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commodore 128DCR stuck after COMMODORE BASIC 128 banner no ready prompt

soviet9922

soviet9922

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Montevideo, Uruguay
Hi a few days ago a friend of mine dropped his non working commdore 128DCR to get some help fixing it.
The computer don't worked in 128 mode the keyboard typed random garbage.
So i replaced the ROM U34 for an eprom (128 basic) and socket both CIA's U4 and U1 replaced them.
This fixed the keyboard issues and the not working 128 mode and now the basic banner shows up in 128 mode.

The issue is that the banner "commodore basic v7 ...." shows but there's no ready prompt.
Read that this can be caused by a bad internal drive so found that cutting pin 1 from U113 disables it the internal drive have a broken head anyway.
Doing this don't changed anything still stuck after banner no ready prompt.
In 64 basic if i try to load from an external floppy drive tried device 8 and 9 nothing computer freezes and there's no activity on the drive.

Wondering what else could be ?.
 

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Are you able to boot into C64 mode, and if so, does that work without issue?

Does the drive boot at all? If you start with a disk in the drive does anything happen?
 
robert_sissco said:
Are you able to boot into C64 mode, and if so, does that work without issue?

Does the drive boot at all? If you start with a disk in the drive does anything happen?
Click to expand...
Yes 64 mode works fine, but if you try to access the drive the system freezes until reset, there's no drive activity.
Tried this agains drives 8 and 9 having also an external drive connected but the same result.
 
Running the computer having U112 removed gets all working fine, of course the internal drive is disabled.
Planning to use the C128 using an external drive so don't care about the internal one.

Thanks for all the help.
 
Just curious, what is U112? I did a bit of research and I found online that this is apparently a common fault on the rendition of the board.

My 128 is the wedge style so I don't have the built in 1571, but the reason I asked was that I know the 128 will boot from disk so I was wondering if you had that fault. All the post on other sites I saw called out the U112, but none of them actually noted what the chip was.
 
>>> Must have been a flaw in that run of the chip.

Most likely.

Commodore would have purchased in bulk a full set of components from various sources for a run of machines. If there is a fault in the manufacturing process for (say) one part, it is highly likely that all of the machines that are built with that particular part will be susceptible to the same failure over time.

Look at the JEDEC website to see how manufacturing and inspection techniques within the semiconductor industry have changed since the 'old days' due to various failure modes.

Dave
 
robert_sissco said:
My 128 is the wedge style so I don't have the built in 1571, but the reason I asked was that I know the 128 will boot from disk so I was wondering if you had that fault. All the post on other sites I saw called out the U112, but none of them actually noted what the chip was.
Click to expand...
Tried replacing the IC but the problem persisted, for that reason ended up removing it altogether.
Think that that chip is enabling some other stuff that is bad in the disk drive related chips.
 
Normally when a C128 computer boots it check to drive if is present a boot disk (ex. CP/M disk), then if not present it says READY. prompt. The issue is caused for more problem, the board have three 7406 logic chip, two of this is used for drive reset and computer reset circuit, if the computer reset circuit works is not sure the drive reset works too...then, the CIA 6526 on board and on drive controller (on DCR are all in a single board) is proably fault, swap 6526 is a simple technique to use.

If in your board you have a logic chip marked with MOS, or manufactured with brown plastic case (ex. EL7406 or EL 7416) at 99% of probablities is in fault.

In my C128D, with separate drive circuit board, i have checked same problem and i have found a 74ls14 fault in it. At the moment i don't have a DCR but probably the similar circuit is integrated in a main board.

Do you have checked if the reset signal arrive to drive circuit chips?

In other rare case the problem is in the C128 ROMs, but very rare cases, but possible.

Emanuel
 
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