28 Jun
2012
28 Jun
'12
12:39 p.m.
(no video, power supply good, video cable good monitor
good). Now what ?=20
There are many possible failures, but one likely one: the PLA chip. Now h=
ow=20
do I figure out if it is the PLA chip (then how do I remove and which too=
Now, I am not a C64 person at all, so these are goign to be genral comments.
There are basically 2 ways of determining if a compoennt is bad. One is
to remove it from the machine and to test it in your own circuit. The
otehr is to see how the machine is mishehaving and to see if the signals
coming from a partocualr component are right fgiven what's going into it.
Of course in complex circuits the signals goin in might well depend on
those coming out (think of a microprocssor, what it does depends on the
instructions being fed to it on the data bus),. so it's not always easy.
Now, IIRC the 'PLA' is a programmed 82S100 or simular. You can get a data
sheet for the unprogrammed part which will tell you the pinuts. It will
also tell you waht it conssits of :
There are 16 inputs. The programming produces 48 internal signals which
are the logical AND of any combination of those 16 inputs and their
inverses. For exampe the first term might be I0.I4.I7/.I9 . A second part
of the programming defines each of the 8 outputs from the chip as the
logical OR of any/all of those48 internal signals. And finally you cna
progam it so that each output is either normal or inverted.
I believe the C64 PLA programming is available somewhere. So from that
you can work out what the states of all 8 outputs should be for each
combination of the 16 inputs. That gives the first way to check it. You
apply eah combination of inputs in turn and check the outputs. Or more
practically you use a computer to do it.
The easier way is to test it i nthe C64. You start by seeign if any of
the outputs are ever chaning state. If not, i would suspect the IC is
bad. Not certain, but certainly a good posibility. If you are getting the
otuput changing, I'd us a logic analyer. Trigger on one such output and
see what the inputs are doing. Are they right? Or trigger the LA on a
combination of inputs that should cause an output to go active. Does that
combination ever occru (is the LA being triggered)? If so, is the output
changing correctly?
As for actualyl replacign the IC, it's a 28 pin DIL device. If you are
lucky, it's in a socekt. You then just flip it out with a small
screwdriver under each end of the IC, and put a replacement in. Actually
being a Commordore, I'd remove the old socket at fit a nice turned-pin
one. The problem chould be bad socket contacts.
If it's soldered driectly to tyhe board you have 2 options really. One is
to carefully desolder all 28 pins with the iron and socler sucker. Then
when each pin is free (use a small screwdriver or pliers to wiggle the
pin around on both sides of the PCB to break any remaing sodler bond),
you pul lthe IC out. Then fit a socket, so it's easier next time.
The second option is that if you _know_ the IC is dead, you cut the pins
off near the body of the IC (you need some fine end or side cutters for
this, which are not cheap!). Then remove the body of the IC and desodler
and remove each pin separately. Clean out the holeson the board (melt the
solder with an iron o nthe solder side, use the solder sucker on the
component side), then fit the socket, etc.
ls=20
do I use etc.) ? Logic probe ? Multi-meter ? Oscilloscope ? I don't know =
how=20
to use any of them. I never asked for "specific" advice, only general=20
advice. Which tools to get etc.
AS I hope you've gatehrerd by now, your question, while certainly valid
and on-topic, doesn't have a simple answer. There is no magic box that
will tell you what to replace. You will end up learning a lot of analogue
and digital electronics, and practicing soldering, etc. I don;t think
_anyone_ can learn this sort of stuff in a few days.
I don;t want to put you off, and I'll certianly help if I can (as will
others here), but I am jsut lettign you know what you are gettign into.
There are two possible approaches to giving help
with such repairs, whic=
h
could be compared to the 'give an man a
fish' .vs. 'teach a man to fish'.
Tony, I understand and agree.
rather 'teach a man to fish'. Rather than
telling them to check this and
then change that IC, I'd ecplain to them how the thing should work, how =
to
check things, and so on. In the end they will
learn enough to be able to
trace faults themselves, and the knowledge of fault diagnosis --=20
soemthign that seems to be rarely taught -- is preserved. None of us are
goign to eb around for ever, so it is a Good Thing to pass on our skills
in this way.
I think a little of both show me how to fix it then have me figure out th=
e=20
next one. A little helping, a little trying on my own. All the while=20
explaining that is going on and adding knowledge. After all you have to=20
start somewhere.