[Coco] 128K CoCo 3 is DOA D:

[jdaggett at gate.net]

Juan

The black border and green screen background is default for the COCO. If that comes up 
then at least part of the GIME chip is working. 

IF you have no prompts then you may have a DRAM or CPU problem. In the COCO3 on 
power up there is a block of code that is copied from the ROM into RAM. If RAM is corrupt or 
the CPU is not working then the GIME reads bad data from the low resolution text screen 
area. Blowing the address and data ports on the CPU in the COCOs is not all that difficult to 
do. Problably the weakest part of the 6809 design and the COCO itself is the lack of robust 
drive capabilities and protection of the address and data buses through the expansion port.   

Try reseating the DRAM chips. I still think the DRAM chips are available through surplus 
outlets like Jameco and others. If not then I may still have the set of DRAMs that I pulled out 
of my COCO3 over twenty years ago. I don't remember if the 6809E wsa socketed or not. If 
so then change CPU if you have a spare. I also have 680B09Es also even thought they are 
still available on the net. 


james

On 6 Oct 2012 at 2:12, Juan Castro wrote:

> On turning on, there's the familiar VDG green with black border -- but
> no text. Turned it off quickly and opened it. There seems to be some
> busted capacitors by the left. Could these be the sole cause of the
> problem? (Please don't tell me I have a busted GIME.)
> 
> https://picasaweb.google.com/lh/photo/HPZWguZCACN3cwolSptlu29XcHdiH80reA6_ygAbi6M
> https://picasaweb.google.com/lh/photo/zSjUBF2jnDgYYnnQ8ZpuOG9XcHdiH80reA6_ygAbi6M
> 
> Intriguing cheat sheets. What the kalamazoo is "JIMDOS3"? Custom ROM?
> 
> https://picasaweb.google.com/lh/photo/mLgGG3Zumnc7GDloyQQO-29XcHdiH80reA6_ygAbi6M
> https://picasaweb.google.com/lh/photo/FylCx2Wh5fArnKY-mNAH429XcHdiH80reA6_ygAbi6M
> 
> Juan
> 
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> 
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2012.0.2221 / Virus Database: 2441/5313 - Release Date: 10/06/12
>