COMMODORE VIC20 COMPUTER - DIAGNOSTICS AND REPAIR 12-07-06.


           COMMODORE VIC20 COMPUTER - DIAGNOSTICS AND REPAIR 
              latest additions and corrections  12-07-06

There are two basic versions of the VIC20. The earlier (1980) model is
 easily recognized from the outside by the two pin connector used for 9VAC
 power. The later (1981/82) versions have a 7 pin DIN connector and use the
 same power pack as a Commodore 64. Inside, the two VIC20 versions use
 different boards. Earlier boards are larger and have two large black heat
 sinks for the bridge rectifier and 5 volt power regulator. Later model
 boards are shorter, have fewer more integrated chips (cost reduced) and
 use an external supply. They are lighter and run cooler.
     Because of the differences in boards and some chip ID numbers, I will
list the two versions separately in the chips vs symptoms portion of this
article. There are other versions but these two represent the majority of
the existing boards. Chips with symptoms shown are considered more common
failures. Although TTL (so-called "glue logic") chips are rather robust,
any part can fail at any time. Most at risk are those directly connected
to external ports such as the VIA and interface logic because of external
voltage "spikes" and surges. Also, any semiconductor device (transistors, 
diodes and ICs) that normally run hot fail sooner than those that run cold. 
     The last part of this article will tell how to troubleshoot problems
and offers some diagnostic procedures you can do with little or no test
equipment. Careful observation is important. As with other Commodore
computers, the easiest way to test suspected chips is by installing them
one at a time in a working board. If a chip is soldered in (no socket), it
makes it that much more difficult to swap them for tests. Minimum equipment
for board rework includes a vacuum desoldering iron, volt-ohmmeter and basic
hand tools.
     Since the VIC20 has no internal RF modulator, you need to either use
the factory external RF convertor box that plugs into the 5 pin A/V port on
the computer, or use a direct A/V connection to a monitor or VCR. The VIC20
opening screen should appear within about four seconds after power up. It
has a cyan border and blue letters on a white background. The full screen
is 22 characters wide by 23 high. At power up, it should look like this:

* * * * CBM BASIC VERSION 2 * * * *
3 5 8 3  B Y T E S  F R E E
R E A D Y
[] <---(flashing cursor)

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

EARLY VERSION BOARDS: ASSY #324003 FAB #324002-01 REV D 1980

UAB1		6522		VIA	INTERFACE, KEYBOARD-SERIAL
  Startup screen normal, but no cursor. Keyboard doesn't work at all.
Partial failure: some keys "stuck" or don't work... may be a whole row or
column. Also can cause serial port (drive access) problems. Can produce
blank screen if shorted (remove to check).

UAB3		6522		VIA	INTERFACE, SERIAL-JOY-USER-CASS
  Startup screen normal, but drive access problems ("SEARCHING FOR.. "
forever). One or more joystick positions don't work. No cassette or user
port access. Can produce blank screen if shorted (remove to check).

UB4		7406		LOGIC	SERIAL PORT BUFFERS
  Normal startup screen. Serial port (disk drive) access problems.

UB6		LM555		TIMER	POWER-ON SYSTEM RESET
  Will not reset on power up. May produce garbled or random screen
characters-graphics, or screen may start up "frozen". If drive resets when
computer is powered up, this chip is OK.

UB7		6560-101	VIC	VIDEO/AUDIO
  White or blank screen, garbled or no video, screen full of (or few random)
"garbage" characters. "Blind" disk commands may still work.
Partial failure: Dark or smeary image, loss of color, garbled or no sound,
game paddles or light pen doesn't work.

UB9		7402 (M53202P)	OSC	MASTER CLOCK OSC
  White screen... no RF interference in AM radio (see text). Partial failure
can produce missing color or incorrect colors, slight or severe tearing of
picture or diagonal lines, sounds play at incorrect pitch. Symptoms may only
appear after warmup.

UC2		74LS04		LOGIC	COLOR INTERFACE
  Screen character colors incorrect (scrambled). If very bad, entire screen
becomes garbled with flashing characters, vertical bars and random colors.
Check also UE1 color RAM and UD1 interface logic. 

UC3		74LS02		LOGIC

UC4		74LS138		LOGIC	MEMORY CONTROL
  Blank screen.

UC5		74LS138		LOGIC	BLOCK CONTROL
  Blank screen.

