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Does anyone know how to repair Commodore computers?

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#1 the-topdog OFFLINE  



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Posted Wed Jan 25, 2012 1:28 PM

I'm scared to root around in the guts of vintage computers. A kid toasted one of my SX-64 computers... all I get is a blank screen now. :(

Does anyone here know how to troubleshoot and fix the problem? If so, I have no problem shipping the SX to you and paying for the service.

#2 save2600 OFFLINE  



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Posted Wed Jan 25, 2012 1:34 PM

I'd open the machine up and inspect for obvious damage before powering it up... but since you already tried... does it operate blind? Have you tried any DOS/disk functions? If so, might be the neck board just popped off the back of the yoke on the CRT...

Or it's just hanging there, not making very good contact. Yoke/tube might be cracked for that matter. To be safe, turn the thing off and wear thick gloves when attempting to put the circuit board back on the back of the CRT. :)

#3 the-topdog OFFLINE  



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Posted Wed Jan 25, 2012 1:55 PM

The crt seems fine... it lights up but just goes to a solid light blue color. The disk drive seeks like normal when powered up.

#4 save2600 OFFLINE  



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Posted Wed Jan 25, 2012 1:58 PM

Hmm... I'm not real familiar with the SX64... so I'm wondering if the main chips are socketed? If so, take a peek at them. The video chip may be under some metal shielding. With the machine off and being careful you stay away from the monitors high voltage, push down on all the socketed chips and see if that does the trick.

#5 the-topdog OFFLINE  



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Posted Wed Jan 25, 2012 2:07 PM

I'll try that when I get home tonight.

#6 98PaceCar OFFLINE  



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Posted Wed Jan 25, 2012 2:57 PM

The PLA chip in the SX is a known failure point that I believe can cause this problem. If you have a spare C64, you can use the same chip but they are rarely socketed in the breadbins.

The best page on the net for debugging tips is here: http://personalpages...arlsen/cbm.html

#7 Mirage OFFLINE  



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Posted Wed Jan 25, 2012 7:27 PM

If you get to a point where you're reasonably certain that you have a certain IC that's bad based on those debugging tips or whatever, PM me as I still have quite a stash of C64 IC's (don't tell anyone else, I don't have time to be rooting around for just anyone ;) ), some pulled, some on boards. Let me know what you need. Give the complete #, because I'm not certain which IC's may be different on an SX vs a normal 64. But if I can help, I will.

#8 Rybags OFFLINE  



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Posted Wed Jan 25, 2012 7:35 PM

The PLA is a common bad point (C64 equivalent to our MMU).

The RAM chips are the same type as used in 800XL and 1-bit 130XEs although need to be fast enough to run at 2 MHz.

There's probably better documents around but here's a copy of "c64-ic.txt" that lists some common problems:

latest additions and corrections: 9-28-07

U1 906108-02 (6526) CIA
Startup screen normal, but no cursor. No keyboard or control port
access. Partial failure: some keys or joystick positions don't work, one
character appears ahead of startup cursor, screen fills at startup as if
keys are being held down. Cartridge works.

U2 906108-02 (6526) CIA
Startup screen normal. No serial or user port access. "File not
found" error when drive accessed. Cartridge works. Characters sometimes
will show as blocks on startup screen.

U3 901226-01 BASIC ROM
Blank screen w/ border. Cartridge works.

U4 901227-03 (early -02) KERNAL ROM
Blank screen, no border. Most cartridges don't work but a few game
carts (example: CBM Kickman and Jupiter Lander) will work with a normal
screen because they bypass the Kernal ROM.

U5 901225-01 CHARACTER ROM
Normal startup border, but "garbage" characters where startup page
characters should be. Cartridge works.

Startup screen shows flickering characters with shimmering colors.
check also PLA U17.

U7 906107-01 (6510) MPU
Blank screen, no border. Cartridge doesn't work.

Blank screen. Partial failure: drive access problems: drive
resets, but "device not present" error.

U9 THRU U12 & U22 THRU U24 8 RAM CHIPS (4164)
note: only 2 4464 RAM chips in C64C
Blank screen, no border. Shorted chips will get warmer than the
other RAM chips. Partial failure: will sometimes produce garbage
screen, abnormal number of bytes free or "out of memory" error on
startup screen.

U13 74LS257 (or MOS7708) LOGIC
Blank screen or less than 38911 bytes free at startup.

U14 74LS258 LOGIC
Blank screen.

U15 74LS139 LOGIC
Blank screen

U16 CD4066 (generic CMOS quad bilateral switch)
Color problems such as random color "checkerboard" pattern on screen
or no color.

U17 906114-01 (82S100PLA) PLA
Blank screen, no border. It can produce colored screen or flashing
color garbage instead of startup screen. It can cause intermittant loss of
cursor, screen freeze and/or program crashes after warmup and it can put
random characters on screen. This chip normally runs hot. It is the most
common chip to fail in the C64 and it should run no hotter than the SID.

U18 906112-01 (6581) SID
Normal screen. No sound or garbled sound. Mouse or graphics tablet
pointer stuck or jitters. If shorted, can cause blank or "garbage" screen.
NOTE: computer will work without a SID plugged in (unplug to check). This
chip normally runs hot.

U19 906109-04 (6567) VIC (PAL version is 6569)
Blank screen (may be light or dark) with no border. Sometimes will
produce garbage or "checkerboard" screen, or screen that lacks contrast.
If screen is blank or garbled from bad VIC, "blind" disk commands from
keyboard may still work.

Blank screen. Computer will not reset, or RESTORE doesn't work.

U25 74LS257 (or MOS7708) LOGIC
Blank screen or less than 38911 bytes at startup.

U26 74LS373 LOGIC
Startup screen has normal border, but characters are scrambled.

U27 74LS08 (or MOS7712) LOGIC
Normal startup border, but screen full of "garbage" characters.

U28 CD4066 (generic CMOS quad bilateral switch)
Normal startup screen. Proportional mouse or graphics tablet
doesn't work in one or both control (joystick) ports.

Startup screen shows normal characters in multicolored "rainbow".

U31 8701 MASTER OSC. (Some C64s use 74LS629N w/ support logic)
Blank white screen, no border. Normal "whine" in nearby AM radio
will be missing if the oscillator is not running. Check also VIC U19

No monitor picture, just diagonal lines on screen.

Can produce many problems like blank screen (red LED on, dim, or off),
program lock-up, garbage screen, hum bars moving on screen, hum in audio,
damaged RAM or other chips, intermittant operation after warmup, etc.

Ray Carlsen CET

#9 krslam OFFLINE  



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Posted Fri Jan 27, 2012 1:31 PM

I'd recommend contacting Ray Carlsen thru his website: http://personalpages....net/~rcarlsen/ Far as I know, he's still repairing Commies and there's no one better. Plus, he's not too far away from you so shipping shouldn't be too outrageous.

#10 schuwalker OFFLINE  



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Posted Sat Jan 28, 2012 5:54 PM

This guy might be able to help out...


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