exxos's Atari, Amiga & retro forum

stephen_usher wrote: 07 Oct 2024 12:03 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.

Yes, that's my understanding too, and I appreciate the offer of one, thanks.

Funnily enough the address lines were next on my list of checks based on this from the service manual:


Basically if the VDP is good it's one or more of the RAM chips, if not then the VDP is bad...

It does then go on to say check the CPU and system ROM (U1 and U2) but since the games are running, I think that they are fine.

However I did just do a continuity test on the address line of the RAM and found pins 6 &10 in the sockets are not connected to each other, however if I flip the board and run the same test, they are. I need to go away and verify that as it seems a bit weird to me and I need to check my work.

OK, so actually the pin 6 &10 thing was just me being an eejit, please ignore, the address bus from the VDP to the RAM and sockets seems to be fine.

No shorts between address lines then.

OK, so the address lines all correctly buzz out to the VDP, all the RAM I'm using tests fine, so I think it can only be the VDP now, nothing else makes sense.

@stephen_usher you mentioned you might have a spare TMS9929A, if so can you let me know how much you'd want for it please?

stephen_usher wrote: 07 Oct 2024 14:38 No shorts between address lines then.

Nope, all clear. I also checked at the VDP as well, no address line shorts there either.

Woo hoo!!!

ColUSB will be here tomorrow :)

Well, it seems I've been well and truly Coleco'd...

The ColUSB power supply arrived and on testing it I get this:


I'm letting it burn in for a bit to see what's what, but looks like maybe I've been fighting a power issue with my PSU lash up :dizzy:

I'm guessing that the Electron PSU was not up to snuff in the 5v or -5V department. Arrrrggghhhh

Well, lets see how the burn in goes...

:crossed2:

:dualthumbup: :yay2: :cheer:

Well, about an hour in and it's still running just fine :)

Given that the VDP only requires 5v I'm assuming that the Electron PSU did not have enough power on the 5v line.

Or the -5V and 12V wasn't enough for the 4116 chips.