Retro SSTV anyone?

[Steve Anderson]

A few minor changes to the demodulator...the output is now deliberately inverted as it makes sync detection and separation easier downstream. The circuit below shows my own version of the low-pass filter which is marginally better than the original Robot version. I very much doubt there would be any visible difference on the 3" screen I'll be initially using.

R131/132 could be either 2x18k or 2x22k, it makes no practical difference, 20k is simply the 'ideal' value.

The sync detection, separation and the timebase ramp generators are nearing completion.

Steve A.

For the circuit diagram see a few posts below...a very slight update.

[Harry Dalek]

That circuit is a thing of beauty Steve ,i will look forward to seeing the rest of the 2015 SSTV monitor circuits , i don't think a P7 crt will have had such well designed circuits for it ,it only had to wait 60 years ?

[Steve Anderson]

Thanks Harry, but as they say, "It's not over until the fat lady sings."

I've been doing a bit of re-work on the sync and ramp circuitry, adding things such as auto-blanking the CRT in the absence of syncs, but that can be defeated if desired. Which means I'm now straying into the CRT grid modulation bit.

I should be plugging in the soldering iron in the morning, hopefully some results from the above during the course of the day.

Steve A.

It will be later in the day - a planned 'net outage by the ISP...again....and that's after several unannounced ones over the past weeks. I'm considering telling them where the put their fibres and go back to copper.

[Steve Anderson]

A very slight update...no electrical changes though...

Also added is the simulator's plot of the two versions of the post-rectification filters. The green trace is the original Robot design, the red is the two-stage Butterworth version shown here. The passband gain of both is deliberately 0.5.

Steve A.

28/02/2015. A few changes to the demodulator circuit. I have also rationalised the component numbering. Updated circuit diagram and recent photo in posting of 28/02/2015 below.

[Harry Dalek]

Well my bet is that fat lady will sing this aria well Steve

like the use of the op amps replacing the normally used bridge rectifiers ,can't say i have seen this idea ever used in past monitors.

Its really good to see an updated SSTV circuit for the original system so i am finding it very interesting to see what you come up with for it .

[gary]

Well my bet is that fat lady will sing this aria well Steve

Let's hope that it's not "Vesti la giubba"...

[Steve Anderson]

Let's hope that it's not "Vesti la giubba"...

I could say, "Stop clowning around.", but I won't.

[Steve Anderson]

Sync detection and separation design proven and rationalised in a few places. Timebases/ramp generators perform much as expected.

Horizontal Timebase 1.gif shows extracted syncs and horizontal ramp.

Vertical Timebase 1.gif shows extracted vertical sync from amongst the horizontal and the eight-second vertical ramp. Note scope timebase set to 1s/div.

Vertical Timebase 2.gif is a zoom-in of the vertical sync pulse and its detection.

Steve A.

[Harry Dalek]

Looks good Steve , when you can can you post up this part of the circuit ?

I think we will have a transistor or 2 for the deflection side ? ,its all op amp so far .


Theres a fair few transistors in this part of the robot 70 circuit ,i suppose these days ics could replace a lot of this .

[Steve Anderson]

I'll probably put up the sync detection/separation and ramp generator circuit later today, it does have four transistors of the BC547/BC557 variety, plus a sprinkling of op-amps, comparators and a dual monostable.

The CRT grid PWM modulation will follow on, a 2N2222 and a 2N4403 plus op-amps and comparators. On the high-voltage grid circuit there's a 6N137 logic opto-isolator, a 2N5401 high-voltage PNP, and a 2N4403.

The deflection amplifiers which actually drive the CRT plates will only use four MPSA42 high-voltage transistors as their active devices. There's also a MPSA42 in each of the focus and astigmatism circuits.

After that lot there's only the power supply, entirely conventional, 78xx's, 79xx's, the high-voltages, both positive and negative, are unregulated.

Steve A.

When complete I will pull all this together with text, circuits, waveforms and photos in one pdf document such that those interested don't have to keep track of all the various bits posted here. This will have the final versions of circuits, those posted individually here should be considered interim.

[Steve Anderson]

A few changes to the demodulator circuit. I removed the Robot version of the post-rectification filter which freed up one section of the original IC102. The original IC101 therefore became redundant and was removed (sorry Bob). This means that the package count is now one less than the original Robot demodulator at three.

I have also rationalised the component numbering.

As usual I don't show supply decoupling/bypassing in the vast majority of circuits, it still needs to be done though. In this case there are 2x100n disc-ceramics and one 47uF electrolytic per supply rail.

Steve A.

[Harry Dalek]

Less is more as they say

Nice seeing it evolve Steve.

[Steve Anderson]

Nice seeing it evolve Steve.

Design of this nature is an iterative process, each subsequent step is smaller than the last until you reach a generally acceptable level of performance. There is one more iteration left for the demodulator but I'll leave that for the final version...nothing major, quite minor really.

I'm holding off on the sync/ramp stuff simply to reduce the number of versions that appear here. So it will 'probably' appear tomorrow.

Steve A,

[Steve Anderson]

Well 'tomorrow' came and went and after a hectic weekend and a busy Monday I decided to sit down and make a start on building the sync detection/separation circuits and the ramp generators. However those good intentions have been thwarted by the fact I'm totally out of the smaller stripboard I was planning to use for this. Some stock control!

I'll move on to other stuff which uses the larger sized stripboard until I can get across town to re-stock.

The sync detection and ramp waveforms posted previously were from a breadboarded version.

Steve A.

[Harry Dalek]

Well 'tomorrow' came and went and after a hectic weekend and a busy Monday I decided to sit down and make a start on building the sync detection/separation circuits and the ramp generators. However those good intentions have been thwarted by the fact I'm totally out of the smaller stripboard I was planning to use for this. Some stock control!

I'll move on to other stuff which uses the larger sized stripboard until I can get across town to re-stock.

The sync detection and ramp waveforms posted previously were from a breadboarded version.

Steve A.

Hi Steve all these electronic goodies are really all in your back yard ,every thing i tend to want to stock up on i order from some Asian country the 3 week wait or so tends to be well worth it, not like i have to pay postage ! and i don't .

ARRR the PCB a great invention i don't know what wire wrapping had going for it ,i am posting as thats a bit like what my sstv circuit board looked like handed to me ,i knew then why the person i got it from couldn't get it going but all the wires were green.

I don't mind strip board but my thing is Matrix PCB you can fit more on.

I would say good luck on the next part of the monitor but i know its not needed you'll get that thing going .