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34561Re: [midatlanticretro] trouble finding a few parts

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  • Dan Roganti
    Feb 24, 2014
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      On Mon, Feb 24, 2014 at 9:00 PM, Dan Roganti <ragooman@...> wrote:



      > On 02/24/2014 04:48 PM, B Degnan wrote:
      >
      >> GD510 diode.  (is there a substitute?)
      >
      >   Now this is a different matter.  That's a pretty well-known diode and
      > it's tough to find.  It's a germanium diode, and the circuit *may*
      > depend on some of its characteristics.  You might get by with something
      > like a 1N34A or a 1N60.  Can you send me the relevant part of the schematic?
      >
      >> I am working to install the Heathkit ETA-3400 expansion unit to a ET3400
      >> Trainer.  To do this they say one must bump up the processing speed to
      >> 4mhz, more RAM, etc. by adding a few modifications to the main unit.
      >


      The diode is used for steering the bidirectional data buffer circuit IC9,IC10 on the expansion ports via the RE signal.  This mod is arranging the connections into a Wired-OR signal to let several different expansion devices have access to this port on the Trainer.  The memory expansion unit generates the RE signal from it's decode logic  - which relies on the CPU signals from the Trainer - when it's time to read or write the Rams or Rom onboard. They also made one for the SS-50 bus.

      The real purpose of the GD150 diode is to provide fast switching for the logic signal from the expansion memory to eliminate any signal delay and a low voltage drop (characteristic of germanium diodes) and minimum capacitance to make sure the logic input signal threshold is satisfied. Not sure why they didn't use a Schottky diode back then, maybe it was too expensive back then ? for only one diode ?


      I left out one little thing, if you did want to connect more expansion devices, that diode would really have to sit onboard each expansion device to actually create the Wired-OR signal. That mod still only supports one expansion device.


      You can also use a schottky diode in place there since it's low power , such as a 1N5817, they have the same characteristics for logic circuits.
      and they're dirt cheap at Jameco, 14cents
      Dan
       _  ____ 
      / \__/   
      Scotty, We Need More Power !!
      \_/ _\__ Aye, Cap'n, but we've only got 80 col's !


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