Ray Carlsen – SX-64 Repair notes

29 May 2019 | Uncategorized | By Berry | 0 Comments
                         THE COMMODORE SX-64
DIAGNOSING PROBLEMS, DISASSEMBLY INSTRUCTIONS, COMMON PROBLEMS, MODS.

           latest updates and/or corrections: 3-06-2019

     The SX-64 is a computer with keyboard, disk drive and 5" color
monitor all compacted into (for its time) a small space. Diagnosis and
repair is therefore more difficult than with "stand-alone" devices.
However, with the info presented here, you should be able to figure out
what the problem is and possibly repair your SX. You need to be
observant for (sometimes subtle) symptoms... what works and what
doesn't, how it failed, checks with external devices, etc. Since most
of the SX-64 computer and drive electronics are virtually identical
with a standard C64 and 1541 drive, you can use those diagnostic repair
articles for further information. Most electrical parts such as the
major IC chips will substitute as well. The major differences are board
and component locations.
     Note that the SX will work with an external monitor and disk
drives, just like a C64 does. This is useful for diagnostic purposes.
The 8 pin DIN A/V connector puts out S-video as well as composite. The 
SX has a 6 pin DIN external serial port. If an external drive is
connected, it should be set up as something other than device 8 (the
default for the SX internal 1541), unless you disable the internal
drive or change its default drive number. See DRIVE SELECT JUMPERS.

DISASSEMBLY:
     The cabinet consists of the front panel, rear panel, and two
half-shells, top and bottom. You will need a medium size Phillips
screwdriver. Hint: set the screws and cabinet parts in a place where
they will not be disturbed after removal. Clear a space large enough
beforehand. If parts are placed in order of removal, you can easily
reassemble the cabinet in reverse order of disassembly. To remove the
cabinet for servicing: remove four small screws in the rear right and
left sides (two on each side). Those screws hold the plastic strips
along each side... those strips will now slide rearward and out of
their slots. Next, remove six small screws (three on each side) and two
large screws in the rear top right and left sides. Remove two large 
screws, one on each side, on the cabinet rear at the top. Lift the top
half-shell up and off starting at the rear. Note the lip at the front 
that must be engaged first when you put the top back on. 
     Most of the boards and chips are accessible from the top. Boards
are held in place with tiny black plastic "plug and socket" fasteners.
When the plug is removed, the underlying fastener will come out of the
hole. Use a knife edge carefully to get under the "cap" (or press on it
from the back side) and lift it up and out, then remove the fastener.
The boards sit in slots at the bottom of the computer, so no fasteners
there, but to completely remove a board, you need to unplug all the
connectors first. Mark each connector as you remove it to prevent
mistakes when you put them back again. In the SX, the plugs are not
marked, but the sockets all have reference numbers. If you have a
camcorder, it wouldn't hurt to set it up on a tripod and record what
you're doing. That way, if you're not sure about something, you can
always play the tape back and see what you did. Add narration to help
with the difficult or hard-to-see areas. If your camcorder is plugged
into a TV during recording, you can see what the camera sees.
     If you need access to the bottom of the computer, the bottom half
shell is removed in same manner as the top, with a few exceptions. You
need to remove four bottom screws that hold the rubber feet, and in 
addition, one screw accessible from the inside. Facing the front of
the computer, that screw is located just behind the drive left rear
side. The one next to it holds the wire bundle in place... no need to
remove that one. Lift up on the back side of the case bottom and pull 
it slightly to the rear, then up and off. 
     It might be helpful at this point to identify the PC boards. They 
are mounted on-edge, so access to some of the components requires board 
removal. The long "CPU board" (main computer components) is on the right 
hand side as you face the front of the SX. The short "I/O board" 
(interface) is mounted just behind the disk drive and plugs into the CPU 
board by means of a double row of pins. Be careful when you plug that 
board back onto the CPU board. It's easy to get it one pin off. The long 
"FDD board" (floppy control) is mounted just behind the I/O board. The 
power supply PC board is mounted to the rear panel heat sink and the rear 
"connector PCB" is mounted along the top edge of the power supply. The 
"expansion PCB" is the small board with the cartridge port connector on 
the top.
    Note that with both top and bottom half-shells removed, there is
nothing to hold the rear panel (power supply section) in place. Rather
than let it flop around hanging by it's wiring, install one small screw
on each side to hold it in place. Of course if you need access to the
power supply, you can just fold the rear panel down for servicing. You
will need to remove connector plugs from the other boards that the
power supply feeds and cut one tiewrap to free the cable bundle.
Careful you don't cut any wires!
     The disk drive assembly (along with the "glovebox" above it) can
be easily removed from the chassis by removing four screws, two on each
side of the drive. NOTE: Some drives can't be easily removed but can be 
serviced by opening the top and bottom case panels and then removing the 
glovebox (two screws). Move the cartridge port connector (two screws) to
get it out of the way. Free the wire bundle from the retainer at the
left rear of the drive. Slide the drive assembly back as far as it will
go, tip up the front of it and lift it out. Don't snag the wires
attached to the right side (CPU) board. The drive wires are long enough
to work on the drive as it sits on top of the computer, but don't
damage the speaker or short anything out with the drive case. If you
need to work on the drive "live", slip a piece of cardboard under it to
protect the computer. The "glovebox" is fastened to the drive assembly
with two screws. That box, by the way, is normally used to store the
power cord and keyboard cable, but can be used to store disks, if
desired. The rumor that disks will be damaged by heat or magnetic
fields while in that box is false. Your disks are safe there.
     You can remove the internal monitor "package" by removing four
screws, one in each corner of that assembly. It can then be lifted
straight up and out, but is held back by by it's wires at the rear. You
can get a bit more length out of those cable bundles by releasing them
from their retainers. If you set the computer on it's left side, the
monitor package can be turned to just about any position for servicing.
It's helpful to elevate it by setting it on a small cardboard box or
other platform about 4" high. Some controls (like horizontal hold) are
only accessible with the monitor in this "service" position. Note: it's
easier to reinstall the monitor if you rock it back and forth to allow
the wire retainers in the back to slide past the rough surface of the
back of the FDD board. IMPORTANT: Make sure you don't pinch any wires
under the metal mounting feet of the monitor package when you're
reinstalling it. A pinched wire can cause a short that's -very- hard to
find... and could cause major damage!