UC6		74LS138		LOGIC	I/O CONTROL

UD1 		CD4066		LOGIC GATE
  (4066 is UC2 in very early version) Some screen characters have incorrect 
colors. Check also UE1 color RAM and UC2 interface logic.

UD2-UD6
UE1-UE6		2114 or TMM314A	RAM	MEMORY
  Blank screen. Partial failure: less than normal "3583 BYTES FREE" at
startup, sometimes will produce "garbage" screen or screen freeze after
warmup. Shorted chips may get very hot.

UD7		901460-03	ROM	CHARACTER
  Startup screen characters missing (just blocks or flashing lines where
characters should be) and alphanumerics missing from games. Game carts that
use mostly graphics characters may look normal while running.

UD8 and UE8	MPS65245 or MOS65245 or 74LS245 TRI-STATE LOGIC
  Blank screen.	Partical failure: garbled video or audio. see UB7

UD9		74LS133		13 INPUT LOGIC GATE
  Blank screen.

UE10		6502		MICROPROCESSOR
  Blank screen. Partial failure: computer may run for awhile, then freeze. 

UE11		901486-01	ROM	BASIC
  Startup screen with borders but no characters. Disk commands do not work,
but game carts still work.

UE12		901486-06	ROM	KERNAL
  Blank screen on startup, no drive access, game carts don't work.
Partial failure: games may still work. 

UF8		MPS65245 or MOS65245 or 74LS245 TRI-STATE LOGIC
  Blank screen.

Q3		2SD880		TRANSISTOR, CASS. MOTOR DRIVER
  Most common problem when cassette motor will not run.

VR1		LM323K		+5V REGULATOR, 3 AMP
  Computer appears "dead" with no red power indicator, or wavy picture with
hum in sound. Shorted regulator will damage chips in the computer and blow
the fuse.

		KBPC802		BRIDGE RECTIFIER, 5A 50V
  Computer appears "dead" with no red power indicator. Partial failure may
produce hum in audio and wavy picture. Partially shorted bridge may make
the power pack (transformer) get hot and may blow the main fuse (3.15A)
inside the computer. Bad solder connections may make this part look
intermittant or open.

F1		3.15 AMP FB	FUSE, MAIN POWER
  Computer "dead"... no red power light. If new fuse blows, suspect short
in bridge rectifier, regulator, or computer chips. Make sure fuse clips are
clean and tight or fuse and holder will get hot and new fuse will blow.

Y1				CRYSTAL, 14.31818MHz in USA version
  See UB9 (7402)
  

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

LATER VERSION BOARDS: ASSY #250403 FAB #251040-01 REV D 1981/2

UAB1		6522		VIA	INTERFACE, KEYBOARD-SERIAL
  Startup screen normal, but no cursor. Keyboard doesn't work at all. Partial
failure: some keys "stuck" or don't work... may be a whole row or column.
Also may cause serial port (drive access) problems. Can produce blank screen
if shorted (remove to check).

UAB3		6522		VIA	INTERFACE, JOY-USER-SERIAL-CASS
  Startup screen normal, but drive access problems ("SEARCHING FOR... "
forever). One or more joystick positions don't work. No cassette or user
port access. Can produce blank screen if shorted (remove to check).

UB4		7406		LOGIC	
  Normal startup screen. Serial port (disk drive) access problems.

UB6		LM555		TIMER	POWER ON RESET
  Will not reset on power up. May produce garbled or random screen
characters-graphics, or screen may start up "frozen". If drive resets when
computer is powered up, this chip is OK.

UB7		6560-101	VIC	VIDEO/AUDIO
  White or blank screen, garbled or no video, screen full of (or few random)
"garbage" characters. "Blind" disk commands may still work. Partial failure:
Dark or smeary image, loss of color, garbled or no sound, game paddles or
light pen doesn't work.

UB9		7402 (M53202P)	OSC	MASTER CLOCK OSC
  White screen... no RF interference in AM radio (see text). Partial failure:
can produce missing color or incorrect colors, slight or severe tearing of
picture or diagonal lines, sounds play at incorrect pitch. Symptoms may only
appear after warmup.

UC2		74LS04		LOGIC
  Screen character colors incorrect (scrambled). If very bad, entire screen
becomes garbled with flashing characters, vertical bars and random colors.
Check also UE1 color RAM and UD1 interface logic.

UC3		74LS02		LOGIC

UC4		74LS138		LOGIC	MEMORY CONTROL
  Blank screen.

UC5		74LS138		LOGIC	BLOCK CONTROL
  Blank screen.

