[Coco] UK PAL CoCo1 Repair

[R Jones]

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.