PROBLEMS, REPAIR TIPS AND MODIFICATIONS...
     Users have reported problems with the Expansion Port in the SX.
Some cartridges do not fit or seat properly, and marginal contact with
the connector results. Those carts therefore may not work reliably or
don't work at all, although they may work fine in a C64. The "cure" in
some cases is to trim off the excess plastic from the cart to make it
fit. If a cart seems to fit, but is intermittant or doesn't work, you
might try lightly cleaning the cart edge connector with a pencil 
eraser. Clean off any debris before inserting it in the computer. A 
dry cotton tipped swab or "Q-tip" works well for that.
     One problem with the User Port (rear panel) is caused by a slight
electrical difference of the SX vs a C64: one of the 9VAC lines is
grounded! That was a factory mistake and it doesn't show on their 
schematic. Both lines are "floating" in the C64. Use of modems or the
Jason-Ranheim Prominade C1 Eprom Programmer unit in an unmodified SX
can damage the computer. A minor modification to the computer will 
prevent that problem. To do it, cut the foil trace between two pins 
(19 and 20) of connector P7 on the underside of the rear panel board.
Remove three screws and lift the board up and out, then turn it over.
See the photo "gnd-cut.jpg". Clean the user port contacts top and 
bottom while you have it exposed. A pencil eraser works fine for that.
     Users have reported that the plug-in I/O board can work itself 
loose from the CPU board (usually during shipment) and cause intermittant 
problems or a blank screen when the computer is powered up. If you have 
a problem with your SX, this is worth investigating as well as reseating 
all the socketed chips while you're in there. Make sure the factory heat 
sink is still attached to the VIC chip on the CPU board. Some users 
report that the metal heatsink had fallen off in transit. It can be 
reattached with fast-set epoxy if you don't have the mounting clips. 
Just be sure to clean the IC and heat sink with solvent first if you must 
glue it on. That chip will quickly self-destruct without a heat sink.
     When repairing SX computers, I use a game cart called ROBOTRON 2084 
to do a quick check of the expansion port. With some of those computers, 
I noticed a partial screen freeze when running that game cart. Doing a 
drive reset will clear it momentarily, but it continues to lock up when 
trying to run the game. I searched my notes for anything similar and 
found something called the "Q-link fix for fast loader problems", which 
was to add a pull-up resistor to the SRQ line. That mod fixed my screen 
freeze problem with those two SX computers running the Robotron game! 
When repairing an SX, I now routinely install a 3.3K resistor from P12 
pin 4 (the keyboard connector header) which is a +5VDC source, and P11 
pin 1, the /SRQ line on the internal serial port connector header on the 
I/O board. Those two points are only an inch or so apart, so the resistor 
installation is easy. I solder it to the back of the board at the header 
connections. See the photo "srq-line.jpg".
     There is no cassette port in the SX, so the version of the Kernal
used does not include the code for it. Note that there is a slightly
different startup screen, and an added feature of SHIFT-RUN/STOP which
will load and run the first program on a disk as if you had typed:  
LOAD":*",8:RUN <RETURN>. For diagnostic purposes, a C64 Kernal ROM will
work in an SX but a bad ROM should be replaced with the correct IC.
     There is no RF modulator in the SX. If you need to feed a TV set
and it doesn't have video and audio inputs, you can run the A/V outputs
of the SX through an old VCR for the TV. Since the A/V 8 pin DIN also 
has S-video, you can feed that output to an S-monitor if desired. There 
are no commercially made S to dual-RCA cables, so you'll have to make 
your own if you want one. If there is a lot of detail in the picture, 
it's hard to see on that little 5" internal monitor, so any external 
monitor offers better resolution. 
     Problems with the cabinet handle can usually be traced to loose
screws. In addition to the two small screws in the handle itself, the
large screws (1 on each side) that fasten the handle to the case can
work loose over time and make the handle "wobbly". Don't over-tighten
those screws or they will strip out the threads in the case. If that
happens, you can put a nut on the inside. Metric hardware is hard to 
find in some places. To get to those large screws, slowly and carefully 
peel off the blue plastic covers. They are held on with double-sided 
tape, so when you peel the covers off, you will need to glue them back 
on. I use a small amount of silicon rubber sealer as glue. If applied 
to the old tape, it allows the cover to be removed again later if 
necessary. Don't get any glue on the screwtop... it's fixed in relation 
to the moveable handle. Missing handle caps can be replaced with bottle 
caps. See the photos in "SX handle caps". 

