99er Forums

Hello all,

Just recently I aquired two TI99/4A computers, one NTSC and one PAL
The NTSC machine came without a power supply and modulator.
The powersupply of the PAL machine seemed to work Ok so last weekend this machine was used on that.
After about 5 hours suddenly the screen showed 'weird' characters instead of the normal startup screen.
Since then it continues to do so.

Is this a GROM0 problem or a DRAM problem (4116) ?

A simple basic program entered seems to run OK. Also some game cartridges I tested seem to run ok but also have garbled graphics.

Thanks in advance.

Gert

Looks like DRAM.. unfortunately that is becoming one of the more common failure modes.

It was indeed bad DRAM...

After painstakingly removing all the old 4116 chips and carefully desoldering the holes I put in 8 DIP16 sockets and removed the power busses next to the ram chips (for easier access to the pads) and also the two inductors carrying the -5 and +12 Volt (these are not needed anymore for the 41256). I created two new power busses with some copper wire and inserted 8 pieces of 41256 chips which I removed from an old ISA memory card. And surprise suprise: it worked ! The unused address pins are connected to GND respectively Vcc and the +5Volt is connected by the small bridge above the 74LS156

Nice little mod!

Very nice!!

http://www.youtube.com/watch?v=xrJ3GRWjdNg&context=C3c5eabeADOEgsToPDskJUVnE3yrgAIgqb7bQ2gBJ3 for a 6809-based computer using a 9918A that has 32k...

Perhaps this could be done with the earlier posters?

The 32k in the YouTube Video is externally bankswitched, so the TMS can still address 16k at once (it has not enough address pins)
Another problem will be the refresh of the DRAM, the 4116 needs only a 6 or 7 bit refresh counter where the 41256 needs at least 8 bit.

The bankswitching is not easily usable on the TI99 as the Basic will have no clue in what bank your program is stored, you could only switch from within assembler code and that is not possible in the 16k of VRAM.
An extra 32k chip on the external or cartridge bus is much more interesting because of the possibility to store assembler programs.