Keep alive for an Economi?

[peteski]

Ok, so the location is confirmed. Then I suspect that those 2 large caps are in the microprocessor/audio circuitry providing some keep-alive functionality to just that section.

As for where to solder the keep-alive, you don't need to specifically add it to the diode terminals.  As you can see in the photos, the positive and negative sides of the diodes are soldered to large copper "planes" on the PC board.  These planes are coated with green solder mask, but you can easily scrape it off away from the diodes and make your own contact areas with bare copper.  Then just solder the keep-alive l leads to where you scraped the solder mask.

For example, for the positive hookup point scrape off the solder mask in the area where the left edge of the copyright symbol is, and for negative hookup point scrape off the solder mask anywhere near the word "Made".  Just on the positive side make sure not to scrape off too far to the right, not to accidentally uncover the adjacent ground plane (under the center of the copyright symbol).

[Maletrain]

Yes, that looks easy on the enlarged photos, but not so easy on the actual tiny part for the area on the left edge of the copyright symbol.  I am not (yet?) an experienced micro-solderer. 

If I carefully scratch off the coating on the pad to the left of the copyright symbol and tin the wire, I am pretty sure I can arrange to just touch the wire with an iron while it is on the bare area.  But, if I need to flux the bare area to get a quick stick, I am concerned that solder might cross the gap line to another area of the board.  I read somewhere that a "lead" (graphite in clay) pencil line will stop flux, but have never seen that done.  Any suggestions? (No liabilities attached.)

[peteski]

Yes, that looks easy on the enlarged photos, but not so easy on the actual tiny part for the area on the left edge of the copyright symbol.  I am not (yet?) an experienced micro-solderer. 

If I carefully scratch off the coating on the pad to the left of the copyright symbol and tin the wire, I am pretty sure I can arrange to just touch the wire with an iron while it is on the bare area.  But, if I need to flux the bare area to get a quick stick, I am concerned that solder might cross the gap line to another area of the board.  I read somewhere that a "lead" (graphite in clay) pencil line will stop flux, but have never seen that done.  Any suggestions? (No liabilities attached.)

If you carefully scrape off the solder mask without exposing the copper in the adjacent ground plane, then the green solder mask will not allow the solder to bridge onto the ground plane.  And if some solder gets onto the pair of diode pads, that is ok - they are all connected together.  But do use flux (rosin-flux, non acidic) to make it easier to solder to the bare exposed copper.  Also, tin the exposed copper first (again use flux), before soldering the wire to it. That will make soldering the wire easier and quicker.

And for any accidental solder bridges, you should have some solder wick to be able to easily remove it.  Radio Shack used to carry it (when they were still around).