ftp.whtech.com/ftp.whtech.com.tar/ftp.whtech.com/articles/archives/limanews.exe/CONSOLE.TXT

Text File | 2006-10-19 | 32KB | 996 lines

.LM 10;RM 70;FI





.CE 3
@&CONSOLE^DEBUGGING^HELP
By John Guion
Dallas TI Home Computer Group








.CE
@PROBLEM^AREAS


.IN +12
1)^^Console will not power up

2)^^Keyboard errors

3)^^Intermittent console lock up

4)^^Module errors

5)^^Joystick port errors

6)^^Video output difficulty

7)^^Sound problems










.IN +0
The &TI-99/4A &Console &and &Peripheral
&Expansion &System &Technical &Data
manual available from Texas Instruments'
Dealer Parts Department [[phone]]
will serve as an excellent source for
schematics and part location guide.

The information contained herein is only
intended for use as a reference for
possible debugging procedures. It is not
intended as a repair guide for the
common user with little or no knowledge
of digital electronics or the basic
structure of the TI-99/4A system. The
author assumes no responsibility for
damages resulting from improper use of
this information.

This information should only be
reproduced in its entirety with credit
to the original author.
.BP
.LM 10;CE
@1)^CONSOLE^WILL^NOT^POWER^UP

.IN +0
@1.1^General^information.
.IN +5

Failure of the TI-99/4A console to power
up and produce the TI title screen is a
common problem that is also the hardest
to track down and fix since failure of
nearly any component in the console or
power supply can cause this.
The following are not intended as
solutions to the problem, but merely as
points to check that may aid in finding
the actual problem and fixing it.
Unless a particular part is suspected,
replace any socketed chips possible with
known working equivalents before
desoldering any components. Since the
socketed chips are common causes of
lock up, eliminating them as possible
problems first may save excess soldering
on the board. The console will power up
if the sound chip is removed entirely,
but not if that chip is shorted
internally.
A simple TTL logic probe can be used for
tracing signals in the circuit. An
oscilloscope may also be used and has
the advantage of being able to check
clock signals for proper frequency. When
a signal should exist as an output from
a particular device, be sure to check
that device's input for proper signals
before attempting to replace the
component.  When checking for locked up
signals, try to trace all signals back
through the circuit to the point of
origin. A set of schematics (available
from several sources, including TI) will
help greatly in this part of debugging.
Tracing locked signals can determine
whether or not the signal is missing due
to a faulty component that it must pass
through or what power up operation was
occurring during lock up.

.IN +0
@1.2^Console^power^up^procedure.

A. TMS9900 CPU resets and addresses low
ROM locations.

B. TMS9900 initializes.

C. TMS9900 sets up workspace registers
in MCM6810 RAM.

D. TMS9900 begins GROM read.

E. TMS9900 enters delay loop for about
1/4 second.

F. TMS9919 sound chip is disabled.

G. TMS9918A VDP chip is initialized.

H. 4116 VDP RAM is initialized (requires
about 1 second).

I. Title screen is loaded into VDP.

J. TMS9919 sound chip emits beep.

K. TMS9900 CPU enters keyboard scan.

L. System is ready for use.
.BP
@1.3^Voltage/signal^checklist.

A. Check power supply for +5V, +12V, and
-5V.  Lack of -5V often results in a
gray flickering screen on power up.
Check for +5V on chips throughout board.
Check TMS9900 for -5V at pin^1; +5V at
pins^2, 33, 59, and 64; and +12V at
pin^27.  If any voltages are missing,
check for shorts on main board. Replace
power supply if necessary.

B. Check TMS9900 pins^8, 9, 25, and 28
for clock signal. If not found, check
TIM9904 clock generator pins^1, 2, 3,
and 4 for clock signal. If not found,
check TIM9904 supply voltages (+5V at
pin^20, +12V at pin^13), crystal, and
tank circuit. If no external problem can
be found, possible TIM9904 failure.

