Heathkit SB-634 display PCB

Fix those blinking and burnt out gas display tubes with new LEDs

The Heathkit Shop SB-634 display upgrade is not for the faint at heart. This upgrade requires removal of several parts from the original Heathkit pcb along with several wires that need to be moved. This project is not for an inexperienced builder.

The concept is simple. The original gas discharge displays have, over forty years old, worn out. Replacements, if found, are extremely expensive. This upgrade replaces those old displays with new LED types.

Here's what has to be done:

  • Stuff the new pcb with SMD parts as well as through hole parts
  • Test the new board
  • Remove six (6) .001 capacitors and replace with .01 capacitors
  • Removed six (6) wires and move them
  • Un-solder six (6) resistors .

Because the old displays worked on 230 V, it is necessary to divorce this high voltage away from the logic level required by the new displays.

That is done by replacing the .001 capacitors and using .01 capacitors. There is one leg of a 1MΩ resistor that needs to be lifted from the pcb. That will allow one of the digit driver wires to be moved from the high voltage end of the circuit to the .01 cap.

The display driver transistors are mounted on the new pcb. They consist of one NPN switch turning on a second PNP transistor. There is one of these switches for each digit for a total of twelve transistors.

SMD parts used here!

This SB-634 display upgrade uses a mix of both through hole and SMD.

This is a challenging project! The instructions are slim, and you'll need a factory copy of the manual.
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Which one is LED and the other original gas discharge displays?

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