[Coco] Weird CoCo3 problem...

[Gene Heskett]

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)

Barker's Proof:
	Proofreading is more effective after publication.