C. Check TMS9918A pin^39 and pin^40 for
the 10.73863 MHz clock. If missing,
check crystal and oscillator circuit.
Otherwise, check TMS9918A pin^36 and
pin^37 for clock outputs. If not found,
remove GROMs and sound processor
(located next to GROMs) and test again
for clock. If missing, possible TMS9918A
failure. Reinsert GROMs and sound
processor after tests.

D. Check TMS9918A pins^14 (-CSW) and^15
(-CSR) for lock up. If locked up, check
memory enable from pin^6 of 74LS32 and
pin^13 of 74LS138 located next to
MCM6810. Trace signal to find possible
failure.

E. Check TMS9918A pin^13 (MODE) for lock
up. If locked up, trace signal back to
TMS9900. Also check for other components
that may be locking up this line (it is
used as A14). If no other fault can be
found on that line, possible TMS9918A
failure.

F. Check TMS9918A pin^1 (-RAS), pin^2
(-CAS), and pin^11 (-R/W) for lock up.
If locked up, possible TMS9918A
failure.

G. Check TMS9918A pins^17 through 24
(data lines) for signals. If missing,
trace to fault. Possible TMS9918A or
TMS9900 failure.

H. Check TMS9918A pins^3 through 10 (RAM
address/data lines) for signals. If
missing, possible TMS9918A failure.

I. Check 4116 RAM pin^14 (DATA OUT) on
each chip for signal. Each chip missing
signal may be at fault as well as
TMS9918A.

J. Check TMS9900 pin^62 (READY) for lock
up. If locked up, check TMS9900 pin^6
(-RESET) for signal. If pin^6 is locked
up low, possible TIM9904 failure. If
high, possible TMS9900 failure. If
TMS9900 pin^6 is not locked up, trace
circuit back from pin^62 to find fault.

K. Check all three GROMs (CD2155,
CD2156, and CD2157) at pin^10 (-CS) and
pin^15 (GREADY) for signals. If either
is missing, remove all three GROMs and
test pin^10 again for signal. If the
signal at pin^10 does not exist, trace
back through circuit to find failure. If
signal exists, replace GROMs one at a
time until GROM that causes lock up on
pin^15 is found.

L. Check all three GROMs for signal on
pin^11 (M0/A14) and pin^12 (M1/DBIN).
If missing, trace circuit to find break
in signal path.

M. Check each GROM for -5V at pin^14,
+5V at pin^9, and -.8V to -.6V at
pin^16.  If missing, check for broken
trace.  If -.8V/-.6V is missing or at
-5V, check diode connected to that
line.

N. Remove sound generator. If console
powers up, check pin^16 for +5V, pin^4
for clock from TMS9918A, pin^5 (-WE) for
signal, and pin^6 (-CS) for signal from
74LS138 closest to MCM6810. If these
signals exist, possible sound chip
failure.

O. Check TMS9918A pin^36 for composite
video output. If missing, check TMS9918A
crystal and clock circuit and pin^16
(-INT) for interrupt signal. If signals
exist, possible TMS9918A failure.

P. Check GROMs for clock on pin^13. If
missing, check clock output on TMS9918A
pin^37. If signal on TMS9918A exists,
check for break in signal path. If not,
check TMS9918A oscillator circuit. If
oscillator operates, possible TMS9918A
failure.

Q. Check pin^20 (-CS) of console ROMs
for lockup. If locked up, trace circuit
back to find fault.

R. Check pins^7 and 9 through 15 of
74LS138 nearest I/O port to determine
memory area accessed during lock up.
Check pin^4 (-MEMEN) for lock up. If no
signal can be found on pin^7 or pins^9
through 15, possible 74LS138 failure.

S. Check pin^11 (-CS) of MCM6810 RAMs
for lock up. If locked up, trace circuit
back to find fault.

T. Check TMS9901 pin^5 (-CE) for lock
up. If locked up, check 74LS138 nearest
I/O port for failure. Check TMS9901
pin^11, 17, and 18 for lock up. If
locked up, trace circuit back to find
fault.
.BP
.CE
@2)^KEYBOARD^ERRORS

.IN +0
@2.1^General^information.

