Heat Sinking ICs in the C-64 2-18-2013.


            Heat Sinking ICs in the C-64     
               latest additions and corrections 2-18-2013


Solid state devices such as Integrated Circuits (ICs) fail for two
basic reasons, assuming they were not defective from the factory:
overvoltage and excessive heat. The Commodore "brick" PS has a history 
of sudden failure, sometimes taking the computer with it. After-market
repairable supplies are much more reliable. Voltage "spikes" and static
electricity can destroy ICs in the computer through any of the accessory
ports. Both of these hazards are preventable to a degree... the use of a
higher quality supply and simple precautions such as never plugging
anything in or unplugging (cartridges, mouse, etc.) when the computer is 
turned on just make good sense. Static electricity buildup on a dry Winter 
day can damage chips in the computer if you touch one of the ports before 
draining off the charge on your body or clothing. You have experienced 
the shock of shuffling your feet across a carpet and touching a doorknob.
That can be a fatal shock for IC chips in the computer. 
     Repeated heating and cooling of semiconductor devices (especially 
ICs and power transistors/diodes) with normal use, even within their 
normal temperature range, will eventually cause them to fail. It is 
usually the devices that generate the most heat in normal operation that 
fail first, such as the PLA chip in the C64. If you bend a piece of metal 
back and forth, it eventually fatigues and breaks. Repeated heating and 
cooling does the same thing to the microscopic components inside an IC 
or transistor. So, is there any way to prevent the damage or at least 
make parts last longer? Absolutely!
     You could leave the computer on 24 hours a day... some systems run
that way and a BBS was a good example. I knew of one system that had run
without a breakdown for over 10 years (except for the old hard drive power
supplies). For most of us however, the computer doesn't get enough use to
justify the expense of a system running all the time. 
     An alternate solution is to reduce the extremes of temperature inside 
the ICs by the use of heat "sinks", and thereby reduce the thermal stress 
on the devices. A heat sink is a strip of metal fastened to the IC to draw 
off its waste heat and allow it to flow out to the surrounding air. 
     The physical design of the C64 limits the airflow over the chips. 
Tiny air vents in several places in the upper and lower half-shells offer
minimal convection cooling. That cardboard and tinfoil shield inside the
early C64s, although of questionable usefulness as an interference filter, 
is very good at -blocking- the airflow and holding all the heat inside the 
case. I have run tests to see how effective that shield is in reducing 
radio and TV interference, and found little difference after removing the 
shield. In fact, a stock 1541 (with all shields in place) generates more 
"noise" than the C64 with or without its "shield". Bottom line: remove the 
tinfoil/cardboard shield from your C64 if it has one. The later C64 has a 
all-metal shield with "fingers" that act as heat sinks. Do not remove it!
     Four of the large ICs tend to run hot normally: the PLA, the SID, the
MPU and the VIC. They would benefit most from heat sinking. That later C64, 
the C64C and the "flat" C128 all have a metal shield with "fingers" that 
touch each of the larger chips. Even so, the fingers barely touch those 
chips when the computer comes from the factory. To provide maximum heat 
transfer, I always remove the shield and bend the fingers down a bit so 
when the shield is replaced, all tabs sit flat on all chips. See the photo 
SHIELD SINKS.JPG. It is a before & after image of the smears of heat 
sink compound on each IC. The larger contact area provides the best effect.
     The C-128DCR (metal box) has no such shield but does have space for a 
small internal fan near the power supply. NOTE: Most 128DCR boxes were 
shipped without a fan installed. Commodore saved a few bucks. Even so, the 
IC's used in the DCR are of a different IC "family" called CMOS and don't 
normally run hot like the earlier NMOS C64 chips do. The SX-64 has one heat 
sink clamped to the VIC chip, and the older model C64 has a metal "finger" 
touching the VIC chip inside the metal box. Other than that, the C64 and SX 
computers have no sinking for their hot chips. However, you can easily make 
and install them yourself to extend the life of your computer. 

