SSTV-625 up-converter MkII

[Steve Anderson]

It's a policy I have that I place a 100nF disc-ceramic capacitor right next the input and output of each and every regulator. And I mean right next to. On stripboard/matrix board in the adjacent 0.1" holes - that close! Disc ceramic only, they're as cheap as chips, buy them by the hundred. I've never had a regulator go unstable.

While on the subject, the same sort of capacitor on every chip, every, no exceptions. Right on the power pins on the underside of the board. Scatter a few more around the board. In this case you'll need two on the D-A as it has +/- supplies.

Usually I'm not quite so 'over the top' as this, but in this case, due to the speeds involved I don't want to take any chances. On stripboard like this and with things as close together as they are, there's no way to have a decent ground-plane.

Steve A.

[acl]

All noted Steve

[Steve Anderson]

I'm sure that 100MHz PICs are in the pipeline - that'll be interesting! Maybe at 1.8V to reduce dissipation. Interesting times ahead. A change to 23A1024 RAM chips, and you're in business...

...D-A may need scrutiny though and potential replacement.

Steve A.

[acl]

Hi there,

Some good feedback on the possibility of producing a PCB for the main processor board. This will simplify the build and ensure that the results are consistence, minimise connection lengths between devices and, will provide suitable ground planes to minimise stray crosstalk. I don't mind putting some money in the pot to get 10 boards built assuming that the layout has been produced between one of the forum member's.

Regards Chris Lewis

[Steve Anderson]

Thanks for the offer Chris, but I'm hoping that this will appeal to the NBTV majority here, the SSTV part could be viewed as a 'bolt-on goody'. So it is possible that more than 10 kits may be purchased. By kit (at this stage) I mean just the two pre-programmed processors and the PCB. The builder would need to source all the other parts.

But if a few ask for a complete kit (just the PCB components, not pots/switches etc) there's no reason why not. Also there may be those that would prefer a built and tested board - ready to go. which is not out of the question.

Anyway, first I need to get the thing going! I had a few doubts yesterday regarding the RAM, but after studying the datasheet I'm satisfied my plan is workable.

I'm still in 'board-build mode', I've had a few distractions over the past week.

Steve A.

[Steve Anderson]

Mentioning kits above sent me looking for sources of the major devices. The two processors and the RAM chips are in current production but The D-A is now a mature device and not many UK/European suppliers stock it. Be careful, it's a DAC-08 (or DAC08) NOT a DAC0808, that's a different device. The TI DAC0800 looks much the same but I need to study the datasheet in detail first.

RS in UK seem to have the DAC-08, but they're not too keen on dealing with hobbyists, preferring business-to-business customers. Cricklewood Electronics have it but they don't show their stock-level on the site. If anyone knows of a supplier where they're at a reasonable price, under five quid, with sane stock levels that would be useful data. E-bay suppliers can be erratic at times.

So it may be worth at least considering a re-design of the D-A and output stages. I would prefer not to go to a R2R array though.

Steve A.

The Texas Instruments DAC0800 is the same device and is a drop-in replacement for the Philips DAC-08 (DAC08). With that supplies should be around for a few years yet. So for the time being I'm going to leave things as they are. However I will be looking to update the D-A/output stages in time.

[kd2bd]

The August and September 1982 issues of "Radio-Electronics" magazine carried a "Picture Phone" project that appeared to be very closely related in design to a Robot 400 SSTV scan converter. It was billed as a Model 535 Picture Phone, and the full schematic is available through archive.org:

https://archive.org/stream/radio_electr ... 6/mode/1up

and

https://archive.org/stream/radio_electr ... 4/mode/1up


73 de John, KD2BD

[kd2bd]

Here's a PDF of the series of Radio-Electronics articles describing the Robot Model 535 SSTV Picture Phone back in 1982.

73 de John, KD2BD

[acl]

During construction of a project I initially mount the equipment on a plywood baseboard. This allows easy access to test equipment and elevates damaging your instrument case. It also reduces the possibility of errors when committing the final project to it's final housing.

Regards Chris Lewis

Image reduced in size to make in-line, Steve A.

[Steve Anderson]

The August and September 1982 issues of "Radio-Electronics" magazine carried a "Picture Phone" project that appeared to be very closely related in design to a Robot 400 SSTV scan converter.

In many ways it appears to be a cut-down version of the 400. Presumably a fixed standard and many features omitted. But it's interesting, I wonder how many were built? (If any). It would be a major undertaking, not for the faint-hearted.

The links to archive.org appear dead, maybe traffic, I'll try again later.

Steve A.

Links now work but the pdf appears to be the same thing...

[Steve Anderson]

During construction of a project I initially mount the equipment on a plywood baseboard...

Not a daft idea, I tend to have mine strewn around the workbench, but it gets things done, even if somewhat messy. Yes, on occasions things do get zapped. Oh well..

I've decided to go with the MkIII version without rebuilding the board and moved quite a few data lines around to make things a bit more sane.

So expect a MkIII thread to pop up soon.

Steve A.

[Klaas Robers]

Steve, Conrad.de sells the DAC 08 of Analog Devices for € 4.22 each. See documemtation: http://www.produktinfo.conrad.com/datenblaetter/1200000-1299999/001247182-da-01-en-IC_DAC_8BIT_MULT_HS_DAC08CPZ_DIP_16_ADI.pdf

[Steve Anderson]

Thanks Klaas. There are still a number of suppliers of this D-A, but they take a little bit of digging to find. There are a number of very similar devices but they either lack one thing or require additional inputs/clocks. The DAC-08/DAC0800 can be viewed as an upgraded R2R ladder where you don't need precision resistors and the slight difference in driver output voltages is taken care of. The interesting thing is there's no direct replacement for it, I guess the demand has simply dropped off.

You could use a 10/12/16-bit D-A as long as it had an appropriate response time and simply ignore the lower-order bits, but these can become expensive.

Steve A.

[Klaas Robers]

I think Steve, it was a simple IC-design that exactly did what was needed: an 8-bit DAC. There never was any reason to make a "better", or at least different design. Then it keeps selling.

There are more of these examples. The BC107 - BC109 and BC177 - BC179 range of transistors from 1970 (?) still exist, although in a plastic enclosure as BC547 - BC549 and BC557 - BC559. Almost 50 years old.

I once used the DAC08, however in reverse. I didn't wat it to give me a negative output. I don't know any more how I did it, but it worked..... Happy that the IC is still made and sold.

[Andrew Davie]

Perhaps AliExpress has them? Not sure if these are the same things?

https://www.aliexpress.com/wholesale?SearchText=dac-08