.IN +5
After much use, the keyboard will
sometimes malfunction and key presses
will not appear to have any effect or
will only work part of the time (either
single keys or groups of keys).  Keys
may also show multiple entries even
though only one key was pressed.
The TI-99/4A's keyboard is set up using
an X-Y matrix to allow encoding of
output signals from a 74LS156 to drive
the interrupt inputs of the TMS9901.
This method requires only 15 lines to
encode all 48 keys.  Keyboard failure is
almost always a mechanical problem, but
occasionally one of the computer's
support components will fail and cause
similar problems.

.IN +0
@2.2^Possible^causes^and^solutions.

A. If only one key works intermittently
or not at all, that single keyswitch is
probably dirty or damaged. Some
keyboards may allow for disassembly and
repair while others make single key
repair less practical than replacement
of the entire unit.

B. If a group of keys has suddenly
failed to work properly, it is likely
that the switches in the keyboard are
good and that some common component has
failed.  Typically, this is a broken
wire or faulty driver chip. Consult a
keyboard schematic to determine control
lines common to groups of keys. When a
common line is found, check continuity
from the keyboard to the main board. If
continuity exists, check loading
resistors on the control lines from the
keyboard connector before replacing any
chips.

C. If the ALPHA LOCK key fails to
operate properly and the console has
been modified with the addition of a
diode in the ALPHA LOCK circuit, remove
the diode and replace with a piece of
wire. The diode is added by some users
to allow use of joysticks with the ALPHA
LOCK depressed, but it sometimes
introduces a timing problem and is not
reliable.

D. If some keys do not work at all and
others result in improper entries, check
the keyboard plug connector for proper
alignment.

E. If a group of keys with a control
line common to the 74LS156 fail to
function and continuity exists to the
main board, use a logic probe to check
for output pulses on pins^4 through 7
and 9 through 12 of the 74LS156.

F. If the entire keyboard fails to
function and continuity exists to the
main board, replace the 74LS156 and test
again.

G. If 74LS156 replacement has no effect,
replace the TMS9901 and test again.
.BP
.CE
@3)^INTERMITTENT^CONSOLE^LOCK^UP

.IN +0
@3.1^General^information.

.IN +5
Occasionally, a console will suffer from
lock up during regular use of software.
This may be caused by either a software
error, a hardware error, or a
disturbance of the system. Assuming that
software errors and outside disturbance
(such as bumping the flex-cable
connector accidentally) have been
eliminated as possible causes, several
conditions may cause random locking up
of the console during use.
Causes for random console lock up fall
into three main categories: power
supply, heat, and poor connections.

.IN +0
@3.2^Possible^causes^and^solutions.

A. Check the computer's supply line
voltage. The transformer input should be
approximately 117VAC. The output of the
consoles internal supply should be +5V,
+12V, and -5V. Aprroximately 5%
variations for each of these are
tolerable.

B. Test the computer in different
surroundings. If the computer
consistently works at another location
or with different equipment attached, it
may be affected by some components
connected to it or by electrical
interference from the 117VAC supply.

C. If lock up only occurs after some
period of use, the problem may be heat
related. Make sure the console's
ventilation slots are not blocked. Check
the heatsink of the TMS9918A for
sufficient heat-conducting grease as
well as the TIM9904 if it has a heatsink
attached. A cooler switching power
supply may be installed to help further
lower operating temperature.

D. If the console fails to run certain
modules reliably (such as Extended
BASIC) and such modules often need
reseating several times, the GROM port
should be changed. This typically occurs
with modules that use contacts on both
sides of the edge connector in the
module, as is the case with Extended
BASIC.  Cleaning the GROM port may also
help, but the problem will probably
appear again shortly unless a new port
is installed. When replacing or cleaning
the port, be sure to remove the felt or
foam wiper from the clip-on cover to the
port. The wiper may be removed with a
small screwdriver and solvent. Replace
the plastic cover once the wiper
material has been removed. Do not use
any type of lubricant since this will
attract dirt and cause further
problems.

