Help finding a video issue on my TI99/4A

[weirdocollector]

Hi and thanks for having me on this board!

Yesterday I took the time to attempt to repair one of my TI99/4a which, judging from the screen displayed and according to this Ninerpeia page (https://www.ninerpedia.org/index.php?title=Troubleshooting) implied a defective 4116 chip and precisely the one corresponding to bit 3 (no TI logo and text and coloured squares of the wrong colours).

I replaced and socketed a new DRAM chip apparently without any issue and without damaging any PCB trace, and after turning on the computer everything seemed OK as the logo and the text were displayed as expected.

But unfortunately it didn't last for long and after about a minute the TI logo disappeared leaving me with a light blue screen.



The computer seems to start and initialise correctly as the usual starting beep is audible.

If the space bar and then the '1' key are pressed (to start TI BASIC) an orange rectangle appears on screen.

Pressing random keys followed by 'Enter' makes the computer to beep as expected for a Syntax Error.

This behaviour seems to be consistent as it repeats every time the TI is turned on.



I've swapped both the DRAM chip the VDP but to no avail.

Have anybody experimented something similar ?

Any help will be really welcome.

Thanks a lot in advance.

 

[VeryVon]

I've been through a couple TI-99/4A video repairs and haven't seen symptoms like this, but my spidey senses are telling me you should check the video connector solder points and VDP clock. Sorry, wish I had something more concrete.

 

[weirdocollector]

Hi and thanks for your suggestions.

I've checked and resolded the DIN connector but without any change.

I've measured with my multimeter the frequency on the leads of the video crystal getting about 16 MHz ( should be about 10) and 0 MHz on the 40 and 39 pins of the VDP.

When the leads of the multimeter touches the VDP pins or the crystal leads the screen goes blank temporarily.

I'm pretty sure I'm doing something wrong when I try to measure the frequency, what should I use and how ?

Thanks for your help!

 

[VeryVon]

Roger that.

I use an inexpensive USB Logic Analyzer along with sigrok pulseview to measure the frequencies. Looks something like this after capture:



If you end up doing that lmk I can give you the settings I use.

 

[weirdocollector]

Roger that.

I use an inexpensive USB Logic Analyzer along with sigrok pulseview to measure the frequencies. Looks something like this after capture:

View attachment 1239510

If you end up doing that lmk I can give you the settings I use.

Hi !

I've got a USB Logic Analyzer too.

How should I use it to measure the VDP clock?

Thanks for your help!