CLEANING THE MONITOR GLASS FACEPLATE...

One user found a solution to the problem of spots on the faceplate:
   "I hope in a small way maybe I can help you and future enthusiasts 
    by saying that I took a chance and tried a brillo pad with glass
    cleaner and water which made a thin paste on the glass. It did appear
    to have almost microscopic scratches in the end when held to light
    but it is MUCH better than it was and the light light scratches are
    not noticeable at all to me with glass back in place, plus they are
    on the inside so it should be fine for years to come. I concentrated
    on the inside of the glass, where the "soot" (for lack of a better
    word) seemed to be imprinted from the CRT (maybe??) and made a light
    film. It took a while, because I would scrub for maybe 30 seconds
    then check for scratches, but I am very pleased with the results!
    Total scrub time I would guess was 4-5 minutes." Dennis

ADDING A RESET SWITCH (two methods described):
     There is no system reset switch on the SX-64. The button inside
the front panel door resets the drive only. The only way to reset the
computer is to turn it off and back on again. If you want to add a
reset switch, connect one end of a momentary contact pushbutton switch 
to ground and the other end to the junction of IC UG7 (LM555) pin 2, 
resistor R23 (1 Megohm), and capacitor C45 (0.1uF). Since the other side 
of that capacitor is grounded, the switch will be connected electrically 
across it. The switch should be a normally open (N.O.) momentary contact
pushbutton. Mount the switch on a convenient place on the cabinet (or 
if you don't want to drill holes in the case, inside the glovebox) and
run the two wires to the CPU board. The IC and other components listed 
are on the lower edge of the CPU board toward the front of the computer, 
so the board must be removed to see how the connections are to be made.
Since the I/O board plugs into the CPU board, they must both be loosened 
and separated, and the power supply panel must be pulled back. Remove the 
three connectors (wire bundles from the power supply, rear connector 
panel, and the monitor) from the CPU board and pull it out for inspection 
and soldering. The layout looks something like this:

