Video problem after apparently successful repair

[weirdocollector]

Hi!

 

Today 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 successfully socketed a new DRAM chip 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 screen passed trough increasing states of degrade and now just display utter garbage accompanied by random beeps. 

 

I've attached  the sequence of degrading screens displayed by the TI99 after the apparently successful repair...

 

I've swapped both the DRAM chip the VDP (my machine is a PAL one and as I only got some TMS9918 I got the same garbage with a pink background) but to no avail.

 

Curiously even if I remove the DRAM chip from the board, nothing changes and the same garbage is displayed on screen.

 

I really do not know what is happening as everything seemed at first OK after changing the DRAM chip.

 

Have anybody experimented something similar ?

 

Any help will be really welcome.

 

Thanks a lot in advance.

 

 

 

 

Edited March 18, 2022 by weirdocollector

[+arcadeshopper]

What did you put in a 4116? Did you test the power supply voltages? You likely need to replace the rest.. see the FAQ on ram issues for pictures

Sent from my Pixel 6 Pro using Tapatalk

[weirdocollector]

1 minute ago, arcadeshopper said:

What did you put in a 4116? Did you test the power supply voltages? You likely need to replace the rest.. see the FAQ on ram issues for pictures

Sent from my Pixel 6 Pro using Tapatalk
 

Hi!

 

I just changed one 4116 chip accordingly to the instructions only and pictures on the Ninerpedia page that pointed to the bit 3 chip as defective.

 

If I remove the new chip I've replaced and socketed the garbage screen do not change.

 

What voltages should I check ? The ones on the DRAM chips?

 

Thanks for your help.

[+arcadeshopper]

Hi!
 
I just changed one 4116 chip accordingly to the instructions only and pictures on the Ninerpedia page that pointed to the bit 3 chip as defective.
 
If I remove the new chip I've replaced and socketed the garbage screen do not change.
 
What voltages should I check ? The ones on the DRAM chips?
 
Thanks for your help.

That's a good start.. you can check at the chips or the 4 pin connector the motherboard connects to on the internal power supply it's labeled clearly on the supply -5 12 gnd 5

Likely another chip was borderline and it failed after you changed the bad one out.

Sent from my Pixel 6 Pro using Tapatalk

[weirdocollector]

13 minutes ago, arcadeshopper said:

That's a good start.. you can check at the chips or the 4 pin connector the motherboard connects to on the internal power supply it's labeled clearly on the supply -5 12 gnd 5

Likely another chip was borderline and it failed after you changed the bad one out.

Sent from my Pixel 6 Pro using Tapatalk
 

I've piggybacked another chip on each of the other 4116 but nothing changes.

 

I'll try to measure the voltages as soon as I can get back to the TI99

 

Thanks again.

[weirdocollector]

Just another question.

 

Is this surely a DRAM problem or it can be caused by another faulty component?

 

When I remove the DRAM chip I've socketed nothing cahnges and the same garbage is displayed.

 

Also the VDP should not be the culprit, as I swapped it with another one with no effect.

 

Thanks again.

Edited March 19, 2022 by weirdocollector

[weirdocollector]

12 hours ago, arcadeshopper said:

That's a good start.. you can check at the chips or the 4 pin connector the motherboard connects to on the internal power supply it's labeled clearly on the supply -5 12 gnd 5

Likely another chip was borderline and it failed after you changed the bad one out.

Sent from my Pixel 6 Pro using Tapatalk
 

Just checked the voltages on the DRAM chips.

 

Everything seems to be ok.

 

Now when I power on the TI I just get a light blue screen and a single beep.

 

Some vertical traparent columns are visible on screen.

 

Hope this helps.

[weirdocollector]

OK, last post...

 

I've had another try connecting the keyboard, and after turning on the TI and hearing the beep when I press the space bar and then '1' (to start TI BASIC) I'm greeted with an orange rectangle.

 

 

This behaviour seem to be consistent, repeating itself at every boot.

 

Again, thanks for any help!

 

Edited March 19, 2022 by weirdocollector

[+Ksarul]

That last bit tells me that the machine is booting successfully, as you are going into the TI BASIC environment (with no screen characters). This leads me back to the video side of things--you may have a problem with multiple 4116 chips, or you may have an issue with your video chip. Do note--that chip REALLY needs to have a good heat sink attached to it when it is running, as it overheats easily. I would check the RAM first though, especially if you had the VDP properly heat sinked during your tests.

[weirdocollector]

14 minutes ago, Ksarul said:

That last bit tells me that the machine is booting successfully, as you are going into the TI BASIC environment (with no screen characters). This leads me back to the video side of things--you may have a problem with multiple 4116 chips, or you may have an issue with your video chip. Do note--that chip REALLY needs to have a good heat sink attached to it when it is running, as it overheats easily. I would check the RAM first though, especially if you had the VDP properly heat sinked during your tests.

Thanks for the suggestions !

 

I also assumed that the machine were starting and initialising correctly but with no screen.

 

During the test I just left on the DVP the metal bar that was fixed with two screws on the PCB metal shield: would that be enough or should I have to attach it back to the shield and test again ?

 

I've tried with another VDP (an NTSC one) but without any change besides the on screen colours.

 

What do you suggest to use to check the RAM chips ?

 

I would like to be able to identify which one are failing without having to change them all.

 

Thanks again.