Hello and Help Needed

[Manxmann]

Hi Folks,

Thanks for the admission! Apologies for my first post being a cry for help.

I have a TC-01 that I'm struggling to get running and would love some help and advice on tracking down this issue.

So the machine is an 'in-box' immaculate example showing very little use over it's life. The problem is the age old 'black screen' but more importantly no response the pressing the ALPA LOCK key i.e. the LED doesn't change.

So far I've :

/ check all voltages OK
/ Using a scope checked Address lines / data lines directly at the CPU and RAM, all look good lots of activity the machine is running and levels are all good.
/ Dumped the ROM and verified it's contents against an image I download, an exact match
/ Check clock - OK
/ Check interrupt - OK
/ No chips are 'hot'
/ Checked various 'random' 74 chips all appear OK
/ Tested data and address bus activity without the ROM and pretty much all activity stopped.
/ Check R/W pins on the VDP no activity
/ Used both the RGB 'set' video out with a HDMI upscaler so the 5v isn't needed and the regular RF, same deal sync from the VDP but blank.

So the machine is running but never get far enough through it's initilisation process to activate the VDP or begin running the basic code to respond to the keyboard. I haven't yet checked if an interrupt is fired when I press a ley, thats a thought I just had so will check tomorrow night.

I've seen a number of reports of blank screens, some on this forum, but never a conclusive 'this fixed it'. I've had a number of MSX machines give similar issues and they could all be tracked down to some fault or other but on this TC-01 everything seems to be working but it isn't

I'm waiting on an eprom eraser so I can program up a 2764 with the diagnostic rom but I'm not hopeful this will reveal anything and the documentation it's great with no info on how it alerts to RAM/compnent failure when there's no video.

Any pointers appreciated.

Simon

[Lardo Boffin]

Hi Simon and welcome!

I can’t offer any help as you have already gone past my level of expertise. Rudedog Retros did a great job of fixing my Vectrex and Mutant Caterpillar Games fixed my Tatung keyboard issues if you end up needing to send it anywhere.

Best of luck!

[arcadesy]

Sounds like you've checked a lot of things already!

I've never had a black screen Albert but additionally to what you've already done i'd be thinking about:

• Are the jumpers set for RGB? Shouldn't affect the RF though
• Have you checked the clock for the VDP at X001 as well as the CPU clock?
• Check activity on the Reset line
• Check the 74LS32 at I032, this is involved with chip select of the VDP
• All the buffers at I002, I003, I004 and I005

Good luck!

[Manxmann]

FIXED!!

Thanks for the suggestions. After checking everything that was recommended and still finding no smoking gun I decided to go with my gut, which said 9 times out of 10 this sort of thing is a RAM issue and it was!

[arcadesy]

Glad you got it fixed! Was it the main or video ram?

[Manxmann]

Thanks, main RAM.

After replacing all the RAM I scoped all the lines and honestly couldn't see any difference in the characteristics of data flow/level etc.

I've worked through the original ram and found 2 faulty chips, thing is with these installed the data 'Q' and address bus look identical to the 'good' chips.

Anyway brute force solved the issue