top of CPU board--------------------------------------
           
          This is the right side of the computer
______________________________________________________
         | metal    ______________________________
part of  | panel  /                                \
handle   |       |       opening in panel           |
_________|_      |                                  |
           |     |                 CPU pc board     |
           |     |     1 2 3 4                      |
-----------'     |     >     |  <--- IC UG7         |
                  \____8_7_6_5_____________________/
   
bottom of CPU board-----------------------------------

     Of course the above mod requires you to drill a hole somewhere in 
the SX case for the pushbutton. Some users are reluctant to do that. 
There is a modification you can install that will allow the existing
drive reset switch to do a full system reset so no new holes need be 
drilled in the SX case. That installation is described in detail in 
the RESET MOD article with schematic and photos that show how it's 
built and connected to the computer. 

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

TROUBLESHOOTING THE COMPUTER...

COMPUTER CHIPS... the following is a list of the major chips in the C64
portion of the assembly, and the likely symptoms when a chip fails.
Most computer chips in the SX are interchangeable with IC's from a C64.
Exceptions such as the Kernal ROM will work for test purposes, but it 
has a slightly different "program". Before you change any chip, try 
reseating it in the socket (assuming it is socketed). Pull up slightly 
at each end and then press the chip back down and try it again. Make 
sure no pins are bent. Note that the following computer chips are 
located on two different boards, namely the CPU and I/O boards.

UA4 THRU UA7 & UB4 THRU UB7 8 RAM CHIPS (4164) on CPU board.
     Blank screen, no border. Shorted chips will get warmer than the
other RAM chips. Partial failure can produce screen or program "freeze"
or "garbage" screen at startup. An abnormal (less than 38,911) number 
of bytes free or "out of memory" error on startup screen is usually a
RAM problem.

UA3 & UB3 74LS257APC or AN or A but not PC (alt MOS7708) on CPU board.
     Blank screen, less than 38911 bytes free or "garbage" startup 
screen. Don't use NS or TI chips for replacement... will not work!

UB2 906108-02 (6526) CIA2 on I/O board.
     Startup screen normal, but no serial or user port access. Cartridge 
works. "File not found" error when drive accessed. Check also UD2 7406 
IC. Partial failure: characters may show as blocks on startup screen or 
screen will fill with lines inside border. 

UB3 906108-02 (6526) CIA1 on I/O board.
     Startup screen normal, but no cursor. No keyboard or joyport access. 
Partial failure: some keys or joystick positions don't work. Cartridge 
works.

UC1 CD4066 QUAD "SWITCH" IC on I/O board.
     Proportional mouse or graphics tablet doesn't work. Check also
SID chip UE3 on CPU board.

UD1 901225-01 CHARACTER ROM on CPU board.
     Blank screen w/ border. "Garbage" characters where startup page
info should be.  Cartridge works.

UD2 7406 LOGIC on I/O board.
     No (or intermittant) serial (drive or printer) access.

UD3 251104-01 KERNAL ROM on CPU board.
     Blank screen, no border. Cartridge works.

UD4 901226-01 BASIC ROM on CPU board.
     Blank screen with border. Cartridge works.

UD7 906107-01 (6510) MPU on CPU board.
     Blank screen, no border. Cartridge doesn't work.

UE3 906112-01 (6581) SID on CPU board.
     Normal screen. No sound or garbled sound. Mouse or graphics tablet
pointer stuck or jitters (check also UC1 on I/O board). If IC shorted,
can cause blank or "garbage" screen. Runs very hot normally. Should be
heat sinked! NOTE: computer will work without a SID plugged in, but
with no sound and no proportional mouse or graphics tablet access.

