6502.org

Hi All,

Just joined in the hope of some help.

My first computer was a SoC MK14, which sadly I don't have any more. My second was a UK101 which I'm now trying to get working. I've replaced three damaged keys (Many thanks RS who still stock them!), sorted out a better 5v PSU and connected it up to the composite input on my TV.

Sadly, all is not well. The screen is full of characters that keep changing. When I hold the reset buttons, the screen is filled with a single character. When I release, its back to the random characters.

I did the 48x32 screen mod years ago and this still appears to be connected.

All help gratefully received.

Dave

If you get a picture then the video generation circuit is working. I'd visually check all the RAM and logic chips for a start. Make sure they have good connections in their sockets.

Is there anyway to determine if the processor is running correctly? Any LEDs that can be toggled (like the BBC's CAPS Lock) or startup beeps?

Thanks for the suggestions.

I'm thinking of taking out as many chips as I can, just keeping monitor ROM, minimal RAM, video chips, processor and any associated logic chips and see what happens. The processor must be trying to do something, as I can at least reset it.

I'm guessing that something is being selected when it shouldn't be, corrupting the data bus and preventing the processor from working?

Dave

What kind of analysis tools do you have?

If you can sample (or latch) the address bus using Sync, you can see where the PC is going. Even if you can't capture a trace, if it gets into a tight loop almost all bits will be stable.

Even checking the frequency of Sync and each of the address bits might tell you something (many inexpensive multimeters will report frequency)

Checking the clock, the reset line, and some decoded chip select signals might also be fruitful.

Cheers
Ed

That sounds like a good idea. Think I'll buy a frequency meter and see what that shows up.

Dave

I've decided to take the easy route and ordered one of these from China:

http://www.ebay.co.uk/itm/271634652740? ... EBIDX%3AIT

It'll take a few weeks to arrive, but I'm in no hurry.

Dave