[sdiy] Odd question about Lexicon 224xl

[Tim Curtis]

Hi Guys -

Here's one that for some reason has me stumped.

I've got a 224XL that has a couple of problems.  The symptoms are:

    1)  Often times, on power up, the unit doesn't come all the way up.  
It's just plain locked up before it starts it's "power on diagnostics".

    2)  Other times, when it comes up from a completely cold state, it 
powers up, runs through it's diagnostics, and begins to function 
normally.  If the unit is then either reset or powered off and then back 
on, it comes up with a set of about a half dozen errors all relating to 
the Arithmetic board.  After going through the list of errors it begins 
to function, but I'm not sure that it's completely functional.

I've gone through just about every IC on the arithmetic board and today 
I found something odd.  This is where I get lost.

The board has eight 4.7uF and two 22uF tantalums spread around between 
+5V and ground.  I put a meter on it while it was out of the mainframe 
and found that between +V and Ground I'm only reading about 170 ohms of 
resistance.  This seems awfully low to me.  Perhaps a leaky tantalum 
that's not completely shorted out?  Or is this possibly correct?

The additional factor is that there are about 5 74S163 ICs on the 
board.  I pulled them one at a time and found that in the case of 4 of 
them, pulling one raised the resistance by about 10 ohms.  However, one 
of the 163s changed the +V to G resistance by about 80 ohms.  This is 
the case no matter where it's used in the circuit - unique to that 
particular 74s163.

I ordered up some replacements for the 163s and also some extra 
tantalums.  Does it sound like I might be heading in the right 
direction?  What kind of resistance would you guys assume I'd normally 
find between +V and G?  I can't get schematics for this thing, so I'm 
not sure just what to expect.

Thanks!

Tim




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