E. A poor connection at the consoles
I/O port on the side may also lead to
occasional problems. If problems persist
after checking for a secure connection,
remove any device connected to the side
port and use alcohol and a stiff piece
of paper to clean the inner contacts on
the device. Remove the main board from
the console and clean the edge connector
with a pencil eraser followed by wiping
with an alcohol-treated pad. Only light
rubbing is needed with the eraser in
order to remove surface oxidation and
produce a clean surface.  Excessive
rubbing will not help and can remove too
much plating from the board, especially
if this procedure is repeated multiple
times.

F. Check the power plug at the back of
the console for tightness. A loose
connection may cause occasional power
failure. If this plug is loose, use
pliers or other suitable device to bend
the pins in the receptacle SLIGHTLY
towards each other. A small piece of
electrician's tape around the plug will
also help secure the connection.

.BP
.CE
@4)^MODULE^ERRORS

.IN +)
@4.1^General^information.

.IN +5
Certain modules may fail to work in the
console either intermittently or at any
time. Although this is usually the fault
of the GROM port, modules do sometimes
fail or have other reasons for not
working.  Before attempting any repair,
however, it is important that the module
in question be tried several times in
the suspect console as well as another
console. Be sure that other modules are
also tested in the suspect console.
This will help in finding the faulty
component.
If the console does not offer TI BASIC
as an option from the module selection
screen and displays an "INSERT
CARTRIDGE" message when no module is
inserted, the console cannot find TI
BASIC in GROMs 1 and 2 (chip numbers
CD2156 and CD2157, respectively).
Replace GROM 1 and test again. If some
of the commands in BASIC fail to
function, replacement of GROM 2 may also
be required.

.IN +0
@4.2^Possible^causes^and^solutions.

A. If various modules have intermittent
problems and often require reseating the
modules in the GROM port, the GROM port
should be replaced or cleaned. This
condition is usually noticed first with
modules that use contacts on both sides
of their edge connectors, although it
may occur with any module. Such a
condition usually worsens with further
use and is commonly a problem with
consoles that have modules removed and
inserted often.

B. If one particular module consistently
fails to place a module selection of the
TI menu, that module is most likely
defective. This is also the case with
modules that consistently lock up the
computer when a specific function is
attempted, indicating some portion of
that module's program is damaged. If the
module is a Texas Instruments produced
device, the GROMs' part numbers usually
indicate their GROM addresses relative
to each other. A memory editing or
debugging program may be useful in
determining a faulty GROM by allowing
viewing of the GROM contents. Since only
five 8K GROM address ranges may exist in
a module (at GROM addresses >6000,
>8000, >A000, >C000, and >E000), the
number of 8K blocks locatable by the
memory editor or debugger should be
equal to the number of GROM chips in the
module. If less blocks are found, GROM
chips should be selectively removed
until the faulty chip is found. If the
module also has a ROM chip in it, the
ROM contents may also be examined with a
memory editor. The console allows for
one 8K block of CPU memory at >6000.

C. With the exception of some
specialized GROM-emulating modules,
nearly all non-TI produced modules
contain only ROM and no GROM. This ROM
is usually a single chip that may not be
repaired. A few companies produced bank
switched ROM modules (TI Extended BASIC
also contained bank switched ROM in
addition to GROM). If a ROM only module
is determined to be at fault, it may
contain bank switching components that
have failed. Internal inspection of the
module must be done to determine if
repair is possible with replacement of
bank switching components. Few, if any,
aftermarket module manufacturers will
sell replacement ROM chips. Module
replacement is usually necessary.

D. If most non-TI modules will not run
in the console, check the TI title
screen for a 1983 copyright notice. If
found, the console contains the TI
operating system which prevented use of
non-TI modules. This operating system
may be bypassed with some software
programs or aftermarket hardware
adaptors. Replacement of GROM 0 in the
console (chip number CD2155) with a chip
from an earlier production console will
replace the newer operating system and
permanently solve the problem.
.BP
.CE
@5)^JOYSTICK^PORT^ERRORS

.IN +0
@5.1^General^information.

