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Next repair project - Commodore SX64

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bitfixer

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Apr 6, 2011
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Hi there,

I recently got hold of an SX-64, in great physical shape but non-working.
Hoping to try repairing it. Currently, when you turn it on, the drive light comes on and the drive motor turns - but nothing on the monitor.
I haven't had an SX-64 before so I'm not that familiar with it yet. Any ideas for first steps on a repair? Thanks!
 
I tried the SX-64 with an external monitor, and it came up to a basic prompt! 38911 Bytes free.
The keyboard works, except some keys are not that responsive.
So at least the main bits of the computer seem to be working. Now to figure out what's up with the internal monitor, and why the drive just keeps spinning. The repair document Tezza points to some possible issues, and the fuse for the CRT seems like a good candidate for a blank screen. Thanks for the help so far, I'll look into the next steps..
 
If the drive keeps spinning, it might be as simple as the 5V line to the drive controller is missing, so the drive never gets to complete its startup routine. However there are far worse causes too, like a bad 6522, ROM, RAM etc. I think you can check Ray's and others guides on 1541 drives and apply most of it to the SX-64 drive.
 
Well, some more good news on this one -
The repair guide indicated that the DOS ROM 2 (901229-05) was a common failure that caused this symptom. So I got a working ROM from a spare 1541 drive, and swapped it into the SX-64, and it worked! At least, the drive startup now looks normal.
The keyboard is sufficiently flaky that it proved difficult to type in commands to test further. I did manage to type a load command, and tried on a game disk, but it got a file not found error. However, I'm not sure the game disk itself was good and will probably need to resolve the keyboard issue to test further. I looked for a fuse near the monitor, but couldn't find it. I'll check again next time I open the machine up..
So that leaves the monitor as a remaining issue.
 
Do you have some cartridge with a built-in wedge like TFC 3, Action Replay etc? If so you could fetch the error channel on startup to see that you at least get the DOS version. You can do that without a cartridge too, but it'd involve more typing on a flaky keyboard:

10 OPEN 1,8,15
12 REM PRINT#1,"UJ" TO RESET DRIVE
15 INPUT#1,E,E$,T,S
20 PRINT E;E$;T;S
25 CLOSE 1

If you got nothing, garbage or the computer hangs when reading the error channel, you have more problems than the ROM you replaced.
 
Thanks. I don't have any of those cartridges, but I managed to get the keyboard to work a bit better. Maybe the keys just needed exercise.
I typed in this program, and got a "DRIVE NOT READY" in response. So at least something is working..
However, I tried reading from several commodore disks, and got "FILE NOT FOUND" errors. Drive would spin, make a cranking noise, then would stop with a blinking red light.
I also tried doing an initialize command, and formatting a disk, and both did not seem to work.
With the format command, the drive spun, made the cranking noise, presumably knocking the head into position, then it stopped. The red light kept blinking. There was no error reported, just a "READY." afterward.
 
From the Ray Carlsen documents, these seem to describe the symptoms I'm seeing:

UH1 NE592N (LM592) READ PREAMP
Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error and red LED flashes. Check
also UJ1 and UK4.

UJ1 NE592N (LM592) READ AMPLIFIER
Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error and red LED flashes. Check
also UH1 and UK4.

UK4 LM311 COMPARITOR (READ AMPL)
Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error, and red LED flashes. Check
also UH1 and UJ1.

That sounds like what's happening on this one. Will check these next.
 
Got a bit of time to work on this last night.
First, I cleaned the drive head with a q-tip and alcohol. No change after that. I tried 'LOAD "$",8' on a few disks, and the drive spun, there was a slight head movement, then a "FILE NOT FOUND" error.

After that I decided to try to replace one of the three chips I mentioned before. None were socketed, so replacing them would require desoldering or cutting them out of the board. I opted to try replacing the LM311 comparator first. I clipped out the chip, removed the legs, and installed an 8-pin socket, and put in a new 311 chip. Connected everything back up, and it worked!
I successfully initialized a disk and read it back, and even loaded and ran choplifter from a C64 floppy I had.

Now the remaining issue is the monitor. When powered on, the monitor is completely dark, with no CRT high-pitched sound. I suspect a fuse. I looked around for the internal fuse, but couldn't find it. Does anyone know where that fuse is located?
Thanks!
 
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