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Hello!

As an x-mas gift my father gave me a "new", never-been-opened TI-99/4A computer. I was pretty excited as this was my first computer back in the early 80s. I had no idea Bill Cosby was pushing these things on TV back then.



Anyway, we took off all the packaging and hooked it up to a little TV set. Once it was powered up, we saw the familiar splash screen and beep, but the characters were significantly garbled (please note the dark horizontal bands are an artifact of my camera and TV - I'm guessing most of you know that ):




After choosing TI Basic, I try to do something simple like print "hello", but this is what I get:



Looks like it says "Incorrect statement". Just about everything simple I try, like print 1 + 1 gives the same error message with a low-freq beep.
The displayed characters are flipping between two garbled states at the same rate as the cursor blinking. The background color is also alternating between blue and green at the same rate. Originally I've seen the display look even worse than depicted in the images I've posted, but I can't re-create the problem anymore for some reason. If I do I'll gather what I can.

Any ideas? Despite the fact its new and fresh out of the box (all original, sealed packaging), it was purchased off ebay and who knows what storage conditions it has endured over the decades. Perhaps it was in a garage or in some otherwise unheated storage facility: 25-30 years of changing temperature and humidity would have to have wreaked some havoc on this things innards.

Bum video chip? Bad ROM holding character data? I'm still learning about this thing, and I'm looking for a spare unit to gut for parts. I have some experience with digital electronics and certainly not new to a soldering iron. If its a bad component, does anyone have any suggestions on where I should start?

Any help is much appreciated!
Thanks for reading!
-Jordan

Sounds to me like an issue with a GROM chip

Thank you for the quick reply!

I did some searching about the GROM, are you referring to one or more of the CD215#NL chips (the ones seated in sockets near the processor)?

Cheers,
-Jordan

Another thing occurred to me regarding the strange behavior - not simply the garbled display - but all the simple commands like print 1 + 1: they shouldn't come back with error messages, I should see a short response like a garbled answer of 2, not something more resembling incorrect statement. Could the memory be bad as well? Which chips would those be? the 24-pin 6810 chips?

You might want to hit the caps lock--looks like you've been using lower case, and that will give the issue with the commands. . .

ksarul wrote:You might want to hit the caps lock--looks like you've been using lower case, and that will give the issue with the commands. . .

No, lower case should work just fine. Only issue with lower case in Basic is that filenames are case-sensitive, and device names are (usually) case-sensitive ("dsk1.filename" will give a bad device name, as DSK1 is the name required, except by some Corcomp controllers that use both upper and lower-case).

So, I am thinking the VDP RAM chips (the two rows of chips near the video processor). Basic stores its program in VDP Ram, and executes statements from there as well, not the static RAM (6810's), so a bad ram chip could change the contents.

enter the statements as a program, then LIST them to see if they change.

Ben Yates wrote:So, I am thinking the VDP RAM chips (the two rows of chips near the video processor). Basic stores its program in VDP Ram, and executes statements from there as well, not the static RAM (6810's), so a bad ram chip could change the contents.

I've taken the computer apart and I'm not really sure where these VDP RAM chips are. If the 9918 is the video processor, there is a single row of 8 chips (plus a decoder) on my board layout. If you view the board length-wise, moving from the DB9 towards the CPU, you have:

8251DY040
D24226-217
8251DY040
8248DY042
8251DY040
D24226-217
8248DY042
8251DY040

I can't seem to find anything meaningful regarding these chips on Google. The nearby components and the traces suggest they're dynamic RAMs, but I'm a little thrown by the part variation.

So from what I understand so far, Basic stores the user-supplied program code in one or more of those those eight chips. If you enter an ad-hoc command like print 1+1, without prefixing it with a program line number, does it go in the same RAM?

Cheers,
-Jordan

Yes, there is no RAM in the computer left over besides the video RAM. The 6810s give 256 bytes (yes, bytes) of CPU RAM, used by the system to interpret BASIC from ROM.

The part # is, I believe, 4164's. Dynamic RAMs.

So, if you execute a statement or enter it in a line #, it is in VDP RAM...

The ram chips are 4116 DRAM' s 16kx1 bit and good ones are quite rare nowadays. I replaced them by a set of 41256 (256kx1) chips, see the message board for more info. The 41256 are single supply so you need to modify the supply lines towards the DRAM chips (and that will void all authenticity).
The chips are a pain to remove though, you can easily damage the board. I would recommend (if you are going to replace them) to cut off the legs near the body of the chip, remove the bodies and then remove the legs pin by pin and then clean up the pads with soldering wick and/or a desoldering pump. Then put in a set of quality sockets.

see: http://www.99er.net/99erbb/viewtopic.php?f=7&t=1141