.IN +5
The TI-99/4A supports two eight-point
joystick controllers, each with a single
fire button control. Both units share a
common port using a 9-pin DB9 male
connector on the side of the console.
With the exception of a single driving
line to each joystick, all five control
lines from each joystick (UP, DOWN,
LEFT, RIGHT, and FIRE) are connected
together at the plug. Interference
between signals is avoided by the use of
diodes on each control line in each
joystick unit. Adaptors for using other
brand joysticks (i.e.^Atari) alter the
pin assignment configuration as well as
supply the diodes in the adaptor unit as
they are not normally used in other
joysticks.
It is suggested that a joystick with a
suspected problem be tested with another
console as well as testing the console
with a different joystick (if available)
since joystick port problems may be
either the fault of the console or of
the joysticks. Joystick errors that are
accompanied by keyboard errors are
usually the fault of the console unit
itself and not the joysticks.
When checking the joystick signal and
driver lines for continuity, check the
circuit then switch the test leads used
and test again.  Since diodes are used,
existing continuity may fail to appear
unless polarity is reversed on the
testing device.

.IN +0
@5.2^Possible^problems^and^solutions.

A. If one position on a single joystick
fails (as opposed to the same position
on two simultaneously connected units),
it is most likely a mechanical problem
related to that unit. Disassemble the
joystick and test for continuity across
the contact points on the position in
question. The circuit should be
completed when the joystick is moved
towards that position. Also check for
continuity from each of the lines from
the joystick to the plug. If continuity
exists, replace the diode connected to
the control line affected by the problem
position and test again.

B. If the joystick will not operate the
UP position, check the ALPHA LOCK key to
make sure it is not depressed. Since the
ALPHA LOCK may be sticking, go to TI
BASIC and make sure that lower case can
be entered as well as upper case when
the ALPHA LOCK is depressed. If not,
consult the section on keyboard repair.
If the console has been modified with
the addition of a diode to allow use of
the UP position on the joysticks when
the ALPHA LOCK is depressed, remove this
diode and replace it with a piece of
wire. In many consoles, the diode
introduces a timing problem that will
prevent proper function of the joystick
and/or the ALPHA LOCK key. This commonly
appears as an ALPHA LOCK malfunction
after the console has been in use for a
few minutes.

C. If one position fails on both
joysticks and the ALPHA LOCK function
has been tested, check for continuity
from each joystick control line to the
plug.  If continuity exists, check each
key on the keyboard for proper function
since all five control lines from the
joysticks are connected directly to the
keyboard control lines. If keyboard
failure also occurs, replace the 74LS156
in the console and test again. If
problems still exist, replace the
TMS9901 and test again.

D. If one joystick unit fails entirely,
but the other unit (or the same unit
plugged into the other port of an
adaptor), check for continuity from the
driver line to the joystick (pin 2 or
pin 7 on the plug, depending on which
joystick is in question). If continuity
exists, check (or replace) the driver
transistor connected to that control
line in the console located near the
joystick port connector. Also check the
appropriate driver line for a short to
ground which would indicate a faulty
capacitor or transistor between the
driver line and ground. If errors still
exist, replace the 74LS156 in the
console and test again.

E. If both joysticks fail entirely,
check each joystick with the above
method. If problem is still not found,
replace the TMS9901 in the console and
test again.
.BP
.CE
@6)^VIDEO^OUTPUT^PROBLEMS

.IN +0
@6.1^General^information.

.IN +5
The TI-99/4A console uses a five-pin DIN
plug at the back of the unit to supply a
composite video output for use with
either a composite monitor or a video
modulator used in conjuction with a
television set. The composite video
signal is created by the TMS9918A VDP
chip and amplified by an external
circuit located within the console.
When diagnosing video output problems,
first eliminate software as a possible
problem. If one particular program
causes problems, it is probably not the
fault of the system and no hardware
repair is possible.
Since either the monitor or television
used may cause many problems, connect
the console in question to a known good
monitor or TV. Also, connect a different
console to the existing monitor or TV.
This will easily determine whether it is
the computer or monitoring device which
is at fault.  Only computer and video
modulator problems are discussed here.

