exxos's Atari, Amiga & retro forum

stephen_usher wrote: 06 Oct 2024 21:33 What does the corruption look like? You(/we) may be able to determine which bit is being affected.

It's the same no matter what RAM is in what socket, and it looks like it's sprites that are bad, so it seems like it's the VDP that's bad.

Just hope your Chinese VDPs will do the job.

HigashiJun wrote: 06 Oct 2024 23:51 Just hope your Chinese VDPs will do the job.

Fingers crossed...

Is it the same VDP as the TI-99?

stephen_usher wrote: 06 Oct 2024 21:33 What does the corruption look like? You(/we) may be able to determine which bit is being affected.

Just to answer your question a bit more, I have 2 games, Subroc and Omega Race.

Subroc looks like this:


It's mostly fine bar what 'looks' to be corrupted sprites that look like an H on the image. The central one should be a circular aiming reticle, the ones to the right should be at the top and area right pointing arrow and a scale that shows your elevation. The majority of the sprites do seem fine and since the program RAM is completely separate from the video RAM, the game actually plays as expected, just with the graphical glitches.

Omega Race on the other hand is in much worse shape:


What I can say is that the game is running just fine underneath all the crud. You can make out certain aspects of the playfield, but it 'appears' as though characters that are supposed to have been redefined simply haven't been. For example, in amongst all the @ symbols there

Some aspects, like the 'fence' around the edge, the word 'PLAYER' and the group of 4 objects just left of centre under the line of B's are correct. You can also just pick out your sprite and the enemy sprite moving around, so again the program code is running fine, it's just the video output that is messed up.

So that leaves 2 components as far as I can tell, the RAM and the VDP:


I now have the service manual so will do some more digging, however the RAM all tests good in my tester and it doesn't matter what order it's installed in, the corruption is the same, which again points to the VDP.

I have a full set of 4164s coming from Retroleum, so that will potentially rule out the RAM once and for all. The VDPs from Ali Express have shipped as well.

However I am not blind to the fact that it could be some sort of issue between the ROM data and how it gets to the video memory. There are logic ICs and one could be bad or marginal, but I need to look at the service manual and schematics to see if there is something there that could be messing things up.

PhilC wrote: 07 Oct 2024 09:32 Is it the same VDP as the TI-99?

Not 100% sure. If the TI uses the TMS9929ANL then yes, though I know there are a lot of similarities between the variations of the VDP.

@rubber_jonnie just done a bit of a Google and it's clear as mud. However, chr king my TI parts draw does indeed contain a TMS9929ANL, so you might be a step ahead if you don't mind borrowing one from your TI-99?

PhilC wrote: 07 Oct 2024 11:01 @rubber_jonnie just done a bit of a Google and it's clear as mud. However, chr king my TI parts draw does indeed contain a TMS9929ANL, so you might be a step ahead if you don't mind borrowing one from your TI-99?

Clear as mud is what I thought too!!!

Whilst the thought of being able to check the CV with the part from my TI is appealing, less appealing is having to desolder the item from my TI. It's always a risk, and if it was socketed I'd probably do so, but I don't think it is.

One other issue I have is that I've got every single socket bar a 40 pin one!!! I have some ordered now and I may pull my TI apart and see if it's socketed, but I don't think it is.

In other news, the ColUSB power adapter is in delivery hell. According to DHL it was delivered to the destination country on Thursday 3rd October


However despite having a valid tracking number, it's still leaving the originating country!!!


So, to me that means that either there is no tracking in the UK, my package is 'in a depot awaiting distribution' or some other scenario I'd rather not consider.

I'm hoping it's the former and it'll rock up soon and I can stop using my 2 power supply lash up that is a bit hinky :)

The hidden face of retro that stretches your patience...

:lol:

Looks like an address line issue. Check for shorts. Probably one of the higher address lines.

What you get on the screen very much depends upon the mode the VDP is in and what graphics planes are being used as to what memory it will use.

The PAL version of the TI99/4A does use the same chip, as does the Memotech MTX, MSX-1, Spectravideo etc.

I should have one in stock if necessary.