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34561Re: [midatlanticretro] trouble finding a few parts

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  • Dan Roganti
    Feb 24, 2014

      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
       _  ____ 
      / \__/   
      Scotty, We Need More Power !!
      \_/ _\__ Aye, Cap'n, but we've only got 80 col's !

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