TI-99/4a - Worth fixing?

[BDW]

Good job, Jmetal!

[Keatah]

More likely a bad RAM chip than a bad VDP.

 

Yeah, you're right - I discovered that a week or so ago, just didn't update the thread.

 

Someone on the vintage computer forum suggested a bad power supply might've caused the ram chips to go bad, but my multimeter is showing the voltages to be pretty steady. I'm thinking about ordering a full set of eight 4116 RAM chips from Jameco to give this another shot.

 

Still though, I wonder why re-seating the VDP helped at first? It doesn't do anything now, but at first it caused the graphics to clear up temporarily.

 

Reseating on chip on the board often flexes the board enough to bend and flex another chip. And at the tolerances of a microprocessor or especially ram; you can have contact made and broken in a faulty etching. DRAM is especially prone to 'leakage' failure, STATIC ram is more reliable.

 

Could also be a solder joint, either on the vdp socket or something nearby.

 

Once a 'trace' inside the chip substrate is cracked, sometimes pressure and flexing can hold it together, but if it is powered up and subject to a current (especially AC in low mhz range from the clock circuit), that gap that has/is being pressed together by you flexing and reseating will start to erode away from electron migration!

 

Well once you get that ram replaced like it was suggested, you should be running just fine. Don't you just love intermittent electronics?

Edited August 13, 2010 by Keatah