UC6		74LS138		LOGIC	I/O

U14-U15	MB8416A			RAM	MEMORY
  Blank screen. Less than normal "3583 BYTES FREE" at startup. Sometimes
will produce "garbage" screen or screen freeze after warmup. Shorted chips
can get very hot.

UD1		CD4066		COLOR INTERFACE LOGIC
  Some screen characters have incorrect colors. Check also UE1 color RAM
and UC2 logic.

UD2		2114		SRAM

UD7		901460-03	ROM	CHARACTER
  Startup screen characters missing (just blocks or flashing lines where
characters should be) and alphanumerics missing from games. Game carts that
use mostly graphics characters may look normal while running.

UD8 & UE8	MPS65245 or MOS65245 or 74LS245	TRI-STATE LOGIC
  Blank screen. Partical failure: garbled video or audio. see UB7

UD9		74LS133		LOGIC

UE1		2114		SRAM	(COLOR RAM) 
  Incorrect screen character colors. Check also UD1 and UC2.

UE2		2114		SRAM

UE10		6502		MICROPROCESSOR
  Blank screen. Partial failure: programs may run for awhile, then freeze. 

UE11		901486-01	ROM	BASIC
  Startup screen with borders but no characters. Disk commands do not work,
but game carts still work.

UE12		901486-06	ROM	KERNAL
  Blank screen on startup, no drive access, game carts don't work. Partial
failure: some game carts may still work.

UF8		MPS65245 or MOS65245 or 74LS245	TRI-STATE LOGIC

Q3		2SD880		TRANSISTOR, CASS. MOTOR DRIVER
  Cassette motor doesn't run. Common failure.

CR2		S10B		BRIDGE RECT, 1A, 50V

F1		1 AMP FB	FUSE, 9 VOLT AC SOURCE
  Opening screen normal and all functions work except cassette motor and
9VAC source to user port.

Y1				CRYSTAL, 14.31818MHz
  See UB9 (master oscillator chip).

