Vintage SSTV transmit converter

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: Vintage SSTV transmit converter

Postby acl » Wed Mar 20, 2024 8:34 pm

I have started static testing on the digital section of the converter. It is so easy to miss slivers of of copper tracking across tracks after breaking. I have hardwired all supplies to chips with tinned copper wire and intend to connect remaining inter-ICs connections with wire wrapping cable underneath the board. The most useful tool in my armoury is the common logic probe. Here I can quickly 'Wizz' round the IC legs and verify power supplies are connected correctly (zero volts green and five volts red) and check for clear connections. By using a resistor connected to a supply line and using the probe the other side of destination I can check continuity of a wire link very quickly.

IMG_4694.JPG
Static testing
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Vintage SSTV transmit converter

Postby acl » Tue Mar 26, 2024 6:49 am

Remaining connections made underneath board with 30 gauge neoprene high temperature wire-wrap cable. Notice I have made 'eyelets' out of tinned copper wire to support cable runs. Next step static testing the remaining connections.

IMG_4696.JPG
Underneath digital board
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Re: Vintage SSTV transmit converter

Postby acl » Wed Mar 27, 2024 8:18 am

Well here we go. Main oscillator on Digital board up and running.


IMG_4699.JPG


IMG_4700.JPG


ttl.jpg
Useful one source supplier of all my TTL chips
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Re: Vintage SSTV transmit converter

Postby Harry Dalek » Wed Mar 27, 2024 5:36 pm

acl wrote:I have started static testing on the digital section of the converter. It is so easy to miss slivers of of copper tracking across tracks after breaking. I have hardwired all supplies to chips with tinned copper wire and intend to connect remaining inter-ICs connections with wire wrapping cable underneath the board. The most useful tool in my armoury is the common logic probe. Here I can quickly 'Wizz' round the IC legs and verify power supplies are connected correctly (zero volts green and five volts red) and check for clear connections. By using a resistor connected to a supply line and using the probe the other side of destination I can check continuity of a wire link very quickly.


Arrrr i see the Beast come to life ! What is that big yellow thing Chis !
Ok similar clock results so far ,i am really curious when how it all go's and the output of the A/D chip infact a few of them will give me an idea what went wrong .
This is not so much a thing i need to work or want desperately but something i would love to see work correctly .
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: Vintage SSTV transmit converter

Postby Steve Anderson » Wed Mar 27, 2024 7:55 pm

Harry, bear in mind the A-D will be converting an analogue value into a 0-63 binary value, the output of which will most likely look like garbage on a simple 'scope. A logic analyser might help here, but unlikely. You simply have to trust it's working as advertised.

A small change (78mV in a 0-5V range) on the input could change the output from 011111 to 100000, and this could be happening at MHz rates, apparently randomly. Aside from confirming logic voltage levels, a 'scope is almost useless here, except under certain and planned triggering arrangements. Not difficult, but requires forethought.

You simply have to trust it 'works' as you/they thought. The 'acid test' of design.

As I've mentioned before, I don't think that 525/625 video is a good source for conversion to SSTV. I could go into a long diatribe why, but for now, just take it as my opinion, for whatever it's worth.

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Am I missing something?

Postby acl » Fri Mar 29, 2024 4:00 am

2.jpg


Following through Monostable U2 with video input

U2 pin13.jpg
Vertical synch timing
U2 pin13.jpg (24.51 KiB) Viewed 268 times


pic_207_2.bmp
U2 pin 13
pic_207_2.bmp (1.1 MiB) Viewed 268 times


U2 pin 5.jpg
Horizontal synch timing
U2 pin 5.jpg (22.97 KiB) Viewed 268 times


pic_207_3.bmp
U2 Pin 5
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Vintage SSTV transmit converter

Postby acl » Fri Mar 29, 2024 7:31 am

pic_209_1.bmp
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Yellow image pin 11 on U3. Blue image pins 8 and 12. Basically showing U3 is dividing the input by a factor of two as described in article. No more waveforms to look at yet as the analogue output stage is required to provide feedback to this board in order to function correctly.
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Re: Vintage SSTV transmit converter

Postby Harry Dalek » Sat Mar 30, 2024 12:34 pm

Steve Anderson wrote:Harry, bear in mind the A-D will be converting an analogue value into a 0-63 binary value, the output of which will most likely look like garbage on a simple 'scope. A logic analyser might help here, but unlikely. You simply have to trust it's working as advertised.

A small change (78mV in a 0-5V range) on the input could change the output from 011111 to 100000, and this could be happening at MHz rates, apparently randomly. Aside from confirming logic voltage levels, a 'scope is almost useless here, except under certain and planned triggering arrangements. Not difficult, but requires forethought.

You simply have to trust it 'works' as you/they thought. The 'acid test' of design.

As I've mentioned before, I don't think that 525/625 video is a good source for conversion to SSTV. I could go into a long diatribe why, but for now, just take it as my opinion, for whatever it's worth.

Steve A.


Well i did give it a go and i am not sure its worth finding new chips for what i think could be faulty depends too on how chris gos on this ,i found problems which should not of been there on the timing ,i was very confused as to how the same TLL ic via different brands gave different results ..again i have to see how its constructed here .
Again its one of those things Steve is what it is better or worse ways a bit of the past i would like to see working correctly chris has much more experience at this sort of thing and you of cause but here its interesting to see how he's build gos to what i saw .
Looks good Chris clean pulses .
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: Vintage SSTV transmit converter

Postby acl » Mon Apr 08, 2024 1:07 am

Well, that's the output stage completed. Ran out of stripboard space so the last IC will have to be hardwired underneath board. I had to 'jiggle' op amp elements to make it easier to layout. When the final chips arrive from China some serious testing can begin. Harry I will have a spare set of LS chips I can send you when testing is complete.

