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Replacing U4 on a Scott Edwards BS1/LCD

The BS1/LCD when bolted together is perfect for slipping the 9 V battery in between the two PCB's. Add a rubber band to hold it in place and you have a self contained unit. Unfortunately you have to disconnect the battery to remove power. Apparently while storing the battery in there the contacts met, with the polarities reversed. The next time I hooked up the battery I got a display of black rectangles.

After putting +5 on the Stamps power and ground and finding that the unit still worked, that left U4, the LP2950 voltage regulator. Here's the relevant circuit:


+9 VDC ------------+-----+     +--------+--- + 5VDC

                   |     |_____|        |

                   |     | U4  |        |

                 --+--   \     /      --+--

              C1 --+--    \___/    C2 --+--

                   |        |           |

                   |        |           |

- GROUND ----------+--------+-----------+

For some reasons LP2950's are scarce in town but you can substitute an NTE977. The NTE 977 integraged circuit 3 terminal positive voltager regulator can be picked up at Fryes for about $1.29. It's pin compatible.

The only surprise was that the holes were blocked on the other side of the board, and there was not enough room to fit a soldering iron and soldering wick in there. The situation was taylor made for some my fishing leader desoldering tool, unfortunately my last one disappeared months ago, and I couldn't remember the right size. Since a web search turned up no results, I'll place the info here:

Run down to a fishing tackle shop and buy some No. 9 stainless steal wire. I found this:
American Fishing Wire
Tooth Proof Stainless Steel Wire for leaders, trolling and casting. 105 lb. Test No.9
.022" Diameter .56mm Diameter.
30 feet will set you back about $1.79
Cut a 6 inch piece and put a bend in one end so you can push on it easily. Solder will stick to it only grudgingly so if you keep wiggling it while heating the PCB it will leave a clean hole.