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quattrohead

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quattrohead last won the day on July 16 2018

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  1. BBC model B uses the same ram chips, clues here ?? http://www.amibay.com/showthread.php?19309-HM4816-RAM-Chips-for-Acorn-BBC http://mdfs.net/Info/Comp/BBC/Circuits/BBC/bbc.gif <Display> Perhaps the cleverest part of the entire BBC Micro’s circuit is the way that the 6845 CRTC is used. If you look at an application sheet for the 6845 you will see that it is supposed to access two different types of memory. An area of RAM, used to store ASCII codes and an area of ROM to generate the dot patterns of the characters to be displayed. The idea is that the output of the RAM is used to select a dot pattern from the ROM and the CRIC provides extra addressing to select each byte of data which make up the characters. Of course the BBC Micro (in modes 0 to 6) doesn’t use a ROM to generate the dot patterns as they are stored directly in RAM. Another interesting feature is the way that the 6845 is coaxed into providing colour information – the raw 6845 is a monochrome device. The key to understanding how the 6845 has been used in the BBC Micro is to notice that although its address lines are segregated into two groups – MA0 to MA13 and RA0 to RA2 – they can just as easily be used to address a single block of memory. In other words, the CRTC simply addresses the memory a certain number of times per TV scan line and what happens to the data that is produced is not its concern. Once the data has been selected by the CRTC it is sent (a byte at a time) to the video ULA (IC6) where it is serialised and applied as the address to a small area of static RAM – the palette RAM. This is responsible for adding the colour to the display. The output of the palette RAM is a four bit word indicating the state of the R(ed), G(reen), B(lue) and Flash signals. The RGB signal is fed (via Q4 to Q6) to the RGB monitor connector and to the PAL colour encoder formed by Q10, IC46, IC47, IC48, Q8 and Q9. The PAL signal is finally applied to a standard ASTEC UHF modulator and brought out through a phono socket. A composite monochrome video signal is also provided by Q7 and Q8 for monochrome monitors. Notice that all the TV sync signals are generated by the CRTC. The cursor signal is mixed into the video signal by the video ULA.
  2. Do we have any schematics of a JPM video system in the downloads ? Anything. Various systems are usually updates of the one before and so may be quite similar. You might get vital clues as to how this system works. Or indeed any video system with a good description of how it works like the Barcrest MPU4 manual. It is superb.
  3. Some old RAM and ROM needed the -5v rails and I think some systems used a -5v for a switch bias...to keep the contacts clean. So might not be needed in this system. Also wondering about the 100Hz signal, this will be 120Hz in the USA lol
  4. quattrohead

    Mpu4 board parts numbers required

    Do you have the schematic diagrams for MPU4 ? Reading those will tell you everything you need to know. Any 1000pf 100v or greater capacitor will do, it may look different. The other things you need are resistor networks, again value is on the diagram.
  5. quattrohead

    Mps2 psu help

    If its the yellow label linear unit it is probably the trim pots as mentioned by Louie
  6. quattrohead

    BARCREST - WAY IN - £3 VIDEO

    Bad language in front of your kid lol I don't remember it always showing "0 nudges held" during each game. What rom version is it ?
  7. quattrohead

    Guitar and my Big 50

    Depends which Big 50 ? Castle I would go after but any other new shit consider it free disposal lol
  8. quattrohead

    Scorpion 4 sound issue

    Just bridge pin's 3 and 5 on the suspect chip, see if the interference still happens. Yes it will be loud.
  9. quattrohead

    JPM sys80 lamp changing

    Yes they do Bob, I had a system80 and an Opel Manta at the same time and used them on both lol.
  10. quattrohead

    Eastenders and coronation street voices

    Alright enough already. Go away.
  11. quattrohead

    just got this

    I have that machine here in the USA LOL
  12. quattrohead

    Jitc mpu4

    I know it's late over there in England, but this sure as hell made to sense to me in Florida at GMT -5 lol
  13. quattrohead

    JPM sys80 lamp changing

    Push and twist, never had a problem with them back in the day. But, as soon as you touch them, the bulb will fail every time lol. So either leave them alone or recon on changing them. You can change just the bulb if you need to.
  14. quattrohead

    my grail ..after all this time is secured .

    Nice one Del Boy, happy days !!!
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