[Coco] Weird CoCo3 problem...

[Brian Blake]

Hi Gene,

Pin 9 of the LS138N is the /SCS, and that reads the same 2.24vdc from pin 9 all the way to the disk controller's LS139AN pin 15.

Be placing a Jameco order tomorrow.

As for soldering iron, I have two. One Radio Shack selectable from 20/40 watts. I also have a Chinese knock-off of a Hakko 852 with adjustable temperature. That's what I used for my first 6309 upgrade almost three years ago, and has soldered countless joints on the Christmas light controllers I've built. The R/S unit was bought this past summer for quick work at home (I keep the other one at work where I have more space) and did the swap over the weekend on the CoCo in question. It hasn't been used 1/100th as much as the 852 has, and it's tip is almost useless. The 852's tip still looks very good, heats up in seconds, and does an overall better job.

Incidentally, I swapped the PBJ upgrade from the problem CoCo3 into the 128k unit I had, plugged the FD-502 into, loaded played Sock's Donkey Kong a few times, and then played it for about two hours - I think that pretty much says the PBJ board is fine. Now I gotta order a couple 512k upgrades from Mark; one for this CoCo and one for my re-pack...

Thanks for all the help guys!!!


----- Original Message ----
From: Gene Heskett <gene.heskett at verizon.net>
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Sent: Tue, January 5, 2010 9:46:32 PM
Subject: Re: [Coco] Weird CoCo3 problem...

On Tuesday 05 January 2010, Brian Blake wrote:
>Okay, I spent some time cleaning the probe and other things this evening.
> I'm happy to say that made an improvement in the waveforms - all waveforms
> between the two systems are almost identical.
>
>The one are I have concern is the /SCS:
>
>On the CoCo3 in which the FD-502 does work on, that is in the neighborhood
> of 4.25v with the blips Mike was talking about.
>
>On the 'Bad CoCo' there is a constant 2.24vdc with no discernible blips.

Bingo!  That would be about what an input would float at if it had no drive 
at all due to no connection.  Now, trace that back to the src chip that 
decodes the /SCS, check it there.  If present as a normal waveform there, 
then there is a broken trace, possibly from a through hole that needs 
resoldered.  If the output pin on that 74ls138 (or 139, my memory is a just 
tottering along on that) is also flat, then locate a fresh chip and a socket 
to fit it.

Then clip the leads on that chip between the board and the chip, toss the 
chip, clean out the holes and install the socket with fresh solder, and put 
the new chip in the socket.  You should be fixed, assuming you have a decent 
soldering iron, a solder sucker, and some decent rosin cored solder.  I have 
an XYtronics static grounded and temperature controlled iron, and for the 
last several years I have been partial to the 3% silver bearing, but 
otherwise eutectic blend of rosin cored stuff that GC sold a few years ago, 
its worked well, and has excellent wetting properties at temps as low as 500 
degrees although I tend to run it at around 700 cuz you can get in, get the 
job done and back out before the heat telegraphs and cooks something else.
The 3% silver also makes it a good deal harder & stronger at room temps.

>Any other areas to check in order to confirm this?

I really think you've found it.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)

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