FOR THE "BREADBOX" 64:
     Remove the screws from the bottom of the computer, but leave the
keyboard in place. Run the computer for 10 minutes or so, lift the keyboard 
and feel the tops of all the chips... some will be quite warm. Those are 
the ones that most need the help! You may be able to purchase (if still 
available) commercial heat sinks that fasten to the top of the chips with 
clamps, adhesive strips or glue. The larger, the better. If you want to 
make your own, any thin metal stock will do. The only consideration is that 
it be flexible enough to bend without breaking, and stiff enough to hold 
its shape when bent. It must be flat on the bottom for intimate contact 
when glued to the chip. I once used strips of "tin" cut from large juice 
cans. The sink ears should be made just high enough to clear all components 
on the PC board, but not touching the keyboard when it is reattached: bend 
the ears over if you need to for clearance. The large surface area of the 
added heat sink is necessary since the airflow inside the case is poor. See 
the photos of the various versions of added sinks. You may notice a string 
around the sink in some of the photos. I always add that as a fail-safe in 
case the sink gets dislodged in transit. Although the epoxy I use is rugged, 
if a sink should break loose for any reason, the string (it's actually 
nylon fishing line) will hold it in place so it can't move around inside 
the computer and short something out.
     I use fast-set epoxy to attach the sink to the IC. If you have several
sinks prepared ahead of time, you can mix one batch and glue them all
before the epoxy starts to set. Print the name or number of the IC on the 
top of the sink with a felt tip pen ahead of time so you can still identify 
the chips when you're done. Be careful not to use too much epoxy (one small 
pea-sized drop is plenty), or it will ooze out the sides and down into the 
socket. (Note: not all chips are socketed.) Spread the epoxy over the sink 
(or on top of the chip) and gently press it into place. I don't recommend 
Superglue as it will eventually break loose from the IC. Heat sink (white 
silicon paste) compound should only be used as for thermal transfer if a 
commercial heat sink is used. It is not glue and can't be used as such. 
Epoxy forms a good bond even with irregular surfaces, as it fills the 
"voids" and provides sufficient heat transfer to the sink. Just make sure 
the metal and IC top is free of dirt, oil, fingerprints, etc. before you 
apply the epoxy. Use grease-cutting solvent on both surfaces, if necessary. 
Paint thinner works well.
     As I stated, the chips that benefit most from sinking are the large
heat producing ones, like the PLA (CBM #906114-XX, 82S100PLA, or 93459),
the SID (CBM #906112-XX or 6581), and the VIC (CBM #906109 or 6567). The
6510 CPU and the Kernal ROM should have sinks as well. The VIC chip is 
already sinked in later C-64s. It's inside a metal box and the cover has 
a tab that touches the chip. If yours doesn't have one, sink it.
Whatever sink design you use, make sure your heat sink fins don't touch 
any circuit components and cause a short! The two VIA (6526) chips run 
cool to the touch and don't really need sinking. Since they connect to the 
external ports, they are more sensitive to static "zaps". The other ROMs 
run warm and could probably benefit from sinks but they rarely fail 
without them. The eight RAM chips (4164) normally run cool, at least until 
the power supply regulator fails. When RAM chips get hot to the touch, 
they're gone! 

SINKS FOR THE 1541:
     Heat sinking can be added to the chips in the 1541 disk drive as
well. Its power supply is internal, making IC heat buildup even worse.
The problem with modifications to the drive is the space limitations
inside the case. It's pretty crowded in there, but it can be done. Find
the chips that get the hottest as described with the C-64 and add sinking
as necessary. The heat sinks you make will have to be cut to fit the space
allowed. The two bridge rectifier blocks get -very- hot and would
certainly benefit from sinking. The easiest way is to cut and form the
metal so it forms a U shape to clamp over the diodes (without touching the
wiring, of course). A small amount of epoxy on one side will hold it in
place. You might consider adding a small fan to the drive to keep the heat
from building up around the hottest chips. Look for instructions in another
article about adding fans to Commodore equipment. Strangely enough, the 
most common chip failure in a 1541 is the DOS ROM (901229-0x), but it 
doesn't run hot! Heat buildup in that drive is one reason for R/W head 
misalignment as the drive hits the zero stop after warmup. 
     The 1571 has an internal switch-mode power supply that all but
eliminates heat problems. The 1541-II and 1581 both use external supplies
and consequently run cooler. No matter what equipment you have, it's
important to keep the vents clear on the computer and accessories. Don't
put papers, books or disks over the vent holes on the computer, drives or
monitor. Don't stack 1541s. Replace your original C-64 supply with one of
the repairable "heavy duty" types if you can find one. A small fan placed 
on the rear of the 1541 drive is probably the best way to fan-cool it. 
Installing a fan in the C128DCR is a good way to extend the life of that 
computer although none of its chips actually run hot. Some users don't 
like the noise a fan generates but I don't mind because I know it helps 
lengthen the life of the internal components. 
     IC chips and other parts for Commodore computers and periferals are
in short supply. Obviously, sinking will not help a chip that is already
failing from excessive heat, but with the simple suggestions and
modifications described here, you can help to extend the life of the ICs
and keep your Commodore system working for many more years. 

Ray Carlsen CET
Carlsen Electronics