HP 9825 16K 09825-66523 memoryboard failure

[Rik Bos]

> -----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
too.

> 
> > 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
that first.
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. 
> 
> -tony
> 
Thanks,
-Rik