UE3 556 (LM556) DUAL TIMER on I/O board.
     RESTORE key doesn't work. SHIFT-LOCK key doesn't work or
appears "stuck down" (keyboard LED on). Check also UE2 on I/O board.

UE4 906114-01 (82S100PLA) PLA on CPU board.
     Blank screen, no border. Sometimes will produce colored screen or
flashing garbage on screen, or cause intermittant screen freeze after
warmup. This chip normally runs hot! Most common chip to fail. Should
be heat sinked!

UE7 2114 SRAM COLOR RAM on CPU board.
     Characters/graphics will have no color or shimmering colors.

UF4 906109-04 (6567) VIC (has heat sink attached) on CPU board.
     Blank white screen, 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.

UG6 CD4066 Quad "Switch" on CPU board.
     Color distortions (lines in background) or no color. Check also 
VIC (UF4) or Color RAM (UE7).

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

DRIVE CHIPS... the following is a list of major chips used in the 1541
drive built into the SX-64, and the possible symptoms displayed if a
chip fails. The ICs are interchangable with those from a stand-alone 
1541. The following components are all on the FDD (floppy disk drive) 
board.

UA1 TMM2016AP-10 16K RAM
	TMM2116AP-15 or MB8128-15
     When drive powered up, spindle motor runs continuously and red
LED flashes about once every two seconds.

UA3 901229-05 DOS ROM 2 (may be earlier -03 version)
     When drive powered up, red LED stays on and spindle motor runs
continuously. This DOS ROM is a common failure. Check also UBC4, UC2,
UB2, UD3, UDC5, UE2, UE3 & UH5.

UA4 325302-01 DOS ROM 1
     When drive powered up, red LED flashes 3 times repeatedly.

UB2 74LS04 LOGIC
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UC2, UD3, UDC5, UE2, UE3 & UH5.

UBC3 901437-01 (6522) VIA (SERIAL INTERFACE)
     Drive powers up and resets normally. When LOAD is attempted,
screen indicates "SEARCHING FOR ...", but no motors run and red LED
does not light. External 1541 will still work (as drive 9, etc.) if
internal drive has not "locked up" the serial bus.

UBC4 901435-02 (6502) MPU
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UC2, UB2, UD3, UDC5, UE2, UE3 & UH5.

UC2 74LS00 LOGIC
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UB2, UD3, UDC5, UE2, UE3 & UH5.

UD2 74LS42 LOGIC
     When drive powered up, spindle motor runs continuously. Red LED
may stay on, or flash three times and go out.

UD3 74LS86 LOGIC
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UC2, UB2, UDC5, UE2, UE3 & UH5.

UDC5 901437-01 (6522) VIA (MOTOR CONTROL INTERFACE)
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UC2, UB2, UD3, UE2, UE3 & UH5.

UE2 74LS14 LOGIC
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UC2, UB2, UD3, UE3, UDC5, and UH5.
Partial failure can affect write protect.

UE3 7406 (or MOS 7707) LOGIC
     Drive does not respond to computer... no reset or disk access. If 
drive accessed, "DEVICE NOT PRESENT" error or "searching for" endlessly. 
External drive may work if internal drive problem has not "locked up" 
serial bus, but if so, unplug the 6 pin cable (white wires) from the 
I/O board (P11) to the FDD board (P19) and see if an external drive 
will work. If so, the problem is on the FDD board. This failed IC can 
also produce "red LED on and drive spinning continuously" symptom. The 
MOS7707 (and other MOS TTL chips) has a very high failure rate!!!

UF2 M54532P TRANSISTOR ARRAY IC (STEPPER DRIVERS)
	SUB: MITSUBISHI ULN2064
     No stepper action or head assembly barely moves. Check also UF3.

UF3 7404 LOGIC
     No stepper action or head assembly barely moves. Check also UF2.

UF5 325572-01 MOTOR CONTROLLER (PLA)
     When drive powered up or reset, red LED comes on and goes out,
but spindle motor does not turn. When LOAD attempted, spindle does not
turn, red LED flickers, screen displays "FILE NOT FOUND" and red LED
flashes.

