5AHP7A Vintage SSTV The Deep image

[Steve Anderson]

Harry, that chart/curve was generated before I added the two pre-set pots.

So I've normalized it to be 0V at 1.2kHz and 1.1V at 2.3kHz. Hopefully you'll get something similar.

Steve A.

A note to the wise...I repeated this three times then averaged the results I obtained, it does help smooth out some of the measurement errors.

[Steve Anderson]

I noticed that in the batch of wave files I created I omitted three...here they are...

Steve A.

[Harry Dalek]

I noticed that in the batch of wave files I created I omitted three...here they are...

Steve A.

Ok Steve will move them to my folder i will put it on a usb stick for when i do the testing.

[Steve Anderson]

As a matter of interest I simulated the demodulator (again) and noted the results. After normalising everything the fit between the measured and simulated results isn't that bad. In the chart below the dark blue curve is the simulated demodulator, the magenta the values measured on the prototype.

From 1200Hz to 2300Hz the fit is amazingly good, the difference is either measurement errors or component tolerances. Although the resistors are 1% in the prototype, the capacitors are 5%. The values in the simulator of course are 'perfect'.

Above 2300Hz probably component tolerances play the most part in the difference between the curves.

Below 1100Hz is quite interesting. The difference is too great to be measurement errors or component tolerances. Something else is happening here. But I'm not worried about it, just interested as to why?

If Harry you can do the same measurements it would be interesting to add a third curve.

Steve A.

[Harry Dalek]

The two presets are:-

VR101 allows you to set the sync-tips to 0V or a small offset if needed.

HI Steve reviewing this post due to a problem ,I don't have much time this early in the week for a lot of testing but i think i have a problem as i tried to look into the test this afternoon and i need to clear up the testing as i forgot to ask the method of testing the instrument .

VR102 sets the overall video amplitude.

[/quote]

Here i have a problem it does not so i must have a mistake in this area ...VR101 moves the waveform up or down on the scope to some degree like a position control ,but VR102 theres no effect on voltage or signal at all .

Only thing that adjusts the amplitude is to adjust the volume control of the laptop.

I tend to think of SSTV as being 1kHz/V so 2.3kHz - 1.2kHz = 1.1kHz, so I chose 1.1V from sync-tips to white level as a standard here in the absence of any other. You can choose what you like, it's the FM sub-carrier that does have standards.

Mine must be working to some degree as the different frequencies do change voltage out at C1
I had it in my head to do this with the scope but with the slight differences frequency to voltage out a multimeter is what we are doing here ? i am also a little confused doesn't take much to get me in this state i have to point out we have both AC and DC at C1 what is needed here if its a meter reading .

Due to the problem of the second trimmer part of the circuit till i have time to fault find ...i gave up on the test just doing what i could to see results .

With volume on the lap top full checking 3 frequencies 1.5khz adjusting the range of VR101 testing one end to the other taking a reading at C1 AC meter reading .......1200hz is 4.1v 8.8v 1500hz that was 4.5v to 9.3v.........2300hz 7.0v 12.2v on DC readings are all about half those AC results

I could not set VR101 on 0 volts even with no audio into the device ...sounds like i have made a mistake some where might be a little one but it is causing enough to confuse me .

Looking at the amplitude of the different frequencies on my scope all on the low end Black look third the size to what i see towards white 2300hz.

Sorry for being a pain here might be best to wait till i check the circuit over again and make sure i track down the problem with the VR102.

[Steve Anderson]

...VR101 moves the waveform up or down on the scope to some degree like a position control ,but VR102 theres no effect on voltage or signal at all.

That's exactly how it should operate! So, that means IC101d is to a large extent doing its job and we can for the moment IC103, both parts are passing signals.

VR102, have you missed the 0V connection?

Measurements at 1C should be taken with a DC voltmeter, but also feed a scope at the same time to make sure there's little/no ripple. With the test tones I uploaded in an ideal world you should simply get pure DC out, different of course for all frequencies. Forget AC measurements, what you're hoping for is no AC at all - at this stage.

Only thing that adjusts the amplitude is to adjust the volume control of the laptop.

