exxos's Atari, Amiga & retro forum

Has anyone seen this pattern on a dragon 32?


Tom

What is the display supposed to look like at this point?

It could be anything, such as a RAM chip with a stuck data line, addressing error, buffer chip problem. There are too many possibilities without probing with an oscilloscope and having a reference system to help know what’s normal.

Generally due to faulty ram I believe however in the two cases I had it turned out to be a VDG chip and then the SAM chip.

Looking at the schematic there's not a lot in there.

The 6847 graphics chip seems to be working fine. The chip is connected to the data bus using a 74LS273 (I think... very poorly photocopied schematic) and 74LS244, so that could be the issue. However, it might just be that the system isn't starting at all so the 6809 isn't initialising memory and so you're seeing the default garbage, which is what I suspect.

I had a similar display pattern on my Apricot F1 when the buffer chip between the CPU and data bus died.

Given that you have a video clock the 74LS783, which is doing the address decoding etc., seems to be working and the video chip is capable of seeing the memory, which sort of rules out the buffer chips, I'd initially look for a bad CPU or iffy ROMs and follow the signals.

P.S. Of course, if RAM is bad in just the wrong place then the CPU won't be able to run the contents of ROM properly and crash, meaning that it won't initialise the memory, which means you get this sort of junk. So, desolder all the 4116 memory, fit sockets and test the memory chips first. Those chips are prone to failure so you'll probably want to socket them anyway.

P.P.S. If you can't get 4116 memory you can use 4164 chips with modification: https://atariage.com/forums/topic/25792 ... liability/

I have to agree with Stephen on this, without any frame of reference to something that works, it's a tricky proposition.

Here looks like a good place to start: http://archive.worldofdragon.org/index. ... =Technical

They have schematics and service sheets, the latter potentially being of more use.

Does this happen on power up? I'd be thinking either a RAM or ROM issue, but it may be worth getting the CPU datasheet and looking at what halt and reset are doing. My bet would be the CPU is halted, though halt is often bi-directional, so could mean CPU halted due to getting nothing or garbage, or another part of the machine initiating a halt. I don't know the 6809E well enough to say.

Also perhaps open it up the case and look for hot ICs or other damage.

With the BBC I recently fixed, I ended up getting another working one for reference. It's an expensive way to go, but sometimes the only way.

@rubber_jonnie
@stephen_usher

I will dig into my spare parts im sure I have a cpu then on to the scope. Yes the schematics are like fuzz I will keep you updated on the battle

Tom

Good stuff.

Just a update found a cap dryed out to a short on the mc6883 chip so it blew the that part i tried a spare and it works again so I ordered some spare parts

Tom

Tomswork wrote: 16 Jun 2021 05:17 Just a update found a cap dryed out to a short on the mc6883 chip so it blew the that part i tried a spare and it works again so I ordered some spare parts

Tom

Ahh nice, be good to see some photos of it up and running.