UG4 7407 (7417) LOGIC (R/W CONTROL BUFFER)
     When drive is powered up or reset from computer, red LED comes
on and goes out, but spindle motor does not turn. When LOAD is
attempted, stepper moves slightly, spindle doesn't turn, and error
message on screen is "FILE NOT FOUND" with flashing red LED. Partial
failure can produce write errors or no write.

UG5 74LS193 LOGIC (COUNTER)
     Drive powers up and resets normally, but when LOAD is attempted, 
screen indicates "SEARCHING FOR ..." red LED does not light and spindle 
runs continuously.

UH1 NE592N (LM592) READ PREAMP
     Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error and red LED flashes. Check
also UJ1 and UK4.

UH4 9602 (74123) LOGIC (MMV)
     Drive powers up and resets normally, but when LOAD is attempted,
screen indicates "SEARCHING FOR ..." red LED does not light and spindle
runs continuously. Check also UE6.

UH5 74LS197 (74177) LOGIC (TIMER)
     When drive powered up, red LED stays on and spindle motor runs
continuously. Check also UA3, UBC4, UC2, UD2, UD3, UE3, UDC5 & UE2.

UJ1 NE592N (LM592) READ AMPLIFIER
     Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error and red LED flashes. Check
also UH1 and UK4.

UK4 LM311 COMPARITOR (READ AMPL)
     Drive powers up and resets normally. Spindle motor runs, stepper
moves slightly, but "FILE NOT FOUND" error, and red LED flashes. Check
also UH1 and UJ1.


DISK DRIVE DIAGNOSTICS: WHAT TO LOOK FOR...

     When you are troubleshooting a drive, it is important to observe
it closely for symptoms. For example, note how the stepper, spindle
motor and indicator LED functions when the computer is powered up, when
the drive is reset, and when it is accessed by the computer. When the
drive is instructed to LOAD a program, note whether the stepper moves,
how much it moves, and if it "chatters". Try various functions like
Initialize and observe the results. Sometimes the clues to a malfunction 
are subtle. Because many chips can cause the same symptom, it's helpful 
to know which ones are most likely to cause a particular failure. As in 
the computer, the drive chips that normally get hottest will usually be 
the first ones to fail. 

DRIVE GOES DEAD SUDDENLY... WHAT HAPPENED?
     One shortcoming of the 1541 is the head getting "lost" past the
directory track. Some disk errors can do that. If your drive suddenly
goes "dead" and you can't read disks without "?FILE NOT FOUND" errors,
try the Initialize command: (example assumes drive is device 8)  
 OPEN15,8,15 <RETURN>
 PRINT#15,"I0" <RETURN>
 CLOSE15 <RETURN> 
This command (or formatting a disk) should pull the head back and may
bring the drive "back to life". The transit card (if you still have one)
will do the same thing and will also spare a program running in the 
computer. Be sure to turn the drive off before using the transit card. 
     One other problem that crops up is a head clog caused by our now 
very old disks. If you try to read a disk and your drive suddenly goes
unresponsive to all known good disks, suspect a head clog. Pull the 
top cover and clean the head with alcohol on a Q-tip. Let it dry and
then try the drive again. 

