HP 9825 16K 09825-66523 memoryboard failure
hp-fix at xs4all.nl
Sat Apr 4 16:00:47 CDT 2009
> -----Oorspronkelijk bericht-----
> Van: cctalk-bounces at classiccmp.org
> [mailto:cctalk-bounces at classiccmp.org] Namens Tony Duell
> Verzonden: zaterdag 4 april 2009 22:05
> Aan: cctalk at classiccmp.org
> Onderwerp: Re: HP 9825 16K 09825-66523 memoryboard failure
> > I have a 16K memoryboard for my HP9825A it suffered from a bad -12V
> > and
> I assume you have schematics.
Yep, thanks to your excellent work.
> > destroyed all the rams TMS4080.
> This puzzles me a bit. The -12V rail is regulated down to -5V
> using a resistor/zener circuit,. and it's the -5V that's used
> by the RAMs. I assume you've checked this voltage.
> Or did the -12 disappear, removning the bias from the DRAMs
> and cooking them?
The -12V was defective (not on this machine but on another witch I fixed
(uA723 replaced) and put a 8k board in it.)
I measured the voltage and there was little or no noise on the power lines.
I did put some extra decoupling caps (100nF and 2.2uF tantalium) on the
power lines just to be sure.
That wasn't the problem, the errors are a bit erratic always the same bit 9
but not always the same address.
Today I replaced the address buffer ('LS368) but that wasn't the solution
> > I bought from ICI 32 pcs. nos uPD411AC-2 replaced all the
> rams but I'm
> > getting an error when testing the board.
> > The HP9825 isn't under suspect because it works okay with
> another 16k board.
> > The systemtest (from the tape) says : 60K board failes Address
> > 60100,60300,60400 not always the same sometimes only 60400.
> > Bit 9 sense 0 or 1 (not consistent sometimes 1 sometimes 0)
> I swapped
> > the
> > LB9 and UB9 chips on the board with other 411's(new ones) But the
> > error stays I replaced the refreshcounters decoupling caps but
> > nothing.
> First check the power lines with a 'scope. Noise on power
> lines causes all sorts of problems with DRAMs.
> I've had vias fail on old HP boards when
> soldering/desoldering components. My next task would be to
> check that all pins on all RAMs go to the rignt places and
> that none are left floating.
I'll think I'm going to do that I got a current tracer, so I'm going to use
And see if something is coming up.
> And then do make sure that all address, data, etc pins are
> being driving nad that none are floating around.
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