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Commodore 128DCR: problem with internal disk drive (resolved)
I have recently unpacked all my old Commodore gear from storage and started testing it to see what works and decide what to keep and what to get rid of. (I have parts of three different 128 systems, plus a bunch of games and other programs.) I'm getting ready to list one of the Commodore 128s for sale, and plan to keep the 128DCR and possibly the other 128, but I've encountered a problem with the disk drive and was hoping someone here might be able to point me in the right direction as to what could be the issue. When I turn on the power, I get the copyright message, but the machine hangs and never displays a READY prompt. The red light on the internal disk drive is blinking slowly, and the motor is spinning.
I found some troubleshooting information here: http://personalpages.tds.net/~rcarlsen/cbm/1571/1571diag.txt
Specifically, I found the following that might be relevant:
U3 SRAM M2128-xx or TMM2016
When powered up, green LED flashes once per second continuously. No drive access, locks up computer.

The problem is, I can't seem to find anything labeled U3 on my board (probably the troubleshooting info is for a discrete disk drive unit) nor can I locate any chip with a M2128 or TMM2016 label on it. Accordingly, I thought I'd post here and see if maybe someone more familiar with troubleshooting these could point me in the right direction. Below is a link to a Photobucket album with detailed pictures of the motherboard and disk drive assembly for reference.

Any pointers or suggestions anyone might have would be greatly appreciated. :) Thanks in advance!
Edited by: Dragget on 2016-02-25 06:43
Possible solution
REMark over on the Commodore 128 forum at freeforums.net sent me a link to the service manual, which identified the chip that is most likely the culprit here. Apparently it's the SRAM at location U103 (a Sanyo LC3517A-15 on my motherboard). Based that info, I found something similar on eBay:

http://www.ebay.com/itm/2-x-KM6816-15-SRAM-2048-x-8-bit-CMOS-static-Ram-IC-24-pin-dip-Korean-made-/261415087835?hash=item3cdd8daedb:g:7hkAAOxyNo9SthyS

Would these work as a replacement?
Datasheet for Sanyo LC3517A-15
Datasheet for Samsung KM6816-15
Looking at the datasheets, they seem to be comptible parts, but I'm no expert, so there could be something I'm missing.
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Moderation in all things... including moderation.
I ordered a replacement SRAM
I have ordered a replacement SRAM chip and in the meantime, I removed the old one and cleaned up the motherboard. I also pulled out the drive unit and hooked it up inside an external 1571 enclosure I have. I tested this with my flat Commodore 128, and it works fine, so I think this is pretty well narrrowed down to the bad chip on the board.
--==--
Moderation in all things... including moderation.
Re: I ordered a replacement SRAM

Dragget wrote:

> I tested this with my flat Commodore 128, and it works fine, so I think this is pretty well narrrowed down to the bad chip on the board.

Ah, that's good that you figured it out.

Truly,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm

Fixed
I just got the parts I needed last night, so I removed the old SRAM chip and replaced it. After I put everything back together, the disk drive works perfectly.
Replacement Chip photo KM6816-15.png
This is the chip I substituted for the original. I hope this is helpful for anyone else encountering similar issues.
--==--
Moderation in all things... including moderation.
The 1571 is a great drive.
The 1571 is a great drive. This problem is rare, but it does indeed happen. Another thing to watch is the ROM version. Bugs in early releases of the 1571 ROM affected access to the second side of the disk, so using the latest ROM is important. An easy way to achieve this is to use Jiffy DOS, since it contains the newest ROM Commodore made. I have to admit I am not 100% sure how this effects the 128D(CR)'s drive, but it should be the same as the stand alone 1571.
C-128 w/2Mb REU, & Video RAM upgrade C-64 w/modern PLA Amiga 2000, '030, 24Mb RAM
Re: The 1571 is a great drive.

motrucker wrote:

> I have to admit I am not 100% sure how this effects the 128D(CR)'s drive, but it should be the same as the stand alone 1571.

The C128DCR doesn't have the old 1571 ROM problems, i.e., it is already upgraded.

Back from the Dave Needle memorial,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm

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