     To properly diagnose a potential problem, you have to know exactly
how the drive should respond when it's working correctly...
  COMPUTER POWER UP: red drive LED should come on and the spindle motor
should spin, then LED should go out and motor should stop within about
two seconds. This indicates completion of the DOS startup sequence.
  DRIVE RESET: the drive should reset when the button inside the
control door is pressed. The drive should react the same as when
powering up the computer. NOTE: this reset button is unique to the SX
and the C128DCR. It resets only the drive, not the computer or other
peripherals. 
  READ DIRECTORY: Insert a known good disk (it is preferable to try
more than one) and type: LOAD"$",8 and hit the RETURN key. The disk
should spin and the head should move to track 18 (if it is not over it
already) and read the directory. The screen will show: SEARCHING FOR $.
If it finds it, the screen will say READY and the drive motor will
stop. Type LIST and hit the RETURN key. The screen should list the
contents of the disk directory. (Exception: some commercial or game
programs will not have a visible directory to list.)
     If the drive "sees" data on the disk, but there are any problems
reading it, DOS will try several times before "giving up". You will
hear the drive chatter as it steps back to track zero in an attempt to
reestablish the correct position over the directory track. If it fails,
it will produce the error message: "?FILE NOT FOUND" and the drive red
LED will flash, the same as you would get if you tried to load a file
that didn't exist on the disk. One very important difference: if the
drive simply cannot find a file, but is otherwise reading the disk
correctly, it will -not- chatter.
     If the disk read fails for any mechanical reason (drive door open,
unformatted or bad disk, bad chip in the drive, etc.), you may hear the
head stepping back and forth (4 clicks) looking for track 18. The drive
red light will flash repeatedly and an error message: "?FILE NOT FOUND"
will appear. If you read the disk error channel, it will say: 74, DRIVE
NOT READY,00,00. That means the drive never saw good data on the disk.
If the drive tries, but the error message "SEARCHING FOR xxx" is
displayed and nothing else happens, it indicates a serial interface
problem between computer and drive. A "?DEVICE NOT PRESENT" error means
that the drive is "invisible" to the computer. In the SX, that's
usually a chip failure or open connection in the computer - drive
serial interface. Of course, if you try to access a non-existant drive
(drive 9 for example), you will get that error. Note that sometimes
just reseating socketed chips or connectors will fix a problem. It's a
good place to start when you're troubleshooting. 
  INITIALIZE: This command from the computer should move the head from
wherever it is to track 18 (directory) and the disk should spin. The
head will not move (but the spindle motor will turn) if it is already
over track 18. If there is no disk in the drive, or you insert an
unformatted disk, or if the drive door is open, it should cause the
spindle motor to run and the head to seek track 18 (directory)  
anyway. When it tries and fails, it will pull the head back to track
zero and "chatter" as it hits the head stop, then advance to where
track 18 should have been. The red light will flash because of the
drive read error. No error message will be shown on the screen, but if
you read the disk error channel, it will say: 21,READ ERROR,18,00.
  FORMAT OR DISK "NEW": When you format a disk, the spindle motor will
turn and the red light will come on. The drive will pull the head back
to track zero and "chatter", then the stepper will advance to each
track as it writes from track 1 to track 35. When it finishes the
format (about 1 minute 25 seconds on an unmodified drive), the head
will return to track 18 (directory). The format will first attempt to
write to track 1, then do a read, and if the read fails, the format
will terminate, and the head will not move from track 1. If it advances
a few tracks and stops, suspect a bad disk, an intermittant connection
to the head, dirty head, missing or worn pressure pad, etc.
     If a format fails, the red drive light will flash, but there will
be no error message on the screen. If you read the drive error channel,
it will say: 21,READ ERROR,00,00. Format failures can be caused by
write protect, drive door open, bad disk, bad or clogged head, or bad
chips in the drive. It could be as simple as a write-protect tab
getting stuck in the drive or dust in the sensor. If the drive will
read OK but fails to format a disk, check IC's that are used for the
"write" function and of course the sensor. The read/write head may test
good with an ohmmeter and read OK, but may still be defective and fail
to format a disk. You can temporarily swap out drive mechanics to check
the head. Substituting a Newtronics for the original ALPS drive may
present a problem as a permanent fix because the twist type of door
lever sticks out about 5/8" and hits the keys when the keyboard is
installed for transport.
     As mentioned above, it is sometimes helpful to read the disk drive
error channel when the drive red light is flashing. If you have 
JiffyDOS installed, just type @ and hit RETURN. If not, here is a small
BASIC program to do it. It reads the channel, displays the error
message, and turns the drive light off. All of the possible drive error
messages are listed in the back of the operators manual.
 10 OPEN 15,8,15 
 20 INPUT#15,EN,EM$,ET,ES 
 30 PRINT EN,EM$,ET,ES 
 40 CLOSE 15

