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Re: CDP1861 resistor question

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  • erd_6502
    ... Yes... especially since I have a quantity of +5V - -12V encapsulated charge pumps - we used them at work for providing -12V for 1488/1489 sets on a board
    Message 1 of 12 , Dec 31, 2004
      --- In cosmacelf@yahoogroups.com, "sbirdasn" <sbirdasn@y...> wrote:
      > --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...> wrote:
      > > I'd love to see a decent amp, even if it's more expensive than a
      > > simple transistor one....
      >
      > Are you willing to provide +/- 12 or 15 Volt power supplies?

      Yes... especially since I have a quantity of +5V -> -12V encapsulated
      charge pumps - we used them at work for providing -12V for 1488/1489
      sets on a board that was placed into a situation where -12V wasn't
      available (and it was 20 years ago, so MAX232s weren't a viable
      solution)... one can also tap off charge pump caps on a MAX232 to get
      the same effect, and those aren't horribly expensive now.

      > If so then sure, use a LM318 and a 2N3904 as a drive transistor on
      > the 318's output. If you really want to get fancy, then changing to a
      > more complicated push-pull output transistor pair will be even
      > better. There are some LM series op-amps that are simple buffers that
      > can drive 50 Ohms directly, without too much GBW. Once again, you
      > need the +/- supplies at about 12-15 Volts.

      OK... nice suggestions.

      > Now if you add up all of the parts including decoupling, feedback
      > compensation (if required), etc., then that cheap transistor solution
      > is looking better all the time.

      Indeed.

      > The real issue with your problems was trying to use the resistor
      > summing node to drive a low impedance video input. The video level
      > got squashed. Just a buffer transistor is all you need (in most
      > cases) to isolate the high impedance summing node from the
      > termination in the monitor/TV's 75 Ohms to ground.

      So I expect, but I'm not enough of a wire-head to be able to figure
      out a solution. I understand the problem, but that's about it.

      -ethan
    • Richard
      At the following link, the TMS9118 board uses a simple amp curcuit. is this about what you are talking about, for the 1861. I also tryed to connect the 1861,
      Message 2 of 12 , Jan 1, 2005
        At the following link, the TMS9118 board uses a simple amp curcuit. is this
        about what you are talking about, for the 1861. I also tryed to connect the
        1861, to a tandy vm-4 green video monitor. and it not work, I figured there
        wasn't enuff signal, to drive it.

        http://www.elf-emulation.com/hardware/tms9918.png
        bottom middle.

        currently I am using a video RF module, pulled out of a VCR, For my ELF II.

        Richard

        ----- Original Message -----
        From: "erd_6502" <erd_6502@...>
        To: <cosmacelf@yahoogroups.com>
        Sent: Saturday, January 01, 2005 12:10 AM
        Subject: [cosmacelf] Re: CDP1861 resistor question


