimensions (not that small!) and the somewhat complex power supply required for all CRTs. I'm hoping to reduce its footprint to 50% of the previous version or better with better planning and layout.

Some 'features' of the original will be omitted for simplicities sake, some weren't essential anyway, but ''nice-to-have' things. I'm of the opinion that this will probably be the last CRT build I will do. Small flat-panel displays are more practical and less expensive than any CRT with its complex power supply requirements.

So look out for a new thread starting soon, entitled, "The Brute MkII". First I need to pull together a conceptual idea/design.

Somewhat later after much musing...maybe just make a simpler version of my SSTV-625 convertor and graft in the NBTV-625 function? A lot simpler, smaller, cheaper and no high-voltage power supplies required. And/or split it into two units...

1) The SSTV and NBTV to 625 convertor and use a standard TV as the display. Much as I did previously, but updated and simplified. I did make a start on this a few years ago, but other things 'got in the way'.

2) A 'scope tube CRT display which accepts a standard 625 video input.

For those in 525-land, I'm not sure what to do. However most TVs made in the last two decades or more will auto-switch between 625 and 525 automatically, though not all. I suggest checking first!

So, my thinking is now this...a SSTV and a NBTV to 625 display 'adaptor'. All monochrome. I did also look into a VGA-based monitor version as that's standard world-wide. That may come into fruition later, though the bandwidth requirements for hobbyist construction methods are somewhat of a challenge. It might be my first foray into PCB work. Normally I pay (or the company does) someone else to do that, but that's an expensive option for a hobbyist project.

There is a possibility that someone here might offer to do the PCB layout, but it's not for the faint-hearted! Maybe two standard Eurocard (160x100mm) double-sided plated through PCBs, solder resist, screen printing etc.. I will avoid any SMD devices if at all possible.

All 'food for thought'...

Steve A.

Add to the above, there'll inevitably be some software required, not for external equipment (PCs and the like), but for the board-mounted microcontroller(s). That I'm quite OK to deal with.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Harry Dalek » Thu Mar 23, 2023 7:30 pm

I see your still thinking about the design of the thing and having a place to put it, that is always a problem as much as you love what you have made can't have every thing on the bench .
I know its a personal choice i would make it all one unit ,if going CRT and you want small it is still possible and 625 line this below sounds cheap and if used sideways or a mirror to reflect if mounted flat might be an idea .
link below and very cheap this one
Screen 00014.bmp
Screen 00014.bmp (676.42 KiB) Viewed 5274 times

https://www.aliexpress.com/item/1005002 ... ainProduct

or this one
https://www.aliexpress.com/item/1005003 ... 1679559651
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Screen 00015.bmp
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Fri Mar 24, 2023 3:27 pm

They are certainly a possibility, a 4" (10cm) screen is a useful portable size, but for 'fixed' applications something larger would be more useful. I'm not sure if those have a baseband (video) input, and not RF? But I'm sure they could be modified if need be.

Anyway, the output of the convertor would be 625 baseband video, so almost any moderately recent TV should be OK. But, as I said, check first.

I'll probably do this as a SSTV/NBTV to 625 convertor as a single unit to drive a separate TV/monitor. I think this would be more useful to others if they wish to copy and/or modify it. A SSTV/NBTV to 800x600 VGA version may follow on later.

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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Harry Dalek » Fri Mar 24, 2023 6:11 pm

Steve Anderson wrote:They are certainly a possibility, a 4" (10cm) screen is a useful portable size, but for 'fixed' applications something larger would be more useful. I'm not sure if those have a baseband (video) input, and not RF? But I'm sure they could be modified if need be.


It says in another one of these sold ,video input no AF .
For some reason there are a lot o these things being sold out of no where .

Anyway, the output of the convertor would be 625 baseband video, so almost any moderately recent TV should be OK. But, as I said, check first.


625 any thing still easy to find some thing

I'll probably do this as a SSTV/NBTV to 625 convertor as a single unit to drive a separate TV/monitor. I think this would be more useful to others if they wish to copy and/or modify it. A SSTV/NBTV to 800x600 VGA version may follow on later.


OK i see your point bit different too.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Steve Anderson » Sat Mar 25, 2023 11:48 am

Harry Dalek wrote:OK i see your point bit different too.


I think that there's a higher chance that others will build the NBTV/SSTV to 625 up-convertor (modified or not) than a unit that also includes a CRT display, i.e. more useful. Though some (myself) would find the published CRT article/design interesting for armchair reading!

All those high-voltages, the size/weight of the thing and sourcing the high-voltage transformers, capacitors and the CRT itself. There's the safety aspect too, many would prefer using a commercial 12V wall-wart/power supply avoiding 110/220V mains wiring too. I'm thinking of using an old laptop PC power supply/charger, something like 18-20V at 2-3A maximum, you can get 'generic' versions off the 'net cheaply.

Even if users aren't interested in the SSTV aspect, it really makes no difference, the hardware is virtually identical for SSTV and NBTV. The only difference is perhaps the requirement for a SSTV demodulator, though that could be handled to a large extent in software.

I think the focus should be a NBTV to 625 convertor with the option of the SSTV element, there wouldn't be a big difference anyway. A large part of the work will getting the software to cope with mechanically derived source NBTV signals with their mechanically induced timing variations...some form of Timebase Corrector (TBC)...that could be a whole new 'can of worms'...