Regards Chris Lewis

IMG_4701.JPG
Analogue section
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Re: Vintage SSTV transmit converter

Postby Steve Anderson » Tue Apr 09, 2024 3:12 pm

I must admit it's tempting to do an update on this device. One way would be to replace a large amount of the logic and the output circuitry with a micro. But let's suppose they're not available yet, though I'm tempted to do just that in the future.

First thing that I thought of was to replace the LS chips with either their HC or AC equivalent. Most do have an AC or HC version. That would drop the +5V current significantly. They're not always a 'drop-in' replacement, so care is needed.

Next would be replace U16 with a ' 574 which is functionally the same but with a 'broadside' I/O, all the inputs are on the left, all outputs on the right side of the chip. Designed for buss-orientated layout. Also change the D-A resistor arrangement to a R-2R ladder, just two resistor values required, say 1k1 and 2k2. Equal loading on each output resulting in more consistant voltage levels.

Fig1 in the original EA article I would simply start over, it surely works but could do with a 2024 make-over and provide black-level clamping as well as colour subcarrier rejection to prevent ailiasing.

There is nothing wrong as far as I can see with the original, it just could do with an update. Also the 566 VCO though good is getting hard to source, at least here. A dual op-amp and a few other passives could replace it. Or use the VCO section of a 4046, though I'm not so sure about that. (Temperature and supply voltage sensitivity headaches.)

Let's see what issues Chris and Harry have with this and deal with them as they arise.

On a general note, the CA3306 could do with upgrading from a 6-bit device to 8-bit, the rest of the circuit is quite capable of handling 8-bits with minor alterations, but 8-bit A-Ds capable of digitising 625 video are few and far between...which is odd and/or expensive these days.

There is a way to 'cascade' two CA3306s to provide a 7-bit output which I suppose is a step in the right direction, and given the source of material might be 'good enough'.

BTW, I haven't forgot 525 sources...though they do have their own 'gotchas'...

I also haven't forgotten stills, JPGs and the like, probably a more varied source of material for most, certainly me, to convert to SSTV. Time/speed problems can be forgotten...but a bit of effort is required in composing the original still for conversion to 128-line SSTV. But the same could be said of 'frame grabbing' video.

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Vintage SSTV transmit converter

Postby acl » Tue Apr 09, 2024 7:25 pm

Thanks for your input Steve,

I'm slowly getting through the testing phase. A lot of static testing before fitting chips. I managed to get the LS devices quite easy on eBay from a quality UK supplier who managed to supply the whole set of devices. The only long lead time device was the 566 supplied from China.

Regards Chris Lewis
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Re: Vintage SSTV transmit converter

Postby Steve Anderson » Tue Apr 09, 2024 8:47 pm

No surprise with the 566 long lead-time, very few places seem to have them now. Easier to replace it (as I did) with an 8-pin micro and some simple DDS (Direct Digital Synthesis) software. But the theme I'm following here is to avoid software and simply bring this closer to the year 2000, not almost 25 years beyond.

But the issue I'd really like to solve is the A-D, surely there's an 8-bit, 10MHz A-D out there at a sane price and in hobbyist friendly packaging? No luck so far...even in SOIC packaging, which is just about 'usable' to the likes of us lot with 1.27mm pin-spacing.

The D-A bit is easy.

Steve A.

If using software it could be shrunk down to perhaps 3-4 chips, and maybe 50mA of current....but don't hold me to that!
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Re: Vintage SSTV transmit converter

Postby Harry Dalek » Tue Apr 09, 2024 9:24 pm

acl wrote:Well, that's the output stage completed. Ran out of stripboard space so the last IC will have to be hardwired underneath board. I had to 'jiggle' op amp elements to make it easier to layout. When the final chips arrive from China some serious testing can begin. Harry I will have a spare set of LS chips I can send you when testing is complete.

Regards Chris Lewis

IMG_4701.JPG


Thanks Chris i would be just happy to see your results and what you used very costly these days to post stuff if your not living in China thats one thing that stopped me using ebay for private buyer or sellers over seas its one thing thats gone backwards in our era .
See how it go's, big problem i had was the correct 555 frequency just didn't match any thing to get a correct frame infact no where near it .
And again different results with different brand LS chips ! for me a LS should give the same result ...fine if its a TTL of a different age type but that really did put me off.
I am keen to see frequency scope results and match them to mine .
Good luck Chris i will keep an eye on this one .
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: Vintage SSTV transmit converter

Postby Steve Anderson » Wed Apr 10, 2024 6:25 pm

Be careful when working out things like monostable timings especially when using any internal components within a chip. We're used to external components being 10%, 5%, even 1% or better, but internal silicon-based components are at best 50% (yes, fifty percent) unless laser-trimmed in production. The datasheet will point that out for sure if true!

Speed and propagation delays also have a wide spread dependant on the quality of the silicon die within and ambient temperature. Board layout, particularly stray capacitance, can also be disastrous, also earth/0v track inductance. Keep layouts small and compact. Many of the logic chips we use, even old LS, can operate into the 100MHz region.

I'll not repeat my usual decoupling/bypassing sermon here...oh heck! I just did!

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Vintage SSTV transmit converter

Postby acl » Wed Apr 10, 2024 6:57 pm

All noted Steve,

I wondered why my timings were out. If critical I can trim externally. Can you check I am using the correct formular for calculating monostable timing. I think it was taken from the 555 data sheet but other sources show calculations are the same for TTL and LSTTL devices.

T =1.1x C1 x R1

Cheers Chris
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