        >
        >
        > --- In cosmacelf@yahoogroups.com, "sbirdasn" <sbirdasn@y...> wrote:
        > > --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...> wrote:
        > > > I'd love to see a decent amp, even if it's more expensive than a
        > > > simple transistor one....
        > >
        > > Are you willing to provide +/- 12 or 15 Volt power supplies?
        >
        > Yes... especially since I have a quantity of +5V -> -12V encapsulated
        > charge pumps - we used them at work for providing -12V for 1488/1489
        > sets on a board that was placed into a situation where -12V wasn't
        > available (and it was 20 years ago, so MAX232s weren't a viable
        > solution)... one can also tap off charge pump caps on a MAX232 to get
        > the same effect, and those aren't horribly expensive now.
        >
        > > If so then sure, use a LM318 and a 2N3904 as a drive transistor on
        > > the 318's output. If you really want to get fancy, then changing to a
        > > more complicated push-pull output transistor pair will be even
        > > better. There are some LM series op-amps that are simple buffers that
        > > can drive 50 Ohms directly, without too much GBW. Once again, you
        > > need the +/- supplies at about 12-15 Volts.
        >
        > OK... nice suggestions.
        >
        > > Now if you add up all of the parts including decoupling, feedback
        > > compensation (if required), etc., then that cheap transistor solution
        > > is looking better all the time.
        >
        > Indeed.
        >
        > > The real issue with your problems was trying to use the resistor
        > > summing node to drive a low impedance video input. The video level
        > > got squashed. Just a buffer transistor is all you need (in most
        > > cases) to isolate the high impedance summing node from the
        > > termination in the monitor/TV's 75 Ohms to ground.
        >
        > So I expect, but I'm not enough of a wire-head to be able to figure
        > out a solution. I understand the problem, but that's about it.
        >
        > -ethan
        >
        >
        >
        >
        >
        >
        >
        > ========================================================
        > Visit the COSMAC ELF website at http://www.cosmacelf.com
        > Yahoo! Groups Links
        >
        >
        >
        >
        >
        >
        >
      • sbirdasn
        ... encapsulated ... But switching supplies (charge pumps included) often inject horrid levels of ground noise spikes into the rest of the circuits, while
        Message 3 of 12 , Jan 1, 2005
          --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...> wrote:
          >
          > --- In cosmacelf@yahoogroups.com, "sbirdasn" <sbirdasn@y...> wrote:
          > > --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...>
          wrote:
          > > > I'd love to see a decent amp, even if it's more expensive than a
          > > > simple transistor one....
          > >
          > > Are you willing to provide +/- 12 or 15 Volt power supplies?
          >
          > Yes... especially since I have a quantity of +5V -> -12V
          encapsulated
          > charge pumps - we used them at work for providing -12V for 1488/1489
          > sets on a board that was placed into a situation where -12V wasn't
          > available (and it was 20 years ago, so MAX232s weren't a viable
          > solution)...

          But switching supplies (charge pumps included) often inject horrid
          levels of ground noise spikes into the rest of the circuits, while
          charge pumps have lousy regulation and/or poor current delivery
          capabilities. It just adds to the problems in many cases when you are
          talking video and audio. Digital/serial communications are rather
          forgiving in this respect.

          You really need to use linear power supplies when dealing with video
          and audio applications unless you are *very* careful, and have things
          like multi-layer boards with ground/power planes, inductors and caps
          galore all over the layout. It is not at all trivial to control
          switching noise from power supplies getting into everything else.
          Trust me on this.

          > one can also tap off charge pump caps on a MAX232 to get
          > the same effect, and those aren't horribly expensive now.

          But the current delivered is rather small. Just the standby current
          for a 318 would probably load down the switch cap supplies by itself,
          and that ignores the current required to drive 1V into a 75 Ohm load.

          > > If so then sure, use a LM318 and a 2N3904 as a drive transistor
          on
          > > the 318's output. If you really want to get fancy, then changing
          to a
          > > more complicated push-pull output transistor pair will be even
          > > better. There are some LM series op-amps that are simple buffers
          that
          > > can drive 50 Ohms directly, without too much GBW. Once again, you
          > > need the +/- supplies at about 12-15 Volts.

          Fer instance, LH0002 buffer amp, obsoleted by National, but others
          still make something like it.

          >
          > OK... nice suggestions.
          >
          > > Now if you add up all of the parts including decoupling, feedback
          > > compensation (if required), etc., then that cheap transistor
          solution
          > > is looking better all the time.
          >
          > Indeed.
          >
          > > The real issue with your problems was trying to use the resistor
          > > summing node to drive a low impedance video input. The video
          level
          > > got squashed. Just a buffer transistor is all you need (in most
          > > cases) to isolate the high impedance summing node from the
          > > termination in the monitor/TV's 75 Ohms to ground.
          >
          > So I expect, but I'm not enough of a wire-head to be able to figure
          > out a solution. I understand the problem, but that's about it.
          >
          > -ethan

          Now with all of the replies with links, anyone having monitor
          problems should be able to add the buffer transistor, and get it
          working OK. If one has the skills to build an ELF, then the skills to
          add this minor change shouldn't be too tough. If still not able to,
          then some one-on-one face time with someone who has the expertise
          would do the trick.

          Tony.
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