Another SX-64 problem for the group.

[Roy J. Tellason]

On Thursday 15 June 2006 06:14 pm, David Vohs wrote:
> I have an SX-64 that has (apparently) a serious problem. On powerup it
> shows nothing on the screen (checked the settings and and also hooked up
> an external monitor) along with an occasional crackling sound from the
> speaker. Keypresses are not registered by the machine.
>
> Anyone got any ideas as to what the problem could be?

When you say it "shows nothing" do you mean that the  screen is as if it's not 
even powered up?  Completely blank?

This is a common problem with C64s.  First thing you want to check is the 
power supplies,  make sure that there's +5V where there's supposed to be, 
easiest place being at the corner pin of pretty much any of the logic chips.

Do you have a scope?  A logic probe if not?  Find the PLA chip,  it'll be a 
28-pin DIP,  mostly marked with the number 906114 though sometimes the 
generic 82S100 will be on there,  and I'm not 100% sure if the SX64 machines 
may have used a variant on that or not.  There will only be two 28-pin parts 
in there,  and the other one is the 6581 SID chip (or variant, possibly).  
Scope (or probe) four pins on each side at the end opposite the notch.  Pin 
14 is ground,  so you want to hit 10, 11, 12, 13, 15, 16, 17, 18.  Each of 
these pins should show a high level,  or a pulse,  but none of them should be 
stuck low -- if one is it'll cause this symptom.  I've also seen some that 
would only go "halfway up",  where a scope would show two levels, ground and 
about 2.5V or so.  This could be the PLA or it could be the chip at the other 
end of that enable line.

If that doesn't turn anything up,  scope the data bus.  Easiest way I found to 
do this was to hit pin 2 of each of the ram chips,  but I don't recall how 
easy that was on the SX,  it's been a really long time since I've seen one.  
Again,  you're looking for a logic pulse there,  a line that's stuck low will 
kill the machine and give you the blank screen symptom.

Unfortunately,  so will a number of other problems,  any number of other chips 
could short a bus line to ground and the only way to track them down 
sometimes is to pull them out and see if the line shows normal transitions 
after that.  The company seemed to have *no* consistency whatsoever when it 
came to what was or wasn't socketed,  though,  which makes it even more fun.  
I used to buy my sockets 50 at a time and the smaller 14- and 16-pin ones 100 
at a time for sticking them in there once I'd unsoldered a chip,  because 
those boards weren't robust enough to take unsoldering more than once.

A couple of other notes,  on c= stuff in general-- bad ram can get HOT!  This 
got bad enough that I found a little thermister that I stuck in the end of an 
old pen barrel with a bit of wire and a dual banana plug to give me a 
homebrew temperature probe.  Burn your fingers,  that can...   The other 
applies much more to c64s than the SX,  since they used those stoopid potted 
"brick" power supplies.  A bad PS that went overvoltage (and they often did 
because that potting compound did NOT let the heat out) was usually a good 
machine-killer,  often taking out the RAM first.  Which is when that 
temperature probe came in handy.  As well as the little box I built with a 
couple of switches and some resistors that'd let me put one of those under 
full load and meter both the AC and DC sides.  :-)  The SX wasn't anywhere 
near as prone to those problems,  though.

Good luck with it,  and feel free to email me if you have any further 
questions on this or maybe need any parts (though I'm not sure about PLAs, 
since somebody has already asked about those and I've gotta do some 
digging :-).


-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
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