[sdiy] Odd question about Lexicon 224xl

[harrybissell]

5V / 170 ohms = 29mA  not too unusual.

The one IC with the odd draw might be suspect.

Also try reversing the ohmmeter probes, and/or
selecting diiode test mode...

Shotgun replacement is an option when there are
no schematics available...

Try to check logic gates for proper states (either high
or low, no half-way voltages)

H^) harry

Tim Curtis wrote:

> 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
>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.308 / Virus Database: 266.9.17 - Release Date: 04/19/05