HP 9825 16K 09825-66523 memoryboard failure

[Tony Duell]

> 
> I have a 16K memoryboard for my HP9825A it suffered from a bad -12V and

I assume you have schematics.

> 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?

> 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. 

And then do make sure that all address, data, etc pins are being driving 
nad that none are floating around. 

-tony