C64C - CHIPS vs COMMON SYMPTOMS 09-28-07.


           C64C - CHIPS vs COMMON SYMPTOMS     
                   last update and/or revision: 09-28-07


There were several revisions to this model that featured a slimline white 
case, white keys and an internal heat sink shield. The earliest versions used 
the original 64 motherboard (250425), the same board as in the brown case C64. 
Interim boards (250466) used two 4464 RAM chips instead of eight 4164, used 
either version SID or CIAs, but retained the old MPU, PLA and VIC-II. The 
latest "cost reduced" motherboards (250469) were narrower and used fewer, more 
integrated chips. Most of those ICs are not interchangeable with earlier models, 
and chip ID numbers are different than earlier boards. For that reason, this 
article deals with the latest board only, although basic troubleshooting is 
similar for all versions of the C64.

U1 	906108-02 (6526/ A or 8521) CIA 
     Startup screen normal but no cursor, or blank screen if chip is shorted 
(remove to test). No keyboard or joyport access. Partial failure: some keys or 
joystick positions don't work, "stuck" line may print a character at startup. 
Cartridge works.

U2 	906108-02 (6526/ A or 8521) CIA 
     Lines or blocks instead of startup screen, but blank screen if chip is 
shorted (remove to test). Partial failure: marginal or no serial / user port 
access, keyboard and cartridges still work. 

U3      74LS08 LOGIC
     Blank screen, no border. 

U4      251913-01 BASIC and KERNAL ROM (same as C128)
     Blank screen, no border. Cartridges don't work except the few that bypass 
the Kernal ROM such as CBM Kickman and Jupiter Lander. Those can be used as a 
diagnostic for a bad Kernal.

U5	901225-01 CHARACTER ROM	
     Blank screen with border or screen full of shimmering lines or characters. 
Partial failure: "garbage" characters or blocks where startup page info should 
be. Cartridge works. 

U6	318012-01 (8500) MPU aka CPU (6510 works as sub, see text)
     Blank screen, no border. Cartridge doesn't work.

U7      318014-01 (8562 or 8565) VIC-II
     Blank -white- screen, no border. Partial failure may produce garbage or 
"checkerboard" screen, or smeary screen that lacks contrast. If screen is blank 
or garbled from bad VIC, "blind" disk commands from keyboard may still work. 
NOTE: earlier 6567 will -not- sub for this chip.
     
U8	251715-01 (LH5062B OR YM3535) PLA, MEMORY CONTROL
     Blank screen, no border. Runs cool, rare failure.

U9  	318013-01 (8580) or 906112-01 (6581) SID (SOUND INTERFACE DEVICE)
     Blank or "garbage" screen if shorted (remove to test), otherwise normal 
screen. No sound or garbled sound. Mouse or graphics tablet pointer stuck or 
jitters. NOTE: for test purposes, computer will work without a SID plugged in. 
NOTE: 8580 and 6581 are pin compatible and somewhat interchangeable, but only 
with board component changes: pin 28 supply voltage either +9 or +12VDC, and 
two capacitors values must be changed for correct voicing.

U10 and U11 (M41464) 2 RAM CHIPS (64K X 4 DRAM)
     Blank screen, no border. Shorted chips may get warmer (sometimes very hot) 
than the other RAM chips. Partial failure: will sometimes produce "garbage" 
screen, abnormal number of bytes free (lower than 38911) or "OUT OF MEMORY IN 0"
error on startup screen.

U18     4066 CMOS QUAD SWITCH (POTX/Y BUFFERS)
     Startup screen normal, but mouse/graphics tablet don't work.

U19     LC3514A or MN2114-2 SRAM 1K X 4 (aka COLOR RAM)
     Shimmering colors on characters.

U20	8701 or 7701 MASTER CLOCK OSC.		
     Blank white screen, no border. 

U21     4066 CMOS QUAD SWITCH
     Wrong colors on characters. 

U22     7406 LOGIC
     Blank screen. No drive reset when computer switched on.

U23     74LS14 LOGIC
     Blank screen. Drive spindle motor runs continuously with computer on. 
RUN/STOP-RESTORE doesn't work. 

M1      252405-02 RF MODULATOR
     Weak or no RF output (snowy screen) to TV. Internal defect in modulator can 
affect A/V outputs (no audio or video) since those signals go -through- modulator.

Q1     2SD313 TRANSISTOR, CASSETTE MOTOR DRIVER
     Cassette motor will not turn when FF/REW or PLAY is pressed. Also check 
fuse inside computer.

F1     1.5 AMP FB FUSE, 9VAC PROTECTION
    If fuse is open, computer appears to work normally, but no cassette 
operation, no 9VAC to user port, and no sound from SID.

POWER PACK: +5VDC at 1.5Amps and 9VAC at 1 Amp
     Can produce many problems like blank screen (red power LED on, dim or 
off), program lock-up, "garbage" screen, hum bars moving on screen, hum in 
audio, damaged RAM chips, intermittant operation after warmup, etc. As common
a failure as it is, the supply should be checked (by substitution) first. 

