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C128 - No serial

NeXT

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Oct 22, 2008
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Location
Kamloops, BC, Canada
I finally had the patience to repair the keyboard on mine and the machine powers on and can switch modes and load from the cartridge fine but I can't use the serial port. The 1571 flashes the DISK light when the machine is switched on but the machine doesn't see the drive or even touch it with a LOAD"$",8. I confirmed the drive and cable were good by pulling out the C64 and booting GEOS with the same drive and cable. I swapped the 6526 and U4 and that didn't fix it either.
 

NeXT

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Oct 22, 2008
Messages
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Location
Kamloops, BC, Canada
I have now replaced the 7406 at U30 and confirmed again that both my 1571 and 1581 and their respective cabling are good and working on my older C64 but not on the C128. The U1/U4 chip swap has been done and nothing changed. Ray Carleson's troubleshoother doesn't list any other components to check.
 

NeXT

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Before I close this machine up again and put it away because I still can't find why it's not talking to the drives, does anyone else have any ideas?
 

dhoelzer

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Oct 20, 2012
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New York
Just a thought... I haven't done a lot of surgery on a 128 but I've done lots of work on 64s... Also, if you commented on this already, please forgive me.. I didn't see it.

I understand that you can't see the drive from 128 mode... can you see it from the 64 mode? I don't remember how much of that serial hardware is shared.

Before I close this machine up again and put it away because I still can't find why it's not talking to the drives, does anyone else have any ideas?
 

dabone

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Feb 26, 2009
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Chattanooga, TN - USA
Just for fun, double check continuity from u30 to the serial cable end.
Also make sure that U60 isn't doing anything to SQRIN signal. (7407)
Also, the SQRIN line also has a diode CR9 to check.


Later,
dabone
 
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NeXT

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Location
Kamloops, BC, Canada
Comparing continuity between the serial port on the 128 and on the 64 found that pin 5 (Serial data I/O) on the 128 has a 170 ohm short to ground.
To see what might of been causing that I flipped over the PCB again to inspect for trace damage and it seems the traces for 1 and 5 got pretty badly burned up. CR9 also appears to be shorted out. I'm going to patch that and see what happens.

Edited: CR7 also appears to be blown.

Edited: Yep. That fixed it. I can now read from both drives on the 128. God, what the hell did someone plug into this??
 
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