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From: Edward Shockley on 12 Apr 2008 11:10
I have a problem with a new addition I have. A Commodore hardware problem
that I haven't been able to identify a repair for yet. And haven't found
anything on google similar to the symptoms. So I hoped maybe someone else
has ran into it.

This machine was working perfectly and passing factory diagnostics loaded
from 8050 just fine for several days earlier. Then it developed the problem
after having been on for two hours while backing up disks on the 8050.

System still boots fine with blinking cursor and BASIC 4 banner. Can type in
the BASIC editor and screen scrolls fine (i.e., you can hold a key down and
fill it and she scrolls up with no garbage). You can CTRL-G and ring the
bell/speaker. You can enter numbered program lines (e.g., 10 print "hello")
and get the cursor for the next line just fine. But, here's the problem, any
BASIC commands with immediate action (e.g., run, new, list) return "?Syntax
error" everytime. So you cannot load/save any programs or run anything you
type in from BASIC. Even more interesting if you remove the BASIC ROMs,
this model default loads to a monitor in the kernal ROM. And from there I
CAN run simple code and load/save to disk just fine so it's something
hardware related that only affects BASIC.

I replaced all the factory socketed chips with known good ones from a
working machine, 6509 CPU, 6526A, BASIC hi and low ROMs, kernal ROM. None of
the DRAM chips are hot to the touch. This is for a B series hi-boy
(700/710/720) machine but I thought if I could find a C64/VIC20/PET/CBM
similar repair story it would help where to debug such a problem on the CBM
256-80 since the architectures are similar enough.

Thanx,




From: Edward Shockley on 12 Apr 2008 13:52
Well I ended up fixing it myself finally so I'll post what it was in case
someone else hits this type of problem in the future. On a whim I decided
it could be segment one memory for the BASIC working space area since the
machine language monitor worked. I've heard a stuck bit rather than
shorted DRAM chip will not be hot.

So I used the piggyback replacement technique until the symptoms changed (in
this case frozen after entering any BASIC command which I figured was timing
related from the piggyback). The 4164 at U50 turned out to be the culprit
and that fixed the ?Syntax Error problem.

"Edward Shockley" <drshock(a)nospam.insectria.net> wrote in message
news:t_6dnX0Nmd9QTZ3VnZ2dnUVZ_jydnZ2d(a)comcast.com...
>I have a problem with a new addition I have. A Commodore hardware problem
>that I haven't been able to identify a repair for yet. And haven't found
>anything on google similar to the symptoms. So I hoped maybe someone else
>has ran into it.
>
> This machine was working perfectly and passing factory diagnostics loaded
> from 8050 just fine for several days earlier. Then it developed the
> problem after having been on for two hours while backing up disks on the
> 8050.
>
> System still boots fine with blinking cursor and BASIC 4 banner. Can type
> in the BASIC editor and screen scrolls fine (i.e., you can hold a key down
> and fill it and she scrolls up with no garbage). You can CTRL-G and ring
> the bell/speaker. You can enter numbered program lines (e.g., 10 print
> "hello") and get the cursor for the next line just fine. But, here's the
> problem, any BASIC commands with immediate action (e.g., run, new, list)
> return "?Syntax error" everytime. So you cannot load/save any programs or
> run anything you type in from BASIC. Even more interesting if you remove
> the BASIC ROMs, this model default loads to a monitor in the kernal ROM.
> And from there I CAN run simple code and load/save to disk just fine so
> it's something hardware related that only affects BASIC.
>
> I replaced all the factory socketed chips with known good ones from a
> working machine, 6509 CPU, 6526A, BASIC hi and low ROMs, kernal ROM. None
> of the DRAM chips are hot to the touch. This is for a B series hi-boy
> (700/710/720) machine but I thought if I could find a C64/VIC20/PET/CBM
> similar repair story it would help where to debug such a problem on the
> CBM 256-80 since the architectures are similar enough.
>
> Thanx,
>
>
>
>


From: Klompmeester on 12 Apr 2008 17:59

"Edward Shockley" <drshock(a)nospam.insectria.net> wrote in message
news:bt6dneNyM7Nxa53VnZ2dnUVZ_quhnZ2d(a)comcast.com...
> Well I ended up fixing it myself finally so I'll post what it was in case
> someone else hits this type of problem in the future. On a whim I
> decided it could be segment one memory for the BASIC working space area
> since the machine language monitor worked. I've heard a stuck bit rather
> than shorted DRAM chip will not be hot.
>
> So I used the piggyback replacement technique until the symptoms changed
> (in this case frozen after entering any BASIC command which I figured was
> timing related from the piggyback). The 4164 at U50 turned out to be the
> culprit and that fixed the ?Syntax Error problem.
>


Thanks for posting how you fixed it, I was about to post that an open DRAM
chip doesn't get hot and a testing procedure (same as what you did).

..


From: bud on 13 Apr 2008 00:59

Group: comp.sys.cbm Date: Sun, Apr 13, 2008, 5:59am (CDT+13) From:
whowhere(a)andwhy.com (Klompmeester)

script:

>Thanks for posting how you fixed it, I
>was about to post that an open DRAM
>chip doesn't get hot and a testing
>procedure (same as what you did).

Yes, thanks Edward. This _is_ a PET group too,… tho' many of us don't
have one.

salaam,
dowcom

To e-mail me, add the character zero to "dowcom". i.e.:
dowcom(zero)(at)webtv(dot)net.

--
http://community.webtv.net/dowcom/DOWCOMSAMSTRADGUIDE

MSWindows is television,… Linux is radar.

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