TI 99/4A problem

TI-99/4A Hardware problems, questions, etc.

TI 99/4A problem

Postby taper3ad » September 3rd, 2011, 10:33 am

I'm mainly a Commodore 64 guy, but today I found a Ti-99/4A on a fleamarket here in Sweden.
They are rather rare here in Sweden (everyone had Commodore's here), but as I like old computer systems
in general I picked it up for like 12$ with a heap of gamecarts and a joystick adapater (to make atari styled
joysticks work I assume).

However, it doesn't seem to work. With or without cart, it boots up with the cyan background and the colored squares (both
upper and lower), but the squares are sometimes garbled, I see no text on the screen (I understand there should be some kind of
boot-text beneath the upper-colored squares). Sometimes when I hit enter a bunch of times, the screen is filled with chars (some
random letters, but mostly garbage chars). The sound seem to work as it says beep now and then when I press enter.

I'm kinda used to repairing broken Commodore 64's, and something tells me this is a memory error. How does the TI99/4A memory layout
look like? Does it have both RAM and graphics memory? If so, what memory circuits are used? Any hints on other possible faults that can
give this symtom?

Any tips are welcome, as I*m eager to repair this marvel-machine. I always treat old hardware with the respect it deserves!
Posts: 1
Joined: September 3rd, 2011, 10:21 am

Re: TI 99/4A problem

Postby Ben Yates » September 9th, 2011, 4:52 am

It has 256 bytes of CPU RAM (two MC6810 chips located near the TMS9900 CPU, IIRC), and a bunch of 4116 chips next to the huge VDP (TMS9981A).

It also has 3 GROMs located near the cartridge port. ISTR GROM 0 contains the character definitions. If there is an issue with the GROM, the character definitions can also be messed up. The difference is that, while in BASIC, everytime you press enter the graphics will change.

If you are 'flying blind', press 1 twice to get into BASIC.
Ben Yates
Posts: 35
Joined: January 25th, 2011, 4:11 am

Re: TI 99/4A problem

Postby gertk » January 4th, 2012, 7:29 am

I know this topic is old but if it is a DRAM problem and you can not find a reliable set of 4116 DRAM chips anymore then have a look at:


I replaced the 4116 with 41256 chips, you need to do some modifications to since the 41256 needs only 5 Volt.
And it will not give you more RAM since the VDP can not handle more than 16k.
Removing the chips is quite a pain but if you simply cut them off first, then remove all the pins and clean up the solder pads with wick and/or desoldering pump it is doable.

Pins 1 and 16 can be connected together and are connected to GND and pins 8 and 9 can be connected together also, they are connected to +5 Volt.
In the pictures you can see that I removed the powersupply 'bus-bars' and simply soldered some copperwire along the left and right hand side of the chips.
You need to remove two inductors which carry the -5 and +12 Volt to the old DRAM chips and you must make a connection from +5 Volt to pins 8 and 9 of the 41256 (above the LS156 it is easily made)
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Joined: November 21st, 2011, 3:30 pm

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