I have no plans to process any audio unless there's a noticeable delay in the video path resulting in loss of lip-sync. I think this is most unlikely.

I still plan to do the 800x600 output VGA version if there's interest, I may do it anyway for myself. It will be in a while though.

Steve A.

I'll start a new thread when appropriate...
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Wed May 24, 2023 1:48 am

Steve, do you still know the resonance frequencies of the two active tuned circuits in the ROBOT SSTV FM discriminator? I want to connect my ROBOT model 70A to the audio tone generator and see what the resonance frequencies are right now. May be both have been shifted in frequency and I remember that you have experimented with the ROBOT discriminator quite a lot in the past.

I have seen that the guys at ROBOT trusted the discriminator very well, as the same circuit is also used in the later ROBOT model 400 scan converter. Happily they used there better resistors, in my model 400 the discriminator works still as it should do.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby kd2bd » Wed May 24, 2023 9:31 am

Hi Klaas.

By reverse engineering the filters, and not taking into account the slew-rate of the op-amps, I get the following:

Low-side filter:
Center Frequency: 987.39 Hz
Voltage Gain: 0.50x (-6.02 dB)
Bandwidth: 964.58 Hz
Q: 1.02

High-side filter:
Center Frequency: 2420.76 Hz
Voltage Gain: 0.50x (-6.02 dB)
Bandwidth: 964.58 Hz
Q: 2.51

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

Postby Steve Anderson » Wed May 24, 2023 11:28 am

Yes, I have quite a few (understatement) resultant plots from the Robot 70 filters. I think the attached is close to the original, I guess I must have been playing around with the Q at the time, hence the third trace, the red & green traces are the originals, ignore the blue. It's theoretical, not measured... However John has crunched the numbers above and I would suggest going with those...the centre frequencies agree quite well...

If slew-rate is a factor, I used TL084s...16V/us on +/-12V supplies...it varies slightly between different manufacturers datasheets...and of course real-world examples...

Steve A.

1k0 Robot 70 Filter 1.gif
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Wed May 24, 2023 8:33 pm

The ROBOT model 70 manual states resonance frequencies of 1000 Hz and 2600 Hz....... I will see.
This morning I had the opportunity to make a photo of the oscilloscope picture of the demodulated video signal of a sawtooth / ramp video picture.

ROBOT_70A.JPG
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This should have been an upgoing ramp. We see that the 2600 Hz resonator is too low in frequency. Also other pictures are bad to look at. I had no time to measure the frequencies, that is the next thing that I am going to do. I have to change my set-up on the desk of my shack, it is now too crowded to add the tone generator.
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Thu May 25, 2023 4:00 am

I measured the resonance frequencies:

The high note resonance circuit: 1830 Hz, (far too low)
The low note resonance circuit: 840 Hz, (something too low)
The 1200 Hz resonance circuit (in 80A, not in 80): 1234 Hz. (slightly too high)

So there is quite something to do. Finrstly I am going to measure the resistor values and replace them (if needed) for the correct values. But may be I firstly replace them by an adjustable resistor, to tune the resonance frequencies. And then solder in a fixed resistor.

I observed that the existing resistors are quite small resistors, that I called in the past as 1/8 watt resistors.

Coming week. This weekend pentecost with a ham fest in the Netherlands..
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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby kd2bd » Thu May 25, 2023 4:17 am

Klaas Robers wrote:This should have been an upgoing ramp. We see that the 2600 Hz resonator is too low in frequency.

That is a strong indication that the resistors that connect between the junction of the two 0.022uF capacitors and ground have increased in value with age. That would be R10 and R119 in the schematic below:

Robot70_demod.png
Robot70_demod.png (96.59 KiB) Viewed 4980 times

In reality, probably all the resistors in the circuit have increased in value with age. I have a Robot 70 monitor that behaves in exactly the same way, very likely due the very same reason.

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

Postby Steve Anderson » Thu May 25, 2023 11:10 am

kd2bd wrote:In reality, probably all the resistors in the circuit have increased in value with age. 73 de John, KD2BD

...I would say more than likely. I agree with John, time to consider replacing all resistors of the same type. Quite an undertaking, but at least no others are going to drift in the future...
Maybe do the same for all PCB-mounted electrolytic caps while you've got the thing in bits...after all the thing is probably 40, 50, or more years old....and like everything, built to a price....life-span, usually (for consumer items) 10 years...though these days it's often planned obsolescence that kills the usefulness of a device...mobile phones being a prime example...

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Re: Robot Research Model 70 SSTV Slow Scan TV working

Postby Klaas Robers » Thu May 25, 2023 7:31 pm

I will see what it brings. But when a resistor did not change its value in the past 50 years, why would it do in the coming 50 years? So I will not replace those resistors. I have seen in the past, when checking my second hand Heathkit SB-101, that quite some resistors of the same make (A&B), did not change their value, and some (most of them the high ohmic ones) did go to a considerably higher resistance.

The same is with capacitors, even electrolytic capacitors. I had very little problems with them. I have still from my time as a student, about 1965, a militairy wireless set no 19, from 1943, that still works as it should do. I specially admire the makers of the vacuum tubes, although that I have a spare for each of the tubes, in an original WS-19 spare tube container of course, they still are working and they still contain their original vacuum. Switch the set to ON, working! Well done!
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