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

     Diagnosis of chip and other failures is oftentimes difficult, but there
are some things you can try to narrow the diagnosis to a specific fault.
Blank screen is the most common symptom, and it can be the most difficult
because there are many potential causes, including power supply (internal
in the early version VIC20) faults. Note the difference in the two terms:
blank screen and white screen. Blank refers to a dark screen (called a
"raster") with no border and no characters. White screen is likewise blank
with no border and no characters, but brighter than a "blank" screen.
     An AM radio tuned to the lowest point on the dial (off station) can be
used to see if the computer is generating RF interference (which is normal).
The digital signals inside a computer are like little transmitters, and an
AM radio held nearby can pick up those signals. Keep the TV or monitor off
for this test as it generates strong interference of its own. The VIC20
startup sequence to opening screen takes about four seconds and produces
characteristic sounds at powerup. If you are used to the sounds it normally
makes, that can be used during diagnostics. For example, if the computer
generates normal interference sounds in the radio, it indicates the
microprocessor and support chips are working, and a blank screen may be a
VIC chip problem. Of course a total failure of the VIC chip will produce a
white screen and little noise in the radio because nothing is running in the
computer except the master oscillator. Clock signals go through the VIC chip
to the rest of the computer.
     Early VIC20 computers have a rather high input current and you need to
make sure the fuse is tight in the fuse holder. If it's loose in the clips,
the fuse and socket will get hot and cause the fuse to fail from the slight
bit of resistance in the contacts. The resulting B+ voltage may be too low
to run the computer properly. Make sure the clips are clean of dirt and
corrosion, and squeeze them together to make a tight fit before the fuse is
installed. Likewise, make sure the power plug is tight in the recepticle. If
necessary, squeeze the plug end gently with a pair of pliers to crimp the
internal contacts (be careful not to crush them), to make a tight fit before 
plugging in the supply to the computer. Loose connections at the socket will 
cause intermittant operation or no power to the computer. Another common 
trouble spot in the early version is the solder connections on the large
bridge rectifier. I always resolder it just to make sure it's making good 
contact. Also, tighten the heat sink on that bridge block. Every one I've 
worked on was loose and the diode was running very hot. If run that way, 
the diode will fail eventually. 
     Speaking of contacts, one preventive (and diagnostic) step you can take
is to reseat all the socketed chips. The contact point between chip and
socket is very small and a bit of corrosion or dust can cause intermittant
operation or a "dead computer" symptom. Lift up each end of the chip with a
small screwdriver and press it back down to wipe the contacts. Don't pry
against the PC board or you may damage the traces (printed "wires"). No need
to remove chips for this procedure, but if you do, make sure the pins are
straight when you reinstall them. A bent pin is sometimes impossible to see
(unless the chip is removed again) and the resulting failure or intermittant
adds yet another problem.
     One diagnostic check you can make on a "dead" computer is to feel each
of the chips and see if any are getting hot. Some chips (the large ones like
the microprocessor and the VIC) will run very warm to the touch after the
computer has been running for awhile. There is quite a bit of variation in
normal chip temperatures, so a bit of experience helps in diagnosis. Some
chips run almost cold and it's perfectly normal, but if you find a cold VIC
after running the computer for ten minutes, it may be bad. If a replacement
also runs cold, the chip is either not getting power (no +5 volts on pin 40)
or the ground has opened up (pin 20). I had such a failure in one board. The
white screen and "radio check" told me the computer was nearly dead. The
master oscillator was running (checked with an oscilloscope) and the chip
had +5 volts on pin 40. The chip was replaced and it still didn't work. It
turned out the socket was at fault. It was likely damaged because of a bent
IC pin on a previous VIC chip installation. The ground return (pin 20) for
the chip was open circuit. I was able to repair the socket by bending the
recepticle for pin 20 back so it made proper contact with the new chip when
it was inserted. I only mention this to show how a "simple" fault can be
VERY hard to find. Unexpected or illogical faults like that one really burn
up bench time and frazzle a technicians nerves. It's not a "normal" failure,
but it happens often enough to keep us humble.
     As a diagnostic, the computer will still start up with a select few of
the chips removed, such as the two VIA chips, the character ROM, UC2, and
UB4. If you suspect one of those chips is shorted and producing a blank
screen, you can remove them to see if the computer will then start up. 
Obviously, without a character ROM you will not have a normal screen 
display but a normal border should appear. Swapping the two VIA chips can 
help diagnose which one is bad... faults will follow the bad chip. A stuck 
(shorted) keyboard key can be isolated by unplugging the keyboard and 
powering up the computer without it, or substituting another keyboard. C64 
keyboards work on VIC20s and (for some versions) internal parts can be 
interchanged as well. Keys that require a hard press to make them work are 
probably worn out (or dirty). If you don't have spare parts handy, the 
conductive rubber plunger can be swapped for a little-used key to get that 
one working again.
     Power supply problems account for quite a few failures. As I mentioned
before, there are two different versions of the VIC20 and each requires the
appropriate power pack. Early version boards use a single 9VAC source
(transformer-only inside the power pack) at a rather hefty 2 Amps. The
rectifier, filter capacitors and regulator are inside the computer. You'll
see the large black metal heat sinks when you open it up. 
     Later version VIC20s used the same supply as a C64, so there are two 
voltages: +5VDC at 1.5 Amp and 9VAC at 1 Amp. It's helpful to know that a 
VIC20 draws more power from the supply, so if the power pack is marginal, 
it may still work with a C64 but not with your VIC20. It's best to use one 
of the "beefier" after-market repairable supplies, especially if you plan 
to use accessories like RAM expansion or game cartridges. They draw power 
from the computer supply. If the 5 volt source is low or missing, the 
computer will not work at all and the red power light will not come on or 
will be dim. If the 9VAC is missing because of a bad supply or open 1 Amp 
fuse in the computer, the red power light will come on and the computer 
will work except for no cassette power and no 9VAC to the user port. 
It might therefore go unnoticed. If the fuse is blown, look for a short on 
one of those ports. Accidental mis-contact while connecting to a port (with 
the computer turned on) is usually the cause of an open fuse. Avoid 
connecting or disconnecting -anything- with equipment turned on!
     The above information may help get your computer going again, but keep
in mind that genuine computer faults can be a challenge even for experienced
techs. There is a limit to what the novice can repair him/herself. I've been
in the repair business for almost 40 years and I still sometimes damage a
chip or board trace when removing components from a Commodore computer or
drive. It's just not that easy to do. So, if you have a major problem with
your VIC20, it's probably best to start looking for another one and keep
your old one for parts. Chips are scarce now, and we need to salvage
whatever we can to keep our systems running as long as possible.

Ray Carlsen CET
Carlsen Electronics... a leader in trailing-edge technology.

Questions and comments are always welcome, especially if you spot a mistake
here. Thanks!

 

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