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ZX81 back porch fix using LM1881

ArtemKuchinArtemKuchin
edited July 2012 in Hardware
Hello!

Has anyone here did the back porch fix for ZX81 using LM1881?
I did it this weekend and instead of clear black on white i got black on dark grey (before it was black on black). So, the solution is unsatisfactory for now.

I have found this thread:
http://www.rwapservices.co.uk/ZX80_ZX81/forums/composite-mod-voltages-t622s20.html-sid=84097263317ce0c752d9e0f76f020a15

and has built a version with just a normal output on LM1881 + BAT46. However, it did not work. Maybe diode is faulty, i don't know.

I have rebuilt it using the original schematic with LM1881 (which used 3 buffers out of 6 on LS07).
This one:
http://www.rwapservices.co.uk/ZX80_ZX81/forums/download/file.php?id=536


It did work. However, when i look at the burst/back porche pin output on LM1881 on my scope here is what i see:
1.gif


The signal is very noisy. I tried it with the original PS and with a switching PS. I also added a small power supply filter made from 10u +0.1u caps. Nothing helps, this noise is always there. Is it normal?

But the main problem is that the picture is still barely visible. It is not black on black anymore, but more like black on dark grey. Also, the picture has a lot vertical aftifacts (darker/ligher vertical lines). Probably from all this noise.

They scope screenshot has 100mv/div. So, the whole signal is 200mv p-to-p. The ULA video output is a lot larger.
I just don't understand from the thread. Am i supposed to feed the output of the circuit to a transistor amp? Or the the RF modulator (cut the track and feed it to the RF modular) or directly to composite input?

One more thought: The voltage divider in the circuit has overall 225 Ohms between 5V and GND, so, it takes 20mA + 1881 takes at least 10mA + LS07 takes at least 10mA. SO, total current consumption is about 40-50mA in idle mode. I guess in normal operation mode it eats up to 0.1A. I'm not sure, but it seems like everything is hotter when i connect the circuit. ULA is over 40C for example. 7805 heat sink cannot be touch for more than 1 second. Is it normal?
Also, the measured voltage from 7805 is not 5V but 4.86. Is it okay?


Best regards,
Artem
Post edited by ArtemKuchin on

Comments

  • bobsbobs
    edited July 2012
    Have you tried posting this on the ZX81 Forums which RWAP hosts?
    https://bobs-stuff.itch.io
    https://twitter.com/bobsstuffgames
    https://www.facebook.com/bobs.stuff
    http://www.bobs-stuff.co.uk
  • ArtemKuchinArtemKuchin
    edited July 2012
    bobs wrote: »
    Have you tried posting this on the ZX81 Forums which RWAP hosts?

    The thread there is way way way old. It shows no activity since then, so, i thought the topic is dead.
  • mcleod_ideafixmcleod_ideafix
    edited July 2012
    I'm building one this video fixers, using a variation of the LM1881 circuit, designed by Jose Leandro. I will post my results once it's complete.
  • ArtemKuchinArtemKuchin
    edited July 2012
    mcleod_ideafix wrote: »
    I'm building one this video fixers, using a variation of the LM1881 circuit, designed by Jose Leandro. I will post my results once it's complete.

    Meanwhile, can you post a link to the schematic you are using?

    I found this one
    http://www.speccy.org/trastero/cosas/JL/zx81%282%29/ZX81-video.html

    And i don't understand the problem the author has. If DC component is the problem then just use AC coupling through a capacitor (10u would find). What's the deal?
  • mcleod_ideafixmcleod_ideafix
    edited July 2012
    That's the one I'm using, slighly modified by adding an emitter-follower.

    I think he's talking about the point in which the original signal mixes with the back porch generated by the LM1881. R1 along with R2 are used to calibrate the DC component of the incoming signal, and the DC level of the back porch, so that the mixing give us a signal with a well-defined black level.

    As he says, without this calibration, the ULA video signal varies between 2V and 4V, but the generated back porch level is 0.7V so the mixing would generate a signal with a black level even lower than the sync level.

    The signal itself is AC-coupled just before entering the LM1881 chip.
  • ArtemKuchinArtemKuchin
    edited July 2012
    Oh well, i see. He ac couples it for video input, but composite out is mixed BEFORE ac cap, so, it suffers from DC shift.

    Look at this:
    http://www.rwapservices.co.uk/ZX80_ZX81/forums/download/file.php?id=536
    The input is AC coupled to everywhere - no problem.

    The thing is, look at my o-scope picture. The backporch level is probably around -50mV.
    So, what 0.7V he means and relative to what - i don't know.
  • mcleod_ideafixmcleod_ideafix
    edited July 2012
    Well... the thing is that this actually works :)

    Here's the tiny board. I didn't measure the room within the modulator case. It doesn't fit into it by a couple of milimeters.
    zx81_video_fix_p1.jpg

    Luckily, there's plenty of space between the Z80 and the RAM.
    zx81_video_fix_p2.jpg

    After poking with the two adjustment pots, here's what I've got:
    zx81_video_fix_p3.jpg

    A couple of demos. One of them for the unexpanded 1K ZX81 (although I had plugged the RAM Pack by then)...
    zx81_video_fix_p4.jpg

    ...and the other for 16K machines (so I can test the RAM Pack)
    zx81_video_fix_p5.jpg

    zx81_video_fix_p6.jpg

    zx81_video_fix_p7.jpg

    And this is the schematic I've used. As I stated before, it's Jose Leandro's version, just with a buffer transistor at its output
    zx81_video_fix_sch.png
  • zxbrunozxbruno
    edited July 2012
    Imagine the "No limits" demo if it had Yerzmyey's music. :)
  • ArtemKuchinArtemKuchin
    edited July 2012
    Good it works. But can you try it on a more modern LCD TV?
  • Jose_LeandroJose_Leandro
    edited July 2012
    Hi
    ArtemKuchin wrote: »
    Good it works. But can you try it on a more modern LCD TV?

    Well, i'm late :-)

    Mcleod explain perfectly how circuit works. Maybe this last point . R1 and R2 also help with the diferent zx81 and ts1000 version you can found. If you see the video signal in a TS1000 you will see a "different" signal that a zx81. The video signal of zx81 ULA is very sensible with the imperance that it founds.

    Yes, you can use with a modern LCD. Don't connect the modulator (you have composite video) and put the buffer transistor like mcleod said.

    bye

    Jos? Leandro
  • mcleod_ideafixmcleod_ideafix
    edited July 2012
    ArtemKuchin wrote: »
    Good it works. But can you try it on a more modern LCD TV?

    Of course. Here you have it :)
    zx81_video_fix_p8.jpg
  • SpiranthoSpirantho
    edited July 2012
    This is very similar to the ZXVid solution I produce. Main difference is that my custom PCB is a bit smaller - we made it to fit into the modulator casing.

    The LM1881 works really well, on all TVs I've seen so far!
  • pmsrpmsr
    edited July 2012
    I will be building this video fix just with through-hole components (using a LM1881N). Don't see any surprises coming up, and the biggest challenge will probably be packing it together as to make it as small as possible. Any tips from the pros?

    /Pedro
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