*******************************************************************

     Troubleshooting any version of the C64 is often difficult because some 
symptoms can be caused by any one of a dozen chips. The most common, a blank 
screen with no border, is sometimes frustrating for even a tech to diagnose. 
We usually start by process of elimination, keeping in mind the chips most 
likely to fail in order. Each board is different in how many of the major 
chips are socketed, but that's a good place to start. Reseat all socketed chips 
by lifting up on each end slightly (pry between chip and socket with a small 
flat-blade screwdriver) and press the chip back down. There is no need to 
remove the chip just for reseating. You risk bending pins if you remove it.
    If you suspect a chip is bad based on symptoms, the best way to test it is 
by installing it in a working computer. If none are available, and/or you have 
no spare parts or boards, the job is nearly impossible. Substitution of chips 
or parts is the only practical way for most people to diagnose a fault. Don't 
overlook the power supply when you are troubleshooting. It deserves its bad 
reputation for damaging computer chips under some fault modes. Check by 
substitution only. The original supply is "potted" with epoxy inside and is not 
repairable. After-market power supplies are "beefier" and usually repairable.
     The 85xx series chips used in the C64C are, for the most part, not 
interchangeable with earlier C64 ICs. They represent a different logic "family" 
and in some cases have a different pinout. Substitutions should therefore not be 
made casually. 
     As indicated above, the computer will work with a few of the major chips 
removed. This is helpful as a diagnostic. For example, the SID can produce a 
blank screen if shorted, but the computer will still work without it, except 
of course without sound, and the POTX/Y inputs will not work (mouse, graphics 
tablet, etc.) but a joystick will still work. The two CIA chips can be removed 
to test for shorted chips (they're bad for sure if they get hot) and the computer 
will come up if the chip is bad although the startup screen will be affected (see 
chip list for U1 and U2). A bad keyboard row or column is usually caused by a bad 
CIA. The two CIAs can be swapped to aid in diagnosis if no other parts are 
available, and the fault will follow the bad chip, but of course with different 
symptoms.
     Unlike the one in the C64, the PLA in the C64C runs cool to the touch. I've 
never seen a bad one. Of course new and old versions are not interchangeable.
The older 6510 MPU seems to work in the C64C as a sub for the 8500. I have tried 
an 8500 in an old C64 and it worked (diagnostic test only), but I don't know how 
well these chips interchange in actual use. All of the 8500 series chips run cooler 
than the older 6500 series. The later C64C uses some of the same chips as a C128. 
     Note that an open fuse in the older C64 computer will make it non-functional 
because the converted 9VAC runs the VIC and SID chips. An open fuse or failed 9VAC 
(bad power supply) with the later C64C may not be immediately apparent... the 
computer will still work but you will have no sound, the user port will not get 
the 9VAC, and the cassette motor will not work. 
     The cabinet screws and the ones inside the computer may be the newer Torx 
(star-shaped) type rather than standard Philips. If so, you may need to get a 
T-10 bit to removed those screws. I've found that a standard small flat-blade 
screwdriver fits well enough if a Torx driver is not available.
     As mentioned before, the "blank screen" symptom is the hardest to troubleshoot 
because many different problems can produce that symptom. It would therefore be 
helpful to know what signals should appear on each pin of the microprocessor, the 
heart of the machine. If, for example, the reset line was being held low for some 
reason, that could be traced down to the specific problem and fixed. With that in 
mind, here is the pinout for the 6510/8500 microprocessor used in the C64 and C64C. 
Readings are taken with computer on and no external devices connected, keyboard 
unplugged. You'll need an oscilloscope or logic probe to make sense of some of 
these readings.
   
1  CLK 1 IN	OSC SIGNAL
2  RDY		DATA, ANDED BA FROM PLA + \DMA 
3  \IRQ		DATA (HIGH)
4  \NMI		HIGH, FROM \RESTORE CIRCUIT
5  AEC		DATA, ANDED AEC FROM VIC + \DMA
6  VCC		ALWAYS +5VDC
7  A0		DATA  \
8  A1		DATA   |
9  A2		DATA   |
10 A3		DATA   |
11 A4		DATA   |
12 A5		DATA   |
13 A6		DATA   | 
14 A7		DATA   |
15 A8		DATA   | ADDRESS LINES
16 A9		DATA   |
17 A10		DATA   |
18 A11		DATA   |
19 A12		DATA   |
20 A13		DATA   |
21 GROUND	0 VDC
22 A14		DATA   |
23 A15		DATA  /
24 P5 		+3V
25 P4		HIGH
26 P3		LOW
27 P2		HIGH
28 P1		HIGH
29 P0		HIGH
30 D7		DATA  \
31 D6		DATR   |
32 D5		DATA   |
33 D4		DATA   |
34 D3		DATA   | DATA LINES
35 D2		DATA   |
36 D1		DATA   |
37 D0		DATA   |
38 R/W		DATA  /
39 CLK 2 OUT	OSC SIGNAL
40 \RESET	HIGH, FROM RESET CIRCUIT
     

Ray Carlsen CET
CARLSEN ELECTRONICS
If you have any questions or comments, please feel free to write,
especially if you spot an error here. Thanks!