Very wrong! Except at very low signal levels it shouldn't make any difference whatever the laptop puts out, that's why IC101a is there, to level-off (limit) all signals. with a signal you should get a square-wave of 20-24V p-p at the output of IC101a.

Mine must be working to some degree as the different frequencies do change voltage out at C1

As above check with a scope, it should be DC with a slight ripple of twice the input frequency upon it. The largest ripple will be at the lowest frequencies, at 1500Hz and above it should almost be invisible.

With volume on the lap top full checking 3 frequencies 1.5khz adjusting the range of VR101 testing one end to the other taking a reading at C1 AC meter reading .......

Until these other errors are sorted out there's little point in these numbers.

I could not set VR101 on 0 volts even with no audio into the device ..

At this stage it could do anything with no signal in, always have a signal present. Set VR101 only when you have a source of 1200Hz, you may be able to set it at other nearby frequencies but that's not the idea.

Steve A.

[Harry Dalek]

HI Steve

That's exactly how it should operate! So, that means IC101d is to a large extent doing its job and we can for the moment IC103, both parts are passing signals.

VR102, have you missed the 0V connection?

I was thinking the first trimmer was ok and running the results by you i am reassured...on VR102 something is up i need to have a good look would i may have missed and yes perhaps i will check .

Measurements at 1C should be taken with a DC voltmeter

OK this is what i started with and same time scope .

, but also feed a scope at the same time to make sure there's little/no ripple. With the test tones I uploaded in an ideal world you should simply get pure DC out, different of course for all frequencies. Forget AC measurements, what you're hoping for is no AC at all - at this stage.

On the scope 1C test point i noticed waveform of one of the frequencies i was testing which i should of checked with the others the sine wave amplitude looked only on the positive with every second wave a half hight ,I should take a photo .

Very wrong! Except at very low signal levels it shouldn't make any difference whatever the laptop puts out, that's why IC101a is there, to level-off (limit) all signals. with a signal you should get a square-wave of 20-24V p-p at the output of IC101a.

OH i forgot we have a limiter still thinking in NBTV mode volume should not change signal ,i will check .

As above check with a scope, it should be DC with a slight ripple of twice the input frequency upon it. The largest ripple will be at the lowest frequencies, at 1500Hz and above it should almost be invisible.

Perhaps thats what i am seeing what i mentioned...i will check again

Until these other errors are sorted out there's little point in these numbers.

Agree need to check for mistakes ,pointless till its all working correctly the limiter as well darn wonder what i did here .

At this stage it could do anything with no signal in, always have a signal present. Set VR101 only when you have a source of 1200Hz, you may be able to set it at other nearby frequencies but that's not the idea.

Steve A.

Well i will make sure next time i turn it on to do this but looks like i was not as lucky as i thought and need to fault find ,let you know what i did !

[Steve Anderson]

Not being 'funny' here Harry, but both Klaas and I have mentioned this in the past - build and test in stages, don't build it all in one go and hope it will work. It usually won't, and you'll be trying to find several faults at once.

Here I would suggest building the limiter first (IC101a), test it's output. If it works - great, if it doesn't it's a simple stage with few components so the fault should be easy to find.

Then add one of the filters, say IC101b, it works? Great. It doesn't? It can only be in the small bit you've added as you know the limiter works already.

Then add the other filter, IC101c...continue as above.

Then add one full-wave rectifier, say IC102c and IC102d - test............and so on. Break it down into bite-sized pieces. It may not seem it but it's quicker and far less frustrating.

If you're not sure what each stage does and what to expect you only have to ask.

Steve A.

[Harry Dalek]

Yes Steve need to slow down my enthusiasm for soldering and take smaller steps i will take that in .
Since i made it i really didn't have much time to check it over more than i should .
Before i disconnected it today to give it a good check over i did one test for the limiter but double checking the C1 test point it works as far as using the reversing wav its pretty much stays same level till the volume gets to low and distorts the waveform .....but on testing 2300hz and lowering the laptop volume it does shrink the sine wave .
More time tomorrow to double check the wiring on the limiter did a Conductivity test where go's where looked fine .
I will check doing the square wave out of it test you mentions a few posts ago but i only wanted to check the wiring now till i know whats correct and whats not more i just need time to sit down and look .

