[Coco] Weird CoCo3 problem...

[Gene Heskett]

On Tuesday 05 January 2010, Brian Blake wrote:
>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.
>
Glad I could help.  And the PBJ board is ok for a memory boost, but I found 
the 2 meg disto kit, using more modern simm type drams but cmos, draws 1/10 
the power of the PBJ.  Virtually no heat with a 63C09 in the cpu socket.  
Mine runs on an old pc power supply with all the shack parts removed, and I 
can toss furniture blankets over it for a day at a time, 2F temp rise right 
over the middle of the grill where the memory sits.  The GIME is by far the 
hottest chip in mine.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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