[Coco] UK PAL CoCo1 Repair

Wed Oct 4 10:58:42 EDT 2017

Yep, I’ve done the ice cube in a ziplock trick too.  :)

Does sound like a faulty IC heating up and failing.  I’d feel the chips and see
if any get noticeably hot in a short time.  It could of course be in the power circuit as well.

- Ed

> On Oct 4, 2017, at 9:44 AM, Salvador Garcia via Coco <coco at maltedmedia.com> wrote:
> 
> Yeah, I was also thinking "heat".
> 
> The ice cube in a bag idea is great! I'll have to keep it in mind for future reference.
> 
> Salvador
> 
>    On Tuesday, October 3, 2017, 4:54:37 PM CDT, Arthur Flexser <flexser at fiu.edu> wrote:  
> 
> 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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