[Klaas Robers]

That was a good idea, to measure the characteristic of the FM-detector. I did this too: Tone generator + frequency counter to limiter -> FM-detector -> digital multimeter, some Excel data logging and a few hours of nice work. This is the result:

FM-det-KLS.GIF (7.93 KiB) Viewed 10960 times

The purple line is the measured output of the FM-detector, the blue line is a straight line that fits the measurements best between 1200 and 2300 Hz.
On the lower side: below 1000 Hz the output becomes somewhat wavy. May be that the slight nonlinearity there is some mismeasuring of the multimeter as a result of the wavyness.
On the higher side: also some deviations, but this line continues until 3800 Hz. Then it falls back suddenly.

[Steve Anderson]

Certainly a nice result Klaas. I assume some form of zero-crossing or pulse-counting technique? I've read many an account where one person says it's 'better' than using filters or tuned circuits and others who say the converse. Without having tried it I don't have an opinion either way. I guess I should have a go at it. It probably would end up being simpler than the re-worked Robot 70 design.

My major concern with any of these demodulators is 'that' output filter. The overshoot, especially on the left-hand edge of the picture, really does deserve some attention. The resolution is to me a secondary concern...at the moment.

Steve A.

[Klaas Robers]

It is indeed a so called "counting detector". Those need a hard limiter as a start, one that outputs a binary signal. Then a digital "one shot", e.g a 74121, fires a fixed width pulse on the positive as well as on the negative edges of the limited input signal. Thats all. The "mean value" of the pulse train is the detected video signal. The low pass filter reconstructs this mean value video signal.
It is rather recently that I posted the circuit diagrams in this thread or in the SSTV thread. You may load that down and look at the diagrams. The limiter is an old TAA350 limiting amplifier for TV sound IF. The one shot is a DTL version of the TTL 74123. Better in this case would have been the retriggerable 74121, which has no fall back at (my) 3800 Hz, but then stays up.
As the pulses are started at both edges, their repetition rate is twice the input frequency, so at 1200 Hz input signal the pulses come at 2400 per sec. That eases the work of the low pass filter, but you also may say: in an FM signal the information is in the zero crossings. That implies that it is in de positive going AND in the negative going crossings. So both crossings should be used to get all information out of the modulation.
Look at the zero crossings as sampling points of the video information. If you use only upgoing zero crossings the sample rate will be too low at e.g. 1500 Hz for a video bandwidth of 900 Hz.

[Harry Dalek]

Had a good look with my meter looks like i missed a resistor R116 10k apart from that no wiring mistakes so far up to ic101d,might explain why the 5k vr102 is not doing much ,i will add the resistor see what happens .
BTW Steve just checking is VR101 a 50k as the zero looks like a D typing mistake just want make sure i made mine a 50k.

BTW just reading Klass post above mine had me wondering why the PLL was not used more on the sync side of things ,i recall it only being used once in a SSTV design would of been very good for DX at the time ,i had on used one for Noaa weather satellites to lock onto the tone it could lock onto the frequency in the snow way before you could hear any thing think it was a 567 tone decoder .

[Klaas Robers]

Steve, this is the circuit diagram of the limiter, frequency doubler and one-shot counting FM-detector:


The FCH191 is a DTL version of the 7404, the FCK101 is a DTL version of the 74123.
The input voltage has a dynamic range of 100 dB. Virtually every input level is acceptable.
The max. input voltage of 30 Veff is limited by the dissipation of the 4k7 input resistor.
The video output is made 600 ohm and goes to a passive (L-C) 7th order Butterworth low pass filter.
The output "lim'd" is the limited audio signal and goes to the 1200Hz sync resonator/detector.

[Steve Anderson]

Certainly omitting R116 won't help, anyway, that's one thing fixed.

VR101 is 50k. Exporting these files into a forum-friendly size sometimes the resolution suffers. The original is definitely 50k, the zero has somehow become distorted. VR102 is 5k.

Thanks for the info Klaas, it's given me an idea that I need to try out. Until I've had a go at it I'm not sure if I'm on the right track. More to come.

Steve A.