System Reset key not working, keyboard OK

[Marius]

Hi,

 

Today I grabbed on of my 800XE's, and the system reset key did not work.

 

The keyboard itself appears to be ok, and all the contacts of the connector work too. To be sure I re-soldered the whole keyboard connector, but it kept failing.

 

What component(s) are involved as soon as the system reset key is pressed?

 

It's an Atari 800XE, 4Bit memory type, PAL.

 

Thanks

M.

 

p.s. where on the mainboard can I usually simulate a keypress of the reset key?

[Rybags]

The Reset key on XL/XE does an actual hardware reset... similar to grounding the reset pin on the CPU+Antic.

 

It's not quite that simple though... IIRC the XE has some sort of circuit where the reset doesn't occur until you release the key, where on the XL holding the key will halt the machine for the entire duration.

[MEtalGuy66]

I've seen that quite a few times. It's usually a cold solder joint on a pulldown resistor, on the reset circuit. Look at all the resistors in front of, to the left of, and behind the keyboard connector. Look for a solder joint where it looks like there is solder on the bottom side, but no solder came through the hole to the top side. These crappy joints worked fine when the machine was new, but over time, resistance increases as oxidization builds up on the wire. The reset problem is pretty common on the later xe boards (2 or 4 ram chip boards) because the resistor in question is going through a ground plane on the top side, and the thru-playing of that hole is not very good. At the factory, their mass soldering process did not get that joint hot enough for solder to flow through the hole to the top side and bond the resistor lead to the ground plane. As a result, since the lead is touching the inside of the hole, it gets a good enough ground when everything is shiny & brand new, but give it 20+ years and the wire of the resistor lead oxidizes a bit.. and now you've got a very poor ground. The solution is just to heat the hell out of that joint, and flow some solder onto the top side of that solder joint. (flux would be a good idea)...