Black screen with High Tone audio

[zonaps2]

Hi,

yesterday I got this TI-99/4A in nice cosmetic condition.

It's a PAL version, 220V PSU. Modulator says VHF Channel 3.

 

I plugged it in an old TV I have and I just got a black screen with a continuous beeeeeeep tone.

I tried a different TV just in case and same problem. I've tried a cartridge (Parsec), with no luck.

 

I have a bunch of old computers, commodore, amiga, etc, but I never owned a TI before. I've read the sticky post for new members and it say that this symptom could be anything on the board.

 

I've already disassembled the machine, on a visual inspection everything looks good, I've re inserted the socketed chips.

 

PSU voltages are:

on pins 1-2 = 19.6V

on pins 2-4 = 9.15V

 

I see that these voltages are higher than specified on the PSU case, I guess it's because it's under no load?

What things should I check? I have no idea beyond this.

 

HERE is a video with the symptom.

 

Edited October 6, 2017 by zonaps2

[mjnurney]

years ago i used to have a few of these ....never fixed them sorry

[Tursi]

I guess the tone means that the sound chip is getting clock, at least. But the system is not executing instructions far enough to silence it. Muting the sound chip happens in GPL code, so it requires that the CPU, scratchpad RAM, ROM, and GROM 0 all be working.

[zonaps2]

Thanks Tursi, I see that I'll have to check out a lot of chips then

 

 

Sent from my iPhone using Tapatalk

[Pepper]

I'm in the same boat as you. I have one unit that has the same issue as yours and another that runs fine for a while then after it warms up the keyboard mappings get messed up.

I'll be interested in what you find. I've been working my way through the IC's looking for issues.

There is a good checklist here http://atariage.com/forums/blog/628/entry-12531-fixing-my-ti-99-4a/ . That steps you through the checks to do.