HP 9825 16K 09825-66523 memoryboard failure NOT ANY MORE !

[Rik Bos]

To keep you posted :

I made a error by stating the -12V was wrong, it was the +12V that's why I
replaced the uA723. (mea culpa etc....)
The TMS4060's don't have a -12V connection they get +5,+12,0,-5V.
The board is working again, the VCC pin of lowerbit 9 had a resistance of
12.5 ohm to the VCC-line.
The A10 pin of the same chip didn't connect to anything. 
A little bypassing did the job, so thanks for your advice.

-Rik

> -----Oorspronkelijk bericht-----
> Van: cctalk-bounces at classiccmp.org 
> [mailto:cctalk-bounces at classiccmp.org] Namens Tony Duell
> Verzonden: zondag 5 april 2009 18:57
> Aan: cctalk at classiccmp.org
> Onderwerp: Re: HP 9825 16K 09825-66523 memoryboard failure
> 
> > > > 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
> 
> 
>