.IN +0
@6.2^Possible^problems^and^solutions.

A. If using a video modulator, testing
the console with a known good composite
monitor will easily determine if the
modulator is at fault. If a monitor is
not available, simply recheck all
connections from the console to the
television and try various switch
combinations on the modulator unit. Due
to the low cost, availability, and
difficulty of repairing video
modulators, problems with the modulator
are best solved by replacement. The
exception is that occasionally the two
leads extending from the modulator to
the television will have broken
connections.  Simply shortening or
replacing these leads may fix a variety
of output problems that occur with this
unit.

B. If the console has no video output
with either a monitor or video
modulator, then either the DIN plug
receptacle, the console's amplifier
circuit, the TMS9918A VDP chip, or a
TMS9918A support component has failed.
Each case assumes that the console is
functioning and has not locked up. This
may be determined by listening for the
"beep" that accompanies power up or
"blind typing" operations that would
activate peripherals such as disk
drives. If no evidence can be found that
the computer is operating with the
exception of video output, consult the
section concerning power up problems.
To locate the faulty component, first
test the monitor cable or modulator for
continuity of signals lines to the DIN
connector. If continuity exists, remove
the main board from the console and
inspect the DIN connector for bad solder
joints or misplaced pins. Test again for
continuity from the cable or modulator
to the main board. If continuity exists,
use either test leads or a soldered wire
to connect pin 36 of the TMS9918A to pin
4 of the DIN receptacle (CAUTION: Be
sure to connect the proper pins since
voltages on certain pins of the DIN
receptacle can damage the TMS9918A).
Remember to remove this jumper after
testing. This procedure bypasses the
external amplifier completely. A proper
video image should appear if the
amplifier circuit is defective, but the
image will appear dimmer than normal.
If the video output appears to function,
any component in the amplifier circuit
may be faulty.  Unless an obviously
faulty component can be located, replace
the two transistors (Q200 and Q201)
first as they are most likely to fail.
If no video image has appeared, check
pin 40 of the TMS9918A for a pulsed
logic signal (this should be 10.73863
MHz) from the video oscillator circuit.
If this signal is missing, first check
other components in this circuit for
damage then replace the crystal located
next to the TMS9918A.  If this signal
exists, check pins 13, 14, and 15 on the
TMS9918A. If signals exist here, replace
the TMS9918A. If the signals are
missing, trace these to their points of
origin to find the break in the signal
path.

C. If the video image is distorted,
first check the computer with another
monitor since this is usually the fault
of the monitor or modulator/TV
combination. If the cause is determined
to be the console, bypass the external
amplifier in the method described above.
If the only effect is dimming of the
image, remove the jumper and continue.
If the distortion is corrected and a dim
image appears, the fault is in the
amplifier circuit. If not, remove the
jumper and adjust the tuning coil near
the TMS9918A VDP chip. If the problem
still is not corrected, replace the
TMS9918A and test again.

D. If a console only outputs monochrome
(black and white) video, adjust the
tuning coil located near the TMS9918A.
If color does not appear anywhere within
its full range of adjustment, replace
the 10.73863 MHz crystal located next to
it. If this has no effect, replace the
TMS9918A VDP chip and test again.

