HP 9825 16K 09825-66523 memoryboard failure

[Tony Duell]

> > > 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 thought hte -12V rail came from a 3-terminal regulator (7912 or 
similar). There's a 723 in the _+12V_ supply IIRC though. 

> 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 think you're going to have to do some measurements before reolacing odd 
components at random. As I mentioned. some of these old HP boards don't 
like being resoldered too often, the through-hole plating fails and leads 
to more problems. Please don't ask how I discovered this!

> > 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.

Using an ohmmeter on the board (with it powered down, of course) will 
verify that all the address lines are connected between the RAMs, etc.

-tony