Does anyone know how to repair Commodore computers?

[the-topdog]

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.

[+save2600]

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.

[the-topdog]

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.

[+save2600]

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.

[the-topdog]

I'll try that when I get home tonight.

[98PaceCar]

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.tds.net/~rcarlsen/cbm.html

[Mirage]

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.

[Rybags]

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:

 

C64 CHIPS and COMMON SYMPTOMS

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.

 

U6 2114 SRAM (COLOR RAM)

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.

 

U8 7406 LOGIC CHIP

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.

 

U20 556 LOGIC, TIMER

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.

 

U29 74LS74 LOGIC, DIVIDER

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

 

U32 4044 CMOS LOGIC

No monitor picture, just diagonal lines on screen.

 

POWER PACK:

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

CARLSEN ELECTRONICS

 

[krslam]

I'd recommend contacting Ray Carlsen thru his website: http://personalpages.tds.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.

[schuwalker]

This guy might be able to help out...

 

 

 

http://www.arcadecomponents.com/index.html