[Coco] UK PAL CoCo1 Repair

Tue Oct 3 17:52:47 EDT 2017

My guess would be that after 10 seconds, one of the chips has overheated
and stops functioning.

I once cured a similar problem in my dad's Commodure 64 using this
technique:

I put an ice cube in a plastic bag and held it against the top of a
suspected chip, a chip that I'd noticed got particularly hot. Then I'd turn
the machine on and watch what happened.  When I held it against this chip,
as I'd hoped, the machine took much longer than usual before the crash
occurred.  Replacing that chip cured the problem.  The chip in question was
the Commodore equivalent to a PIA, so certainly try this with the PIA
chips.  Come to think of it, before ordering the new chip, I tried swapping
the two identical  PIA-equivalent chips with one another, and the symptoms
of the crash changed, so that was a confirmation.  If your two PIAs are
identical and socketed, try swapping them and see if that affects anything.

Art

On Tue, Oct 3, 2017 at 5:27 PM, R Jones <rjones at biketech.org> wrote:

> Just a quick update. I replaced some of the other logic ICs and replacing
> the 74ls02n seems to have improved the situation. The computer now starts
> up and brings the usual initial text and the prompt, the keyboard works and
> commands seem to work fine, but after 10 seconds or so the screen fills
> with garbage and crashes.
>
> I have uploaded a quick video to show what is happening on youtube, here's
> the link https://www.youtube.com/watch?v=JsPAIc3ixcw
> In the video it shows it working, I press 'enter' which works it then
> crashes after a few seconds, I press the reset button, this does not bring
> the prompt back. Just before the end of the clip I power off and then back
> on, the computer crashes straight away this time. If it is left off for 5
> minutes or so it will power on back to the prompt but crash after a few
> seconds again.
>
> Has anyone seen this behaviour before and what could be causing it? I am
> still waiting on a couple of other logic ICs to arrive, so maybe they will
> help. I am not sure what is causing the glitching of the video output.
>
>
> Thanks.
>
>
> Subject:        [Coco] UK PAL CoCo1 Repair
> Date:   Sat, 23 Sep 2017 17:18:23 +0100
> From:   R Jones <rjones at biketech.org>
> To:     coco at maltedmedia.com
>
>
>
> Thanks for all the help and suggestions, much appreciated.
>
> It is possible have could have made an error while removing some of the
> ICs to check and socket them. I will try find time this next week to go
> over each and double check no tracks have been damaged or any other faulty
> traces.
>
> I did notice that the 10k pot just under the 2n6594 seemed loose, once I
> had desoldered it one of the legs fell off, so that may have been one
> issue. Replacing it has not changed the situation. This is strange because
> I don't remember adjusting this at all.
>
> As most of the simple logic ICs are still available and cheap, I have
> ordered replacements for them just in case. Although I am trying not to
> just replace every component on the board! I could of course just look on
> eBay for a working CoCo, but I would like to get this one running without
> replacing the whole mainboard.
>
>
> Rob
>
>
> On 21/09/2017 13:58, Zippster wrote:
> Sorry meant to send this to the list.....
>
> On 21/09/2017 13:58, Zippster wrote:
>  > Well, if power seems goods, and the IC’s seem good, I think I’d take
> a close look at
>  > the reset circuit if you haven’t already.
>
> Yep one of the problems I had with the CoCo and some Dragon motherboards
> during the development of DragonMMC was that the Xilinx CPLD would pull
> the reset line low due to the rather weak pullup on it leading to the
> machine not coming out of reset and having a screen full of random junk.
>
> Fixed it with a diode and reistor between CPLD and reset line.
>
> Also worth checking that the IRQ, FIRQ, NMI and HALT lines are high, (or
> possibly pulsing at 50Hz in the case of IRQ)
>
>  > If that looks good I might start poking around with the scope,
> probably starting at
>  > the CPU/SAM looking for the expected signals and bus activity and
> working out
>  > from there.
>
> Yep check for E, Q pulsing, R/W will generally be pulsing though less
> regularly, ass should Address and Data lines.
>
>  > It sounds as if the video system is working ok, so you’re probably
> good there.
>  > It does sound like a CPU/memory problem of some sort.
>
> Agreed if you are getting a picture with junk on the screen then the
> 6847 and probably the SAM also are working in this board. (I know you
> verified them in another machine).
>
>  > You may want to verify the BASIC ROM just to make sure it hasn’t been
> corrupted/damaged
>  > in some way.  Just to make sure the system isn’t simply crashing on
> bad ROM code.
>
> I think he said he tried it in a Dragon and it worked there, which would
> seem to mean it's probably working.
>
> Have you tried blindly typing MOTORON and listening for the relay click?
> If that works the CPU is working and your fault lies somewhere between
> the RAM and the VDG, I believe there is an LS373 latch there.
>
> Something that I have also seen fail is the LS244 buffer between the RAM
> data outputs and the CPU data bus, can't remember if you checked those.
>
> May also be worth checking for shorts / continuity on the data and
> address lines and between the pins of any chip that you have desoldered
> and replaced as it's all too easy to solder splash short between tracks
>
> Note that on the CoCo and Dragon, unlike almost all other micros of the
> period, the data in and data out of the RAM chips are not connected
> together, so need tracing separately.
>
> Cheers.
>
> Phill.
>
>
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