     Suspect a drive alignment problem if the drive can read it's own
recently formatted disks, but other (commercial) disks fail to work. A
slightly misaligned drive will chatter when loading programs as it
encounters disk read errors, and it will fail to load if severely
misaligned. Note that some commercial programs have intentional errors
(copy protection) on the disk(s) that will make a normal drive chatter
and flash the red LED while loading.
     The SX drive doesn't suffer from the heat related alignment
problems that a stand alone 1541 does, but it can be "sluggish" and
look like it's out of alignment. That's usually caused by sticky head
rails. Any 1541 drive can become sluggish if it sits for a long time
without being used. The head assembly should slide back and forth
easily (drive turned off, of course). If rails are sticky, the head has
trouble finding the correct track quickly enough and you end up with
intermittant read errors, especially when the drive is cold. It may
find files close to track 18 but throw errors if the files are further
out on the disk. One other common problem of the ALPS mechanisms is a
sticking spindle clamper, the spring-loaded device that clamps the disk 
to the spindle. The grease in the clamp assembly hardens and gets 
sticky as it ages. That causes the disk rotation speed to vary and will
produce random read and write errors... very hard to track down! The
fix is to disassemble the clamper, clean it out with solvent, and relube
it with oil. Be careful you get all the washers back in the same place. 
The assembly will come apart when you remove the "C" washer, so don't 
let it, the spring, or other parts fly off and become lost.

DRIVE SELECT JUMPERS:
     Disabling the internal drive can be used as a diagnostic check
(example: when you're not sure what is "locking up" the serial port).
To disconnect the internal drive (and make it "invisible" to the
computer) so an external drive can be used as device 8, unplug P19, a
(six wire: five white and one black) cable on the FDD board, or the
other end at P11 on the I/O board.
     If you simply want to change the device number of the internal
drive, there is a set of jumpers on the FDD board, just as you would
find inside a 1541. They are located near P22, the 2 wire connector for
the drive reset switch.

FDD board top edge--||------------------||||||----------
                    ||                  ||||||
               P22 [||]  (2) (1)       [||||||] P19
                         drive select
                           jumpers

As in the stand-alone 1541, the SX drive has both drive jumpers closed
from the factory, so the drive is device #8. The jumpers, by the way, 
are the two tiny silver half-circles with a bar in between them. To open 
a jumper, cut the bar between the half-circles with an Xacto knife. To 
close an open jumper, apply a blob of solder to reconnect the 
half-circles. To change the drive to device #9, cut jumper 1. For device 
10, cut jumper 2, and for device 11, cut both jumpers. If you want to 
add a drive select switch, each wire of the added switch will go to one 
side of the cut jumper. If the solder pads are damaged, you can wire 
directly to the IC pins. Jumper 1 grounds pin 15 of IC UBC3 VIA on the 
FDD board and jumper 2 grounds pin 16 of that chip.

POWER SUPPLY...
     Fortunately, power supply problems are rare, unless the supply 
is overloaded by adding power-hungry accessories like modified RAM
expanders or hacking a second drive into the SX. The supply consists
of a +5 volt switcher (3.15 Amps), a +12 volt switcher (2.8 Amps), and
a transformer to generate the 9VAC (0.2 Amps) for the User Port. Note
that the TOD clocks do not run off the 60Hz power line frequency as in
a C64. In the SX, there is a crystal oscillator (UA1) on the I/O board
that feeds a 60Hz signal to the two CIA chips UB2 and UB3. The large
and heavy metal rear panel is the heat sink for the power supply. Air
should be allowed to circulate around it when the computer is running.
     The internal monitor gets its power from the +12 volt supply. Most
of the computer chips use the +5v source for power, but a few like the
SID and VIC also need +12 volts, as does the internal 1541. Loss of
only the 5 volts would make the computer appear dead. The monitor would
still have power but of course would have no picture. The dim orange
glow of the filaments in the picture tube neck (under the speaker) is
an indication that the monitor is running and probably has high voltage
since the heater current is "scan-derived" directly off the flyback
transformer. Since the +5 volt supply is "stepped down" from the +12
volt supply, loss of 12 volts would kill power to everything. There is
only one fast-blow fuse. It's located on the rear panel, is wired in
series with the power line, and is rated at 3.15 Amps. Power supply
problems should really be left to a technician. Switching power supplies 
are tough to diagnose and repair even for the experts.

Ray Carlsen CET
CARLSEN ELECTRONICS... a leader in trailing-edge technology.

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