[Coco] UK PAL CoCo1 Repair

[Camillus Blockx]

If you have the correct frame on display, then the sync is ok. this tells
you that the vdg,en cpu areworking. because you have sync also the pia
inquestion is ok too. garbage on screen is what the cpu reads from ram from
hex400 startiing in standard screen. if you can swap memory chips one by
one. i think that,s the problem here...bad ram


On Sep 20, 2017 12:54 PM, "Paul Shoemaker" <paulfe3 at swbell.net> wrote:

> Have you checked the power supply?  In my (limited) experience
> troubleshooting PCs, dodgy power supplies can often make it appear like
> other components are the problem (sneaky).
> According to the Color Computer Technical Reference manual:
> A "dead computer (garbage on the display or no display) indicates a major
> malfunction of the machine.  In this case, the first thing to check is the
> power supply voltage levels.  These may be checked at the following points.
> +5 volts - test Point 12-5 volts - Test Point 11+12 volts - Test Point
> 9-12 volts - Test Point 10Ground - Test Point 3
> Or if you are having problems finding these test points, check the voltage
> son one of the RAM chips (U20 - U27).
> Pin 1 = -5 volts (minus 5)Pin 8 = +12 voltsPin 9 = +5 volts
>
>
>
>       From: R Jones <rjones at biketech.org>
>  To: coco at maltedmedia.com
>  Sent: Wednesday, September 20, 2017 9:25 AM
>  Subject: [Coco] UK PAL CoCo1 Repair
>
> Hi,
>
>
> I'm trying to fix a fault on my Color Computer 1 (ECB), I broke this
> over 30 years ago while playing with a diy cartridge slot board. I put
> it to one side and never got round to fixing it. It now powers up and
> show random characters on screen, but no prompt. I'm just trying to work
> out the power line requirements for the major chips. Are schematics
> available for this version of the board?
>
> I have now socketed all the ICs on the board. As I have a Dragon 32,
> which uses mostly the same chips, I have swapped out most of the ICs in
> to the Dragon and seem to work fine, such as the cpu, 74ls783, 6821,
> 6822, 6847, 74ls138, 74ls273, 74ls244. I also have tried 2 other sets of
> working ram chips which do not seem to make any difference. I put the
> meter on most of the resistors and they check fine, I have also taken
> most of the capacitors out of circuit and they check fine on my basic
> tester.
>
>
> This is the board, a 26-3003: http://www.biketech.org/coco_board2.jpg
>
>
> I have checked the power and ground to all ICs and they are fine with
> the exception of this chip which I cannot find a datasheet for
> http://www.biketech.org/unknown_ic.jpg I see that Dave Jones of EEVBlog
> recently did a teardown of a CoCo1 with what looks to be an identical
> board but this chip has a different number on his.
>
>
> I have a scope and checked for the signals suggested in the SAMS
> diagnostic pdf and they are present.
>
>
> There doesn't appear to be any damaged or broken traces. I have tried
> the basic rom in the Dragon and it power up with the usual Tandy Basic
> text and appears to work fine.
>
>
> Any advice on where to check next would be appreciated.
>
>
> Thanks.
>
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> https://pairlist5.pair.net/mailman/listinfo/coco
>
>
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> https://pairlist5.pair.net/mailman/listinfo/coco
>