E. If the TI title screen appears
"jumbled" on power up and shows unwanted
or oddly shaped characters, but not at
other times, console GROM 0 (CD2155) may
be faulty. This may also be accompanied
by errors in using module software.
Replace GROM 0 and test again. If this
has no effect or if screen display is
disturbed with unwanted or improper
characters when running assembly
programs, a 4116 RAM chip in the console
may be faulty. Faulty RAM is usually
accompanied by an inability to run
programs in console BASIC. There are
eight identical RAM chips in the
console, and any one may cause the
described symptoms (these chips, located
in a row below the TMS9918A, may have a
different number than 4116, such as
8216, but are still compatible). One of
the easiest ways to find the faulty RAM
is to acquire an extra 4116 dynamic RAM
chip.  Bend the legs together slightly
so that it may be "piggy-backed" on top
of an installed 4116 RAM (it may need to
be held in place by hand). Put the chip
on top of each 4116 chip in turn until
some effect on the screen is noticed (be
sure that all the pins are touching the
lower chip or this method will not
work). Test all the 4116 RAMs in the
console. If one chip shows to have some
effect on the display, check it
repeatedly to ensure that the 4116 chip
used for testing is making good contact.
If the effect is repeatable, replace
that RAM. Also test any remaining chips
since multiple RAMs may be faulty. If no
effect can be found when testing the
RAMs in this manner, replace the
TMS9918A and test again.
.BP
.CE
@7)^SOUND^PROBLEMS

.IN +0
@7.1^General^information.

.IN +5
The TI-99/4A uses the TMS9919 sound
processor to produce one noise and up to
three tone outputs for use by the
monitor or television's amplifier and
speaker. This chip also has the ability
to accept an external sound signal input
that may be joined with its output, as
in the case of the TI Speech
Synthesizer. Output level from the sound
chip is programmable, but this signal
must still be amplified in order to
drive a speaker.
Three versions of the sound processor
chip were used in the TI-99/4A, each
being numbered differently. The earlier
consoles used chips numbered SN76489 .
The later chips were numbered either
SN76494 or SN94624. The earlier chips
required a 3.58 MHz clock signal derived
from the TMS9918A VDP chip and supplied
to the sound processor through a piece
of hand-wired coaxial cable. In later
units, the sound chips were revised to
use the same 447.443 KHz clock as the
GROM chips and no coaxial cable was
used. The different clocks are not
interchangeable.
Essentially, four problems may exist
that concern the sound processor:
console lock up due to chip failure (see
section on power up problems), improper
sound frequency, lack of sound output,
and inability to accept sound input from
another device.

.IN +0
@7.2^Possible^problems^and^solutions.

A. If the sound chip produces tone
frequencies excessively higher or lower
than normal, first check the chip number
to to see if the proper chip has been
installed for the clock wiring on the
board. A board wired for the later clock
should have continuity between pin 14 of
the sound processor and pin 37 of the
TMS9918A VDP chip. A board wired for an
earlier clock will have continuity
between pin 14 of the sound processor
and pin 38 of the TMS9918A VDP chip. If
this type of sound problem has developed
after a period of proper operation, this
check is not necessary since the chip
could not have properly operated with
incorrect wiring.  If proper wiring
exists, check pin 14 for the appropriate
clock signal frequency. If the clock
signal is missing, check the clock
output pin of the TMS9918A VDP chip
corresponding to the proper clock
frequency. If missing, replace the
TMS9918A chip. If the signal exists at
the TMS9918A, inspect wiring between the
sound processor and the TMS9918A.  If
the sound processor does have the proper
clock on pin 14, replace the sound
processor and test again.

B. If no sound output from the system
exists, check the sound processor using
the method described above. Check pin^6
(-CS) and pin^5 (-WE) of the sound
processor with a logic probe for
activity during a CALL SOUND command
from BASIC.  Also check or replace the
100 microfarad capacitor between pin 7
of the sound processor and pin 3 of the
five-pin DIN output port on the back of
the console. If no sound exists, replace
the sound processor and test again.

C. If the sound processor will output
proper sounds to the monitor but will
not accept sound input from another
device (usually the Speech Synthesizer),
check for 330 ohm resistance between pin
9 of the sound processor and pin 44 of
the I/O port on the side of the console.
If this circuit is open, replace the
resistor between these two points. If
this has no effect, check for a short to
ground from pin 4 of the cassette
interface port. If this is shorted,
replace the .001 microfarad capacitor
between this line and ground. Also
inspect transistor Q400 (connected
through two resistors to pin 4 of the
cassette port) as well as the capacitor
across it for shorting. Replace if
necessary.