[Coco] 128K CoCo 3 is DOA D:

[Retro Canada]

juan its a 512 ram coco. check the ram chips first. the gime is a hard chip to remove. if you do so remember its orientation since there are no marks.

i would say its a memory problem since you see the emulated vdg screen.

check the voltage on TP1 and TP2 (there are 2 small test points on board labelled as) and they must be IIRC 8v and 5v

good lucks

Sent from my iPhone

On 2012-10-06, at 9:16 AM, jdaggett at gate.net wrote:

> 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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>> 
>> -----
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>> Version: 2012.0.2221 / Virus Database: 2441/5313 - Release Date: